Jan Terje Lifjeld

Publications

• Øksnebjerg, Daniel Bilyeli; Nielsen, Sascha Dreyer; Chen, Guangji; Andersen, Michael J.; Almagro, Vanessa & Ambrosini, Roberto [Show all 93 contributors for this article] (2026). A comprehensive reference genome assembly dataset of birds inhabiting Denmark, Greenland, and the Faroe Islands. GigaScience. 15, p. 1–9. doi: 10.1093/gigascience/giag078.
• Chen, Guangji; Wang, Shuang; Bilyeli Øksnebjerg, Daniel; Nielsen, Sascha Dreyer; Dai, Wei & Jiang, Wei [Show all 69 contributors for this article] (2026). Draft assemblies for 177 bird species enhance genus-level coverage. GigaScience. 15. doi: 10.1093/gigascience/giag045. Full text in Research Archive
• Cramer, Emily R. A.; Grønstøl, Gaute; Benham, Phred M.; Cicero, Carla; Bowie, Rauri C. K. & Tobiansky, Daniel J. [Show all 7 contributors for this article] (2025). Unusual Sperm Morphology in Two Sedentary Songbird Species. Ecology and Evolution. 15(8). doi: 10.1002/ece3.71873. Full text in Research Archive Show summary

  Sperm morphology can differ dramatically among closely related species. Within songbirds, the typical filiform sperm has a slender, corkscrew‐shaped head and an elongated midpiece coiled around the flagellum. However, three songbird species are known to have an unusual tadpole‐like sperm morphology with a round or ellipsoid head and short, uncoiled midpiece, which may arise developmentally via neoteny. Here, we describe tadpole‐like sperm phenotypes from two additional songbird species, the white‐breasted nuthatch ( Sitta carolinensis ) and the wrentit ( Chamaea fasciata ). These five species with tadpole‐like sperm share several ecological characteristics that can inform hypotheses for the evolution of this unusual phenotype: They are largely non‐migratory, form long‐lasting pair bonds, and have high paternal investment, small testes (no data for wrentit), and short and highly variable sperm lengths. These characteristics could indicate particular natural selection pressures driving physiological states such as low testicular testosterone levels. Additionally, though direct measures of female promiscuity are lacking in these species, these characteristics are consistent with weak post‐copulatory sexual selection. Further study of these similar yet independent evolutionary events, across all levels of analysis, would be valuable for understanding how such dramatic shifts in phenotype evolve.

• Lifjeld, Jan Terje; Garcia-del-Rey, Eduardo; Grønstøl, Gaute; Valle, Carlos A & Leder, Erica Helen (2025). Weak sperm differentiation in Darwin’s finches. Biological Journal of the Linnean Society. ISSN 0024-4066. 146(2). doi: 10.1093/biolinnean/blaf103. Full text in Research Archive Show summary

  Spermatozoa may provide insights into the evolutionary history, reproductive isolation, and mating systems of species. Here we combine sperm and genomic data to conduct the first comparative analysis of sperm differentiation among Darwin’s finches, an iconic adaptive radiation with considerable gene flow across species borders. All eight study species had the typical form of songbird spermatozoa, but shorter than most other species in the Thraupidae family. There was no detectable differentiation in sperm length among four ground finch species (Geospiza) and two tree finch species (Camarhynchus). In both genera, autosomal single nucleotide polymorphisms (SNPs) revealed signatures of genetic admixture. The grey warbler-finch Certhidea fusca and the vegetarian finch Platyspiza crassirostris had significantly shorter sperm than the sister genera Geospiza and Camarhynchus from which they diverged about 0.90 and 0.43 Mya, respectively. The largest intergeneric divergences in sperm length were of the same magnitude as divergences observed within the speciation continuum in other songbirds over similar time spans. Relatively high among-male variation in sperm length indicates a moderate-to-low level of extrapair paternity and a divergence rate in sperm length that is lower than in more promiscuous songbirds. We conclude that sperm size evolution is too slow to drive prezygotic isolation in this radiation.

• Bagdonaite, Laima; Leder, Erica Helen; Lifjeld, Jan Terje; Johnsen, Arild & Mauvisseau, Quentin (2025). Assessing reliability and accuracy of qPCR, dPCR and ddPCR for estimating mitochondrial DNA copy number in songbird blood and sperm cells. PeerJ. 13. doi: 10.7717/peerj.19278. Full text in Research Archive Show summary

  Mitochondrial DNA (mtDNA) copy number varies across species, individuals, and cell types. In birds, there are two types of cells with a relatively low number of mitochondria: red blood cells and spermatozoa. Previous studies investigating variation of mitochondrial abundance in animal sperm have generally used quantitative PCR (qPCR), but this method shows potential limitations when quantifying targets at low abundance. To mitigate such issues, we investigated and compared the reliability and accuracy of qPCR, digital PCR (dPCR) and droplet digital PCR (ddPCR) to quantify high and low concentration DNA. We used synthetic DNA targets, to calculate the limit of detection and the limit of quantification and found that with both dPCR and ddPCR, these limits were lower than with qPCR. Then, to compare quantification accuracy and repeatability, we used DNA extracted from blood and sperm cells of Eurasian siskin. We found that qPCR, dPCR and ddPCR all reliably quantified mitochondrial DNA in sperm samples but showed significant differences when analyzing the typically lower levels of mtDNA in blood, with ddPCR consistently showing lower variation among replicates. Our study provides critical insights and recommendations for future studies aiming to quantify target mtDNA and indicates that dPCR and ddPCR are the preferred methods when working with samples with low abundance of mtDNA.

• Lifjeld, Jan Terje; Kleven, Oddmund; Amundsen, Trond & Slagsvold, Tore (2025). Extrapair copulations in coal tits (Periparus ater): female insurance against male infertility? Behavioral Ecology and Sociobiology. ISSN 0340-5443. 79. doi: 10.1007/s00265-025-03621-4. Full text in Research Archive Show summary

  The evolutionary causes of female extrapair mating in socially monogamous birds are poorly understood. Adaptive hypotheses focus on direct benefits, i.e. fertility and paternal care, or indirect, genetic benefits, but the empirical support is generally sparse and mixed. Here we report a study of extrapair paternity in a Norwegian population of coal tits (Periparus ater), a species in which previous studies have reported a high frequency of extrapair paternity, but with no clear support for any female benefits. We confirmed a high occurrence of extrapair paternity (32% of offspring) and found that most extrapair sires were breeding neighbours. Extrapair offspring were heavier than their withinpair nestmates during the early nestling period, indicating that they were among the earlier eggs in the laying sequence. We evaluate various hypotheses for female extrapair mating and conclude that a direct benefit in the form of insurance against mate infertility is a plausible explanation to our results. There is empirical evidence for a low, but presumably biologically significant rate of infertile males in several passerine species, including other tit species. Fertility insurance may have a wider relevance as a direct benefit of female extrapair copulation than previously acknowledged, and male infertility warrants more scrutiny in wild birds in general. Significance statement Nearly one third of all offspring in coal tit broods were sired by a male neighbour. Data on early nestling body mass hierarchy indicate a paternity skew in the egg-laying order. The results are consistent with the fertility insurance hypothesis, which states that female birds engage in extrapair copulations before the start of egg laying to safeguard against a potentially infertile mate. However, direct evidence for male infertility is lacking and further studies are needed.

• Bagdonaite, Laima; Mauvisseau, Quentin; Johnsen, Arild; Lifjeld, Jan Terje & Leder, Erica Helen (2025). Sperm mtDNA Copy Number Is Not Associated With Midpiece Size Among Songbirds. Ecology and Evolution. 15(3). doi: 10.1002/ece3.71055. Full text in Research Archive Show summary

  Tremendous variation in sperm morphology is observed across the animal kingdom. Within avian taxa, the songbirds (infraorder Passerides) have the largest variation in sperm morphology. Their spermatozoa move by using energy generated in the midpiece, which is formed by multiple mitochondria fusing together during spermatogenesis. However, very little is known regarding the number of mitochondria required to form the songbird midpiece. Based on previous research showing an association of midpiece length and mitochondrial DNA (mtDNA) copy number in the zebra finch Taeniopygia guttata, we hypothesize that songbird species with longer sperm midpieces have more copies of mtDNA. We estimated the sperm mtDNA copy number in 19 species from 10 families within Passerides, covering a broad range of midpiece sizes. Mitochondrial and nuclear DNA abundance were determined using droplet digital PCR (ddPCR) and the ratio between mitochondrial and single-copy nuclear genes was used to estimate mtDNA copy number per spermatozoon. We found that species differ in their average mtDNA copy number, but the variation was small and not significantly related to midpiece length. A possible explanation is that mitochondrial genomes are eliminated in the spermatids during spermatogenesis.

• Rowe, Melissah; Hooper, Daniel M.; Hofgaard, Antje; Hurley, Laura L.; McDiarmid, Callum S & Pataraia, Ioanna [Show all 8 contributors for this article] (2024). Independent evolution of atypical sperm morphology in a passerine bird, the red-browed finch (Neochmia temporalis temporalis). BioRxiv. ISSN 0362-4331. doi: 10.1101/2024.09.30.615754. Full text in Research Archive Show summary

  Spermatozoa exhibit striking morphological variation across the animal kingdom. In passerine birds, sperm exhibit considerable variation in size, yet the basic sperm phenotype is highly conserved; sperm are filiform, the head is corkscrew-shaped, and the midpiece is elongated and twisted around the flagellum. A significant departure from this typical sperm morphology has been reported in the sister species, the Eurasian bullfinch (Pyrrhula pyrrhula) and Azores bullfinch (P. murina). Here, we report a second evolutionary shift in passerine sperm phenotype in the nominate subspecies of the red-browed finch (Neochmia temporalis temporalis); sperm are non-filiform, with an ellipsoid head, and an extremely short midpiece restricted to the nuclear-axoneme junction. Additionally, we show that the sperm phenotype of the red-browed finch is similar to the putatively neotenous sperm described in the two bullfinch species. Using whole-genome data, we found no evidence that the unusual sperm phenotype of the red-browed finch is associated with reduced genetic variation or a population bottleneck. In contrast, we find some support for the hypothesis that relaxed post-copulatory sexual selection may, at least in part, explain the unusual sperm of the red-browed finch. We also discuss the possible roles of mutation, genetic drift, and genetic hitchhiking, in the evolutionary origins and maintenance of neotenous sperm phenotypes. Finally, we suggest that these dramatic evolutionary shifts in sperm phenotype warrant further investigation and highlight the need for a greater understanding of the developmental and genomic basis of sperm phenotype.

• Malick-Wahls, Sarah Lou; Byrkjedal, Ingvar; Ganter, Barbara; Lifjeld, Jan Terje; Marthinsen, Gunnhild & Rösner, Hans-Ulrich [Show all 7 contributors for this article] (2024). Morphometric differences between sexes and populations in Norwegian Dunlins Calidris alpina. Bird Study. ISSN 0006-3657. doi: 10.1080/00063657.2024.2392214. Full text in Research Archive Show summary

  Capsule The sex of Dunlins from two widely separated Norwegian breeding populations can be determined using morphometrics, with the Dunlins’ sizes differing between populations. Aims We aimed to understand the Dunlin’s physical characteristics within and outside of a zone of intergradation between the subspecies C. a. schinzii and C. a. alpina, and to provide a resource for determining the sexes. Methods We tested for morphometric differences between Dunlins from Finnmark, inland South Norway and coastal South Norway to determine if any or all of these populations had unique characteristics. We also compared morphometrics from our study with those in published sources. We performed discriminant function analyses on morphometric data from Dunlins of known sex to identify variables that could be used for sex identification in captured birds. Results Dunlins from Finnmark were significantly larger than those from South Norway. However, birds from inland South Norway, and a small sample from coastal South Norway, overlapped in multiple measurements. Our comparison with previously published sources also found no clear differences in morphometrics between birds from South Norway and C. a. schinzii from other populations. Our discriminant functions for both Finnmark and inland South Norway accurately predicted the sex of Dunlins using only the wing chord and bill length measurements. Conclusion There was no clear evidence that Dunlins from the subspecies intergradation zone in South Norway differed in size from subspecies C. a. schinzii. The sex of Norwegian-breeding Dunlins can be accurately determined by morphometrics alone, using population-specific discriminant functions.

• Bagdonaite, Laima; Leder, Erica Helen; Lifjeld, Jan Terje; Johnsen, Arild & Mauvisseau, Quentin (2024). Assessing reliability and accuracy of qPCR, dPCR and ddPCR for estimating mtDNA copy number in songbird blood and sperm cells. BioRxiv. ISSN 0362-4331. doi: 10.1101/2024.11.08.622696. Full text in Research Archive Show summary

  Mitochondrial DNA copy number varies across species, individuals and cell types. Two avian cell types carrying a relatively low number of mitochondria are the red blood cells and spermatozoa. While previous studies investigating variation of mitochondrial abundance in animal sperm have generally used quantitative PCR (qPCR), this method shows potential limitations when quantifying low abundant targets. To mitigate such issues, we investigated and compared the reliability and accuracy of qPCR, digital PCR (dPCR) and droplet digital PCR (ddPCR) to quantify high and low concentration DNA. Using synthetic DNA, we found that both dPCR and ddPCR displayed lower Limit of Detection and Limit of Quantification than qPCR. Using DNA extracted from blood and sperm cells of Eurasian Siskin, we found that qPCR, dPCR and ddPCR reliably quantified mitochondrial DNA in sperm samples, but showed significant differences when analyzing typically lower levels of mtDNA in blood. We found that ddPCR consistently showed lower variation among replicates. These analyses provide critical insights and recommendations for future studies aiming to quantify target mtDNA. Our study indicates that dPCR and ddPCR are the preferred methods when working with samples with low abundance of mtDNA.

• Lifjeld, Jan Terje; Cramer, Becky; Leder, Erica Helen & Voje, Kjetil L. (2024). Sperm as a speciation phenotype in promiscuous songbirds. Evolution. ISSN 0014-3820. 79(1), p. 134–143. doi: 10.1093/evolut/qpae154. Full text in Research Archive Show summary

  Sperm morphology varies considerably among species. Sperm traits may contribute to speciation if they diverge fast in allopatry and cause conspecific sperm precedence upon secondary contact. However, their role in driving prezygotic isolation has been poorly investigated. Here we test the hypothesis that, early in the speciation process, female promiscuity promotes a reduction in overlap in sperm length distributions among songbird populations. We assembled a data set of 20 pairs of populations with known sperm length distributions, a published estimate of divergence time, and an index of female promiscuity derived from extrapair paternity rates or relative testis size. We found that sperm length distributions diverged more rapidly in more promiscuous species. Faster divergence between sperm length distributions was caused by the lower variance in the trait in more promiscuous species, and not by faster divergence of the mean sperm lengths. The reduced variance is presumably due to stronger stabilizing selection on sperm length mediated by sperm competition. If divergent sperm length optima in allopatry causes conspecific sperm precedence in sympatry, which remains to be shown empirically, female promiscuity may promote prezygotic isolation and rapid speciation in songbirds.

• Lifjeld, Jan Terje; Hernando, Miguel de Gabriel; Marcos, Benito Fuertes; Grønstøl, Gaute; Anmarkrud, Jarl Andreas & Matschiner, Michael [Show all 7 contributors for this article] (2023). Rapid sperm length divergence in a polygynandrous passerine: a mechanism of cryptic speciation? Evolution. ISSN 0014-3820. 77(11), p. 2352–2364. doi: 10.1093/evolut/qpad146. Full text in Research Archive
• Johannessen, Lars Erik; Johnsen, Arild; Koppetsch, Thore; Lifjeld, Jan Terje; Matschiner, Michael & Søli, Geir Einar Ellefsen [Show all 7 contributors for this article] (2023). Arctic Specimens in the Zoological Collections at the Natural History Museum, University of Oslo, Norway (NHMO). Collections: A Journal for Museum and Archives Professionals. ISSN 1550-1906. doi: 10.1177/15501906231167574. Full text in Research Archive Show summary

  In the NHMO zoological collections, specimens from the Arctic include about 9,000 mammals and 7,100 birds, whereas the Insect Collection holds about 105,000 specimens plus more than hundred jars with unsorted material. The Fish Collection contains approximately 1,400 specimens, while the Herptile Collection (amphibians & reptiles) holds only thirty-one specimens of three taxa. Many of these specimens originate from expeditions to E Greenland, N Canada, Svalbard, Novaya Zemlya, Finnmark, and NE Siberia in the period 1898 to 1966. Furthermore, the DNA Bank has about 5,600 tissue and extracted DNA samples, mostly sampled from wild animals during the last decades but also from specimens in the voucher collections. Most of the Arctic specimens have been digitized and are available in online data portals like GBIF, except for the Insect Collection, where only the type material and about 30 percent of the total specimens are digitized.

• Rekdal, Silje Larsen; Anmarkrud, Jarl Andreas; Lifjeld, Jan Terje & Johnsen, Arild (2023). Do female bluethroats without extra-pair offspring have more MHC-compatible social mates? Behavioral Ecology and Sociobiology. ISSN 0340-5443. 77(3). doi: 10.1007/s00265-023-03311-z. Full text in Research Archive Show summary

  Abstract Genes of the major histocompatibility complex (MHC) are crucial for adaptive immunity in jawed vertebrates, and theory predicts that there should be mate choice for optimizing MHC constitution in the offspring. In a previous study, we demonstrated a non-random female choice of extra-pair males in the bluethroat ( Luscinia svecica ), yielding offspring that was closer to an intermediate MHC class II (MHCII) allele count than their within-pair halfsiblings. The present study tests whether social pairs with only within-pair young (WPY) in their brood, in the same study population, had a combined MHC-constitution closer to a presumed intermediate optimum, than social pairs with extra-pair young (EPY), with a corresponding pattern in their offspring. As expected, we found that WPY from pure WPY-broods were more MHC-optimal than WPY from mixed broods, but only in broods of young (second year) males. Correspondingly, there was a tendency for social pairs with only WPY in their brood to be more MHC-compatible than social pairs with EPY in their brood, when the male was young. Older bluethroat males have considerably larger testes than young males, and their higher sperm competitiveness could help them secure paternity in their own brood, also when they are not MHC-compatible. In other words, in the sexual conflict over paternity, females may be more likely to realise their preference for a MHC-compatible mate when paired to a young male. As a possible fitness indicator, immune responsiveness to an injected antigen (PHA) was elevated for offspring closer to “the golden mean” in MHCII allele count. Significance statement This study contributes to our understanding of MHC-based mate choice in extra-pair mating systems, by showing that female bluethroats ( Luscinia svecica ) with an MHCII-compatible social mate tend to have no extra-pair young in their brood, but only when the social male is young. This elucidates a possible sexual conflict, in which older social males are able to override female preferences and prevent other males from gaining paternity in their brood through higher sperm production. Studying systems in which extra-pair paternity occurs offers an insight into the genetic benefits of mate choice, as extra-pair males, in contrast to social males, generally contribute only sperm. Further, the strict and thorough genotyping scheme applied in this study enabled us to demonstrate a preference for “the golden mean” in MHC-diversity in a species with one of the highest MHC class II-diversity known to date.

• Viitaniemi, Heidi M.; Leder, Erica Helen; Kauzál, Ondřej; Stopková, Romana; Stopka, Pavel & Lifjeld, Jan Terje [Show all 7 contributors for this article] (2023). Impact of Z chromosome inversions on gene expression in testis and liver tissues in the zebra finch. Molecular Ecology. ISSN 0962-1083. 33(24). doi: 10.1111/mec.17236. Full text in Research Archive
• Lifjeld, Jan Terje; Kleven, Oddmund; Fossøy, Frode; Jacobsen, Frode; Laskemoen, Terje & Rudolfsen, Geir [Show all 7 contributors for this article] (2022). When older males sire more offspring—Increased attractiveness or higher fertility? Behavioral Ecology and Sociobiology. ISSN 0340-5443. 76(5), p. 1–11. doi: 10.1007/s00265-022-03170-0. Full text in Research Archive Show summary

  In birds with extrapair mating, older males usually have higher fertilization success than younger males. Two hypotheses can potentially explain this pattern: 1) females prefer older, and often more ornamented males, or 2) older males invest more in reproduction and fertility than younger males. Here we studied factors associated with age-related male fertilization success in a population of barn swallows Hirundo rustica in Canada. We document that male fertilization success increased gradually up to a minimum age of four-year old. The age efect was especially strong for the number of extrapair ofspring obtained and the occurrence of a second brood. The higher fertilization success of older males was also associated with an early start of breeding in spring. The length of the elongated outermost tail feathers, a postulated male ornament preferred by females, also increased with age (in both sexes), but it was not a signifcant predictor of male fertilization success within age classes. Male fertility traits, especially testis size, but also sperm motility and sperm velocity, increased signifcantly across age groups. Our results suggest that the higher fertilization success by older males is due to their higher reproductive investments and that their longer tails are an adaptation to early arrival on the breeding grounds. Extrapair paternity · Life history · Sexual selection · Sperm traits · Tail length · Testes size

• Rowe, Melissah; Oort, Annabel van; Brouwer, Lyanne; Lifjeld, Jan Terje; Webster, Michael S. & Welklin, Joseph F. [Show all 7 contributors for this article] (2022). Sperm Numbers as a Paternity Guard in a Wild Bird. Cells. 11(2). doi: 10.3390/cells11020231. Full text in Research Archive Show summary

  Sperm competition is thought to impose strong selection on males to produce competitive ejaculates to outcompete rival males under competitive mating conditions. Our understanding of how different sperm traits influence fertilization success, however, remains limited, especially in wild populations. Recent literature highlights the importance of incorporating multiple ejaculate traits and pre-copulatory sexually selected traits in analyses aimed at understanding how selection acts on sperm traits. However, variation in a male’s ability to gain fertilization success may also depend upon a range of social and ecological factors that determine the opportunity for mating events both within and outside of the social pair-bond. Here, we test for an effect of sperm quantity and sperm size on male reproductive success in the red-back fairy-wren (Malurus melanocephalus) while simultaneously accounting for pre-copulatory sexual selection and potential socio-ecological correlates of male mating success. We found that sperm number (i.e., cloacal protuberance volume), but not sperm morphology, was associated with reproductive success in male red-backed fairy-wrens. Most notably, males with large numbers of sperm available for copulation achieved greater within-pair paternity success. Our results suggest that males use large sperm numbers as a defensive strategy to guard within-pair paternity success in a system where there is a high risk of sperm competition and female control of copulation. Finally, our work highlights the importance of accounting for socio-ecological factors that may influence male mating opportunities when examining the role of sperm traits in determining male reproductive success.

• Grønstøl, Gaute; Danielsen, Marie; Cramer, Becky; Johannessen, Lars Erik; Johnsen, Arild & Whittington, Emma [Show all 7 contributors for this article] (2022). Effects of fixatives and storage duration on avian sperm morphology. Journal of Ornithology. ISSN 2193-7192. 164(1), p. 171–181. doi: 10.1007/s10336-022-02015-x. Full text in Research Archive Show summary

  Abstract Sperm cells are variable both within and among species. To be able to accurately measure sperm cells and understand their function, it is important that sperm cells are preserved in a manner that maintains their structural integrity. Formalin is a widely used fixative and storage medium for sperm cells, but few studies have examined the effect of fixation and long-term storage on their morphological integrity. Ethanol is also a common fixation and storage agent for tissue samples, and here we examine if fixation and storage in formalin or ethanol alters sperm cell size and structural integrity. We found no significant effects of the fixation process on fresh sperm cells fixed in formalin or ethanol. Further, there were no consistent length changes in sperm cells stored in formalin or ethanol over a period of 227 days, or in sperm cells stored in formalin for three years. A comparison across 13–14 years of storage time showed a small but significant reduction in sperm cell length of 0.93%. Furthermore, sperm cells initially fixed in formalin remained quite stable in dry storage on glass slides for a minimum of six months (we found a mean reduction in sperm cell length of 0.18% after 6 months). The proportion of sperm cells with head damage was, however, much higher for samples stored in ethanol than for those stored in formalin. Overall, 70% of sperm cells had acrosome damage in ethanol versus only 3% in formalin. Finding intact sperm cells for measuring length, therefore, required greater effort in ethanol samples than in formalin samples. Our findings indicate that use of sperm cells from long-term storage for the study of sperm morphometrics is justified for either fixative, although formalin clearly preserves the sperm cells better.

• Cramer, Becky; Grønstøl, Gaute & Lifjeld, Jan Terje (2022). Flagellum tapering and midpiece volume in songbird spermatozoa. Journal of morphology. ISSN 0362-2525. 283(12), p. 1577–1589. doi: 10.1002/jmor.21524. Full text in Research Archive Show summary

  In contrast to numerous studies on spermatozoa length, relatively little work focuses on the width of spermatozoa, and particularly the width of the midpiece and flagellum. In flagellated spermatozoa, the flagellum provides forward thrust while energy may be provided via mitochondria in the midpiece and/or through glycolysis along the flagellum itself. Longer flagella may be able to provide greater thrust but may also require stronger structural features and more or larger mitochondria to supply sufficient energy. Here, we use scanning electron microscopy to investigate the ultrastructure of spermatozoa from 55 passerine species in 26 taxonomic families in the Passerides infraorder. Our data confirm the qualitative observation that the flagellum tapers along its length, and we show that longer flagella are wider at the neck. This pattern is similar to mammals, and likely reflects the need for longer cells to be stronger against shearing forces. We further estimate the volume of the mitochondrial helix and show that it correlates well with midpiece length, supporting the use of midpiece length as a proxy for mitochondrial volume, at least in betweenspecies studies where midpiece length is highly variable. These results provide important context for understanding the evolutionary correlations among different sperm cell components and dimensions.

• Cramer, Becky; Yilma, Zelealem Belaineh & Lifjeld, Jan Terje (2022). Selection on sperm size in response to promiscuity and variation in female sperm storage organs. Journal of Evolutionary Biology. ISSN 1010-061X. 36(1), p. 131–143. doi: 10.1111/jeb.14120. Full text in Research Archive Show summary

  Sperm cells are exceptionally morphologically diverse across taxa. However, morphology can be quite uniform within species, particularly for species where females copulate with many males per reproductive bout. Strong sexual selection in these promiscuous species is widely hypothesized to reduce intraspecific sperm variation. Conversely, we hypothesize that intraspecific sperm size variation may be maintained by high among-female variation in the size of sperm storage organs, assuming that paternity success improves when sperm are compatible in size with the sperm storage organ. We use individual-based simulations and an analytical model to evaluate how selection on sperm size depends on promiscuity level and variation in sperm storage organ size (hereafter, female preference variation). Simulations of high promiscuity (10 mates per female) showed stabilizing selection on sperm when female preference variation was low, and disruptive selection when female preference variation was high, consistent with the analytical model results. With low promiscuity (2–3 mates per female), selection on sperm was stabilizing for all levels of female preference variation in the simulations, contrasting with the analytical model. Promiscuity level, or mate sampling, thus has a strong impact on the selection resulting from female preferences. Furthermore, when promiscuity is low, disruptive selection on male traits will occur under much more limited circumstances (i.e. only with higher among-female variation) than many previous models suggest. Variation in female sperm storage organs likely has strong implications for intraspecific sperm variation in highly promiscuous species, but likely does not explain differences in intraspecific sperm variation for less promiscuous taxa.

• Lifjeld, Jan Terje (2022). Experimental manipulation of sexual traits in barn swallow populations—No evidence for divergent sexual selection. Evolution. ISSN 0014-3820. 76(9), p. 2199–2203. doi: 10.1111/evo.14505. Full text in Research Archive Show summary

  Safran et al. (2016a) manipulated two sexual traits (ventral plumage coloration and tail streamer length) in male barn swallows (Hirundo rustica) and reported divergent effects on paternity change between two study populations, in Colorado and Israel. They concluded that geographical variation in the two phenotypic traits is maintained by divergent sexual selection. However, the response variable they used, the longitudinal change in paternity from a pre-treatment clutch to a post-treatment clutch, does not reflect an unbiased effect of the treatment. Here, I show that the magnitude of the change in paternity is influenced by variation in the initial paternity score among the treatment groups, which is presumably due to stochastic variation from low sample sizes in the treatment groups. When the bias was accounted for in re-analyses of the Israeli dataset, the statistical significance of one of two treatment effects disappeared. Similar re-analyses of the American population were not possible due to inaccessibility of raw data for individual clutches, but an assessment of the mean scores indicates that the two significant treatment effects in this population were similarly biased in their initial paternity scores. The conclusion of divergent sexual selection on male phenotypic traits between the two populations does not seem to be supported.

• Johannessen, Lars Erik & Lifjeld, Jan Terje (2022). Type specimens of birds in the Natural History Museum, University of Oslo, Norway. Zootaxa. ISSN 1175-5326. 5150(4), p. 451–486. doi: 10.11646/zootaxa.5150.4.1. Full text in Research Archive Show summary

  A list of all known bird type specimens in the collection of the Natural History Museum, University of Oslo, Norway is presented. The specimens originate from northern Australia, eastern Indonesia or Tristan da Cunha, and have either been collected and described by personnel associated with the museum or sent from other museums in exchange for other specimens. The catalogue is not intended as a taxonomic revision of the treated taxa, and consequently no taxonomic revisions are made, but the type status of some specimens formerly considered to be types is refuted. The collection holds name-bearing types of 11 taxa, comprising one holotype and 26 syntypes; two paratypes from the same type series as the holotype; and one paralectotype of a twelfth taxon. The catalogue also includes two specimens of one taxon here refuted as being type specimens.

• Cramer, Emily Rebecca A; Garcia-del-Rey, Eduardo; Johannessen, Lars Erik; Laskemoen, Terje; Marthinsen, Gunnhild & Johnsen, Arild [Show all 7 contributors for this article] (2021). Longer Sperm Swim More Slowly in the Canary Islands Chiffchaff. Cells. 10(6). doi: 10.3390/cells10061358. Full text in Research Archive Show summary

  Sperm swimming performance affects male fertilization success, particularly in species with high sperm competition. Understanding how sperm morphology impacts swimming performance is therefore important. Sperm swimming speed is hypothesized to increase with total sperm length, relative flagellum length (with the flagellum generating forward thrust), and relative midpiece length (as the midpiece contains the mitochondria). We tested these hypotheses and tested for divergence in sperm traits in five island populations of Canary Islands chiffchaff (Phylloscopus canariensis). We confirmed incipient mitochondrial DNA differentiation between Gran Canaria and the other islands. Sperm swimming speed correlated negatively with total sperm length, did not correlate with relative flagellum length, and correlated negatively with relative midpiece length (for Gran Canaria only). The proportion of motile cells increased with relative flagellum length on Gran Canaria only. Sperm morphology was similar across islands. We thus add to a growing number of studies on passerine birds that do not support sperm morphology–swimming speed hypotheses. We suggest that the swimming mechanics of passerine sperm are sufficiently different from mammalian sperm that predictions from mammalian hydrodynamic models should no longer be applied for this taxon. While both sperm morphology and sperm swimming speed are likely under selection in passerines, the relationship between them requires further elucidation.

• Cramer, Emily Rebecca A; Grønstøl, Gaute; Maxwell, Logan; Kovach, Adrienne & Lifjeld, Jan Terje (2021). Sperm length divergence as a potential pre-zygotic barrier in a passerine hybrid zone. Ecology and Evolution. 11(14), p. 9489–9497. doi: 10.1002/ece3.7768. Full text in Research Archive Show summary

  The saltmarsh sparrow Ammospiza caudacuta and Nelson's sparrow A. nelsoni differ in ecological niche, mating behavior, and plumage, but they hybridize where their breeding distributions overlap. In this advanced hybrid zone, past interbreeding and current backcrossing result in substantial genomic introgression in both directions, although few hybrids are currently produced in most locations. However, because both species are nonterritorial and have only brief male–female interactions, it is difficult to determine to what extent assortative mating explains the low frequency of hybrid offspring. Since females often copulate with multiple males, a role of sperm as a postcopulatory prezygotic barrier appears plausible. Here, we show that sperm length differs between the two species in the hybrid zone, with low among-male variation consistent with strong postcopulatory sexual selection on sperm cells. We hypothesize that divergence in sperm length may constitute a reproductive barrier between species, as sperm length co-evolves with the size of specialized female sperm storage tubules. Sperm does not appear to act as a postzygotic barrier, as sperm from hybrids was unexceptional.

• Rekdal, Silje Larsen; Anmarkrud, Jarl Andreas; Lifjeld, Jan Terje & Johnsen, Arild (2021). Elevated phytohaemagglutinin‐induced skin‐swelling response at an intermediate number of MHC class II alleles in bluethroat nestlings. Journal of Avian Biology. ISSN 0908-8857. doi: 10.1111/jav.02734. Full text in Research Archive Show summary

  Genes of the major histocompatibility complex (MHC) are crucial for adaptive immunity, and theory predicts that the optimal number of alleles is intermediate rather than maximal. Support for such an intermediate optimum in MHC allele count has mainly been demonstrated in studies on parasite load and reproductive traits. In this study we tested if an intermediate number of MHC class II (MHCII) alleles based on positively selected sites was correlated with an increased skin-swelling response to two consecutive injections of phytohaemagglutinin (PHA) in bluethroat Luscinia svecica nestlings. We found that nestlings that were closer to the population mean in their number of alleles had a higher response to the first injection, supporting an intermediate, optimal number of alleles. After the second PHA injection, we found that the skin swelling was associated with MHCII allele count only at the among-brood level, and with paternity (extra-pair offspring had a larger swelling than within-pair offspring). We therefore conclude that the PHA response is partly correlated with MHCII allele count, but that additional factors must be involved in both the PHA response and in female extra-pair mate choice in this species.

• Bodawatta, Kasun H.; Synek, Per; Bos, Nick; Garcia-del-Rey, Eduardo; Koane, Bonny & Marki, Petter Zahl [Show all 12 contributors for this article] (2020). Spatiotemporal patterns of avian host–parasite interactions in the face of biogeographical range expansions. Molecular Ecology. ISSN 0962-1083. 29(13), p. 2431–2448. doi: 10.1111/mec.15486. Full text in Research Archive
• Omotoriogun, Taiwo Crossby; Albrecht, Tomas; Gohli, Jostein; Horák, David; Johannessen, Lars Erik & Johnsen, Arild [Show all 12 contributors for this article] (2020). Sperm length variation among Afrotropical songbirds reflects phylogeny rather than adaptations to the tropical environment. Zoology. ISSN 0944-2006. 140. doi: 10.1016/j.zool.2020.125770. Full text in Research Archive Show summary

  Sperm cells vary tremendously in size and shape across the animal kingdom. In songbirds (Aves: Passeri), sperm have a characteristic helical form but vary considerably in size. Most of our knowledge about sperm morphology in this group stems from studies of species in the Northern temperate zone, while little is known about the numerous species in the tropics. Here we examined sperm size in 125 Afrotropical songbird species with emphasis on the length of the major structural components (head, midpiece, flagellum), and total sperm length measured using light microscopy. Mean total sperm length varied from 51 μm to 212 μm across species. Those belonging to the Corvoidea superfamily had relatively short sperm with a small midpiece, while those of the three major Passeridan superfamilies Passeroidea, Muscicapoidea and Sylvioidea showed large interspecific variation in total sperm length and associated variation in midpiece length. These patterns are consistent with previous findings for temperate species in the same major clades. A comparative analysis with songbird species from the Northern temperate zone (N = 139) showed large overlap in sperm length ranges although certain temperate families (e.g. Parulidae, Emberizidae) typically have long sperm and certain Afrotropical families (e.g. Cisticolidae, Estrildidae) have relatively short sperm. Afrotropical and temperate species belonging to the same families showed no consistent contrasts in sperm length. Sperm length variation among Afrotropical and Northern temperate songbirds exhibits a strong phylogenetic signal with little or no evidence for any directional latitudinal effect among closely related taxa.

• Thomas, Jessica E.; Carvalho, Gary R.; Haile, James; Rawlence, Nicolas J.; Martin, Michael David & Ho, Simon Y.W. [Show all 37 contributors for this article] (2019). Demographic reconstruction from ancient DNA supports rapid extinction of the great auk. eLIFE. 8:e47509. doi: 10.7554/eLife.47509. Full text in Research Archive Show summary

  The great auk was once abundant and distributed across the North Atlantic. It is now extinct, having been heavily exploited for its eggs, meat, and feathers. We investigated the impact of human hunting on its demise by integrating genetic data, GPS-based ocean current data, and analyses of population viability. We sequenced complete mitochondrial genomes of 41 individuals from across the species’ geographic range and reconstructed population structure and population dynamics throughout the Holocene. Taken together, our data do not provide any evidence that great auks were at risk of extinction prior to the onset of intensive human hunting in the early 16th century. In addition, our population viability analyses reveal that even if the great auk had not been under threat by environmental change, human hunting alone could have been sufficient to cause its extinction. Our results emphasise the vulnerability of even abundant and widespread species to intense and localised exploitation.

• Cramer, Emily Rebecca A; Rowe, Melissah; Eroukhmanoff, Fabrice; Lifjeld, Jan Terje; Sætre, Glenn-Peter & Johnsen, Arild (2019). Measuring sperm swimming performance in birds: effects of dilution, suspension medium, mechanical agitation, and sperm number. Journal of Ornithology = Journal fur Ornithologie. ISSN 0021-8375. p. 1–11. doi: 10.1007/s10336-019-01672-9. Full text in Research Archive Show summary

  Sperm swimming performance, including swimming speed and the proportion of motile cells, may strongly affect fertilization success. However, little is known about how methodological factors affect in vitro measurement of these parameters. We compare the swimming performance of sperm from House Sparrows (Passer domesticus) and Spanish Sparrows (Passer hispaniolensis) in two standard suspension media, at two different dilutions, and with different degrees of cell agitation. Further, we conduct a resampling analysis to investigate sample size effects. Sperm performance was generally reduced when sperm were diluted, or when suspended in phosphate-buffered saline (PBS) rather than a medium containing additional nutrients. Sperm performance was particularly low when they were diluted more and suspended in PBS, suggesting that seminal fluid may provide compounds that enhance performance but that these are less available following dilution. Mechanical agitation of the cells by vigorous pipetting increased the proportion of motile cells. Between-male repeatability, assessed on a single sample measured in multiple conditions, was moderate and significant, suggesting that similar results may be obtained regardless of the methodology used to assess sperm motion. We found no evidence of biased results when low numbers of cells per male were used in analysis, though precision increased substantially as sample size increased from five to 20 or more cells per male. We recommend using the same suspension media and similar sperm concentrations and levels of agitation, to the greatest degree possible, and including as many individuals as possible in analyses, even when some individuals are represented by few sperm cells.

• Støstad, Hanna Nyborg; Rowe, Melissah; Johnsen, Arild; Tomasek, Oldrich; Albrecht, Tomas & Lifjeld, Jan Terje (2019). Sperm head abnormalities are associated with excessive omega-6 fatty acids in two finch species feeding on sunflower seeds. Journal of Avian Biology. ISSN 0908-8857. 50(3), p. 1–12. doi: 10.1111/jav.02056. Full text in Research Archive Show summary

  In a rapidly changing world, it is important to understand how urban environments impact wildlife. For example, supplementary feeding of birds, though well‐intended, might have unexpected negative effects on the health of individual animals. Sunflower seeds are commonly provided in garden bird feeders, but they contain high levels of linoleic acid (LA), an omega‐6 polyunsaturated fatty acid (PUFA). Omega‐6 PUFAs are associated with increased oxidative stress, which can damage cell membranes, and in particular sperm cells. We assessed the level of LA in the blood of two seed‐eating finch species, greenfinches Chloris chloris and hawfinches Coccothraustes coccothraustes , caught in and in environments with direct access to sunflower seed feeders (Norway), and compared these with the level of LA in a smaller number of individuals sampled in in a rural area with low incidence of sunflower seed feeders (Czech Republic). Furthermore, we investigated the relationship between the proportion of LA in the blood (as well as the proportion of 10 other fatty acids) and sperm quality (the frequency of sperm head abnormalities and sperm swimming speed). We found that both finch species, but particularly greenfinches caught near feeders, exhibited levels of LA that were considerably higher than those previously reported for other wild birds. We also found that the proportion of LA was positively correlated with the frequency of abnormal sperm heads (sperm missing the acrosome), while there was no significant effect of fatty acid composition on sperm swimming speed. Our results indicate that the sperm quality of finches may be negatively affected by a high intake of sunflower seeds, adding to a growing body of research showing that supplementary feeding may have detrimental side effects for urban animals. This is particularly relevant for the greenfinch, which is currently affected by disease and population declines.

• Støstad, Hanna Nyborg; Rowe, Melissah; Johnsen, Arild & Lifjeld, Jan Terje (2019). Sperm head abnormalities are more frequent in songbirds with more helical sperm: A possible trade-off in sperm evolution. Journal of Evolutionary Biology. ISSN 1010-061X. p. 1–9. doi: 10.1111/jeb.13446. Full text in Research Archive Show summary

  Sperm morphology varies enormously across the animal kingdom. Whilst knowledge of the factors that drive the evolution of interspecific variation in sperm morphology is accumulating, we currently have little understanding of factors that may constrain evolutionary change in sperm traits. We investigated whether susceptibility to sperm abnormalities could represent such a constraint in songbirds, a group characterized by a distinctive helical sperm head shape. Specifically, using 36 songbird species and data from light and scanning electron microscopy, we examined among‐species correlations between the occurrence of sperm head abnormalities and sperm morphology, as well as the correlation between sperm head abnormalities and two indicators of sperm competition. We found that species with more helically shaped sperm heads (i.e., a wider helical membrane and more pronounced cell waveform) had a higher percentage of abnormal sperm heads than species with less helical sperm (i.e., relatively straight sperm) and that sperm head traits were better predictors of head abnormalities than total sperm length. In contrast, there was no correlation between sperm abnormalities and the level of sperm competition. Given that songbird species with more pronounced helical sperm have higher average sperm swimming speed, our results suggest an evolutionary trade‐off between sperm performance and the structural integrity of the sperm head. As such, susceptibility to morphological abnormalities may constrain the evolution of helical sperm morphology in songbirds.

• Lindsay, Willow R.; Andersson, Staffan; Bererhi, Badreddine; Höglund, Jacob; Johnsen, Arild & Kvarnemo, Charlotta [Show all 16 contributors for this article] (2019). Endless forms of sexual selection. PeerJ. doi: 10.7717/peerj.7988. Full text in Research Archive Show summary

  In recent years, the field of sexual selection has exploded, with advances in theoretical and empirical research complementing each other in exciting ways. This perspective piece is the product of a ``stock-taking'' workshop on sexual selection and sexual conflict. Our aim is to identify and deliberate on outstanding questions and to stimulate discussion rather than provide a comprehensive overview of the entire field. These questions are organized into four thematic sections we deem essential to the field. First we focus on the evolution of mate choice and mating systems. Variation in mate quality can generate both competition and choice in the opposite sex, with implications for the evolution of mating systems. Limitations on mate choice may dictate the importance of direct vs. indirect benefits in mating decisions and consequently, mating systems, especially with regard to polyandry. Second, we focus on how sender and receiver mechanisms shape signal design. Mediation of honest signal content likely Depends on integration of temporally variable social and physiological costs that are o measure. We view the neuroethology of sensory and cognitive receiver biases as the main key to signal form and the `aesthetic sense' proposed by Darwin. Since a receiver bias is sufficient to both initiate and drive ornament or armament exaggeration, without a genetically correlated or even coevolving receiver, this may be the appropriate `null model' of sexual selection. Thirdly, we focus on the genetic architecture of sexually selected traits. Despite advances in modern molecular techniques, the number and identity of genes underlying performance, display and secondary sexual traits remains largely unknown. In-depth investigations into the genetic basis of sexual dimorphism in the context of long-term field studies will reveal constraints and trajectories of sexually selected trait evolution. Finally, we focus on sexual selection and conflict as drivers of speciation. Population divergence and speciation are often influenced by an interplay between sexual and natural selection. The extent to which sexual selection promotes or counteracts population divergence may vary depending on the genetic architecture of traits as well as the covariance between mating competition and local adaptation. Additionally, post-copulatory processes, such as selection against heterospecific sperm, may influence the importance of sexual selection in speciation. We propose that efforts to resolve these four themes can catalyze conceptual progress in the field of sexual selection, and we offer potential avenues of research to advance this progress.

• Rekdal, Silje Larsen; Anmarkrud, Jarl Andreas; Lifjeld, Jan Terje & Johnsen, Arild (2019). Extra‐pair mating in a passerine bird with highly duplicated major histocompatibility complex class II: Preference for the golden mean. Molecular Ecology. ISSN 0962-1083. doi: 10.1111/mec.15273. Full text in Research Archive Show summary

  Genes of the major histocompatibility complex (MHC) are essential in vertebrate adaptive immunity, and they are highly diverse and duplicated in many lineages. While it is widely established that pathogen‐mediated selection maintains MHC diversity through balancing selection, the role of mate choice in shaping MHC diversity is debated. Here, we investigate female mating preferences for MHC class II (MHCII) in the bluethroat (Luscinia svecica), a passerine bird with high levels of extra‐pair paternity and extremely duplicated MHCII. We genotyped family samples with mixed brood paternity and categorized their MHCII alleles according to their functional properties in peptide binding. Our results strongly indicate that females select extra‐pair males in a nonrandom, self‐matching manner that provides offspring with an allelic repertoire size closer to the population mean, as compared to offspring sired by the social male. This is consistent with a compatible genes model for extra‐pair mate choice where the optimal allelic diversity is intermediate, not maximal. This golden mean presumably reflects a trade‐off between maximizing pathogen recognition benefits and minimizing autoimmunity costs. Our study exemplifies how mate choice can reduce the population variance in individual MHC diversity and exert strong stabilizing selection on the trait. It also supports the hypothesis that extra‐pair mating is adaptive through altered genetic constitution in offspring.

• Lifjeld, Jan Terje; Gohli, Jostein; Albrecht, Tomas; Garcia-del-Rey, Eduardo; Johannessen, Lars Erik & Kleven, Oddmund [Show all 10 contributors for this article] (2019). Evolution of female promiscuity in Passerides songbirds. BMC Evolutionary Biology. 19(1). doi: 10.1186/s12862-019-1493-1. Full text in Research Archive Show summary

  Background: Female promiscuity is highly variable among birds, and particularly among songbirds. Comparative work has identified several patterns of covariation with social, sexual, ecological and life history traits. However, it is unclear whether these patterns reflect causes or consequences of female promiscuity, or if they are byproducts of some unknown evolutionary drivers. Moreover, factors that explain promiscuity at the deep nodes in the phylogenetic tree may be different from those important at the tips, i.e. among closely related species. Here we examine the relationships between female promiscuity and a broad set of predictor variables in a comprehensive data set (N = 202 species) of Passerides songbirds, which is a highly diversified infraorder of the Passeriformes exhibiting significant variation in female promiscuity. Results: Female promiscuity was highly variable in all major clades of the Passerides phylogeny and also among closely related species. We found several significant associations with female promiscuity, albeit with fairly small effect sizes (all R2 ≤ 0.08). More promiscuous species had: 1) less male parental care, particularly during the early stages of the nesting cycle (nest building and incubation), 2) more short-term pair bonds, 3) greater degree of sexual dichromatism, primarily because females were drabber, 4) more migratory behaviour, and 5) stronger premating sexual selection. In a multivariate model, however, the effect of sexual selection disappeared, while the other four variables showed additive effects and together explained about 16% of the total variance in female promiscuity. Female promiscuity showed no relationship with body size, life history variation, latitude or cooperative breeding. Conclusions: We found that multiple traits were associated with female promiscuity, but these associations were generally weak. Some traits, such as reduced parental care in males and more cryptic plumage in females, might even be responses to, rather than causes of, variation in female promiscuity. Hence, the high variation in female promiscuity among Passerides species remains enigmatic. Female promiscuity seems to be a rapidly evolving trait that often diverges between species with similar ecologies and breeding systems. A future challenge is therefore to understand what drives within-lineage variation in female promiscuity over microevolutionary time scales. Keywords: Extrapair paternity, Life history, Mating system, Pair bond, Parental care, Sexual selection

• Lifjeld, Jan Terje (2019). The Avian Sperm Collection in the Natural History Museum, University of Oslo. Alauda. ISSN 0002-4619. 87(3), p. 93–101. Full text in Research Archive
• Støstad, Hanna Nyborg; Johnsen, Arild; Lifjeld, Jan Terje & Rowe, Melissah (2018). Sperm head morphology is associated with sperm swimming speed: a comparative study of songbirds using electron microscopy. Evolution. ISSN 0014-3820. 72(9), p. 1918–1932. doi: 10.1111/evo.13555. Full text in Research Archive Show summary

  Sperm exhibit extraordinary levels of morphological diversification across the animal kingdom. In songbirds, sperm have a helically shaped head incorporating a distinct acrosomal membrane or “helical keel,” the form and extent of which varies across species. The functional significance of this helical shape, however, remains unknown. Using scanning electron microscopy, we quantified inter‐ and intraspecific variation in sperm head morphology across 36 songbird species (Passeriformes: Passerida). Using phylogenetic comparative methods, we investigated the relationship between sperm head morphology and both sperm swimming speed and the frequency of extra‐pair young (EPY). We found that species whose sperm had a relatively more pronounced helical form (i.e., long acrosome, short nucleus, wide helical membrane, and a more pronounced waveform along the sperm head “core”) had faster‐swimming sperm. We found no evidence of a relationship between interspecific variation in sperm head morphology and EPY, although we did find that among‐ and within‐male variation in sperm head traits were negatively correlated with EPY. Applying principles of fluid mechanics, we discuss how the helical form of the sperm head may influence swimming speed, and suggest that further studies considering aspects of sperm morphology beyond sperm length are needed to improve our understanding of sperm structure‐function relationships.

• Králová, Tereza; Albrecht, Tomas; Bryja, Josef; Horák, David; Johnsen, Arild & Lifjeld, Jan Terje [Show all 10 contributors for this article] (2018). Signatures of diversifying selection and convergence acting on passerine Toll-like receptor 4 in an evolutionary context. Molecular Ecology. ISSN 0962-1083. 27(13), p. 2871–2883. doi: 10.1111/mec.14724. Full text in Research Archive Show summary

  Positive selection acting on Toll‐like receptors (TLRs) has been recently investigated to reveal evolutionary mechanisms of host–pathogen molecular co‐adaptation. Much of this research, however, has focused mainly on the identification of sites predicted to be under positive selection, bringing little insight into the functional differences and similarities among species and a limited understanding of convergent evolution in the innate immune molecules. In this study, we provide evidence of phenotypic variability in the avian TLR4 ligand‐binding region (LBR), the direct interface between host and pathogen molecular structures. We show that 55 passerine species vary substantially in the distribution of electrostatic potential on the surface of the receptor, and based on these distinct patterns, we identified four species clusters. Seven of the 34 evolutionarily nonconservative and positively selected residues correspond topologically to sites previously identified as being important for lipopolysaccharide, lipid IVa or MD‐2 binding. Five of these positions codetermine the identity of the charge clusters. Groups of species that host‐related communities of pathogens were predicted to cluster based on their TLR4 LBR charge. Despite some evidence for convergence among taxa, there were no clear associations between the TLR4 LBR charge distribution and any of the general ecological characteristics compared (migration, latitudinal distribution and diet). Closely related species, however, mostly belonged to the same surface charge cluster indicating that phylogenetic constraints are key determinants shaping TLR4 adaptive evolution. Our results suggest that host innate immune evolution is consistent with Fahrenholz's rule on the cospeciation of hosts and their parasites.

• Rekdal, Silje Larsen; Anmarkrud, Jarl Andreas; Johnsen, Arild & Lifjeld, Jan Terje (2018). Genotyping strategy matters when analyzing hypervariable major histocompatibility complex-Experience from a passerine bird. Ecology and Evolution. 8(3), p. 1680–1692. doi: 10.1002/ece3.3757. Full text in Research Archive
• Anmarkrud, Jarl Andreas & Lifjeld, Jan Terje (2017). Complete mitochondrial genomes of eleven extinct or possibly extinct bird species. Molecular Ecology Resources. ISSN 1755-098X. 17(2), p. 334–341. doi: 10.1111/1755-0998.12600. Full text in Research Archive Show summary

  Natural history museum collections represent a vast source of ancient and historical DNA samples from extinct taxa that can be utilized by high-throughput sequencing tools to reveal novel genetic and phylogenetic information about them. Here, we report on the successful sequencing of complete mitochondrial genome sequences (mitogenomes) from eleven extinct bird species, using de novo assembly of short sequences derived from toepad samples of degraded DNA from museum specimens. For two species (the Passenger Pigeon Ectopistes migratorius and the South Island Piopio Turnagra capensis), whole mitogenomes were already available from recent studies, whereas for five others (the Great Auk Pinguinis impennis, the Imperial Woodpecker Campehilus imperialis, the Huia Heteralocha acutirostris, the Kauai Oo Moho braccathus and the South Island Kokako Callaeas cinereus), there were partial mitochondrial sequences available for comparison. For all seven species, we found sequence similarities of >98%. For the remaining four species (the Kamao Myadestes myadestinus, the Paradise Parrot Psephotellus pulcherrimus, the Ou Psittirostra psittacea and the Lesser Akialoa Akialoa obscura), there was no sequence information available for comparison, so we conducted blast searches and phylogenetic analyses to determine their phylogenetic positions and identify their closest extant relatives. These mitogenomes will be valuable for future analyses of avian phylogenetics and illustrate the importance of museum collections as repositories for genomics resources. The final version of this research has been published in Molecular Ecology Resources. © 2017 Wiley

• Johnsen, Arild; Pauliny, Angela; Lifjeld, Jan Terje & Blomqvist, Donald (2017). Is telomere length associated with mate choice in a songbird with a high rate of extrapair paternity? PLOS ONE. 12(8). doi: 10.1371/journal.pone.0182446. Full text in Research Archive Show summary

  Telomere length is related to aging in many eukaryotes and the rate of telomere attrition has been suggested to reflect individual genetic quality. Telomere length could thus have implications for mate choice. We investigated telomere length variation in bluethroat Luscinia svecica families with mixed paternity, including social parents, extra-pair fathers and nestlings, testing whether telomere length is associated with social and/or extra-pair mate choice through assortative mating or selection of mates with relatively long telomeres. In adults, relative telomere length (rTL) did not differ between the sexes, nor between two age categories. In chicks, however, rTL decreased with body mass at sampling (an index of nestling age). We found a positive correlation between the rTL of social mates, suggesting assortative mating with respect to telomere length or a correlative thereof. However, extra-pair males did not differ from social mates in rTL, and accordingly there was also no difference between within- and extra-pair young (i.e. half-siblings) when controlling for the effect of mass. We found no relationships between telomere length, age and fitness-related traits in adults, but an intriguing year-difference in telomere length in both sexes. In conclusion, we found no support for the idea that females choose extra-pair males based on their telomere length, but social mate choice seems to be influenced by rTL, possibly through its co-variation with aspects reflecting individual quality, like early arrival at the breeding grounds.

• K, Supriya; Rowe, Melissah; Laskemoen, Terje; Mohan, Dhananjai; Price, Trevor & Lifjeld, Jan Terje (2016). Early diversification of sperm size in the evolutionary history of the old world leaf warblers (Phylloscopidae). Journal of Evolutionary Biology. ISSN 1010-061X. 29(4), p. 777–789. doi: 10.1111/jeb.12826. Full text in Research Archive Show summary

  Sperm morphological traits are highly variable among species and are commonly thought to evolve by post-copulatory sexual selection. However, little is known about the evolutionary dynamics of sperm morphology, and whether rates of evolutionary change are variable over time and among taxonomic groups. Here, we examine sperm morphology from 21 species of Old World leaf warblers (Phylloscopidae), a group of generally dull, sexually monochromatic birds, which are known to have high levels of extra-pair paternity. We found that sperm length differs markedly across species, spanning about 40% of the range observed across a larger selection of passerine birds. Furthermore, we found strong support for an ‘early-burst’ model of trait evolution, implying that the majority of divergence in sperm length has occurred early in the evolutionary history of this clade with subsequent evolutionary stasis. This large early divergence matches the early divergence reported in ecological traits (i.e. body size and feeding behaviour). Our findings demonstrate that rates of evolution in sperm morphology can change over time in passerine taxa, and that evolutionary stasis in sperm traits can occur even in species exhibiting characteristics consistent with moderate-to-high levels of sperm competition. It remains a major challenge to identify the selection mechanisms and possible constraints responsible for these variable rates of sperm evolution. The final version of this reseach has been published in the Journal of Evolutionary Biology. © 2016 Wiley

• Omotoriogun, Taiwo Crossby; Laskemoen, Terje; Rowe, Melissah; Albrecht, Tomáš; Bowie, Rauri C.K. & Sedláček, Ondřej [Show all 9 contributors for this article] (2016). Variation in sperm morphology among Afrotropical sunbirds. IBIS. ISSN 0019-1019. 158, p. 155–166. doi: 10.1111/ibi.12334. Full text in Research Archive Show summary

  Birds show considerable variation in sperm morphology. Closely related species and subspecies can show diagnostic differences in sperm size. There is also variation in sperm size among males within a population, and recent evidence from passerine birds suggests that the coefficient of inter-male variation in sperm length is negatively associated with the level of sperm competition. Here we examined patterns of inter- and intra-specific variation in sperm length in 12 species of sunbird (Nectariniidae) from Nigeria and Cameroon, a group for which such information is extremely limited. We found significant variation among species in sperm total length, with mean values ranging from 74 μm to 116 μm, placing these species within the short to medium sperm length range for passerine birds. Most of this variation was explained by the length of the midpiece, which contains the fused mitochondria and is an important structure for sperm energetics. Relative midpiece length was negatively correlated with the coefficient of inter-male variation in sperm total length across species, suggesting that sperm competition may have selected for greater midpiece length in this group. We also mapped sperm lengths onto a time-calibrated phylogeny and found support for a phylogenetic signal in all sperm length components, except head length. A test of various evolutionary or tree transformation models gave strongest support for the Brownian motion model for all sperm components, i.e. divergences were best predicted by the phylogenetic distance between lineages. The coefficients of inter-male variation in sperm total length indicate that sperm competition is high but variable among sunbird species, as is the case with passerine birds at large. The final version of this research has been published in Ibis. © 2016 Wiley

• Omotoriogun, Taiwo Crossby; Albrecht, Tomas; Hořak, David; Laskemoen, Terje; Ottosson, Ulf & Rowe, Melissah [Show all 8 contributors for this article] (2016). Sperm size evolution in African greenbuls (Passeriformes: Pycnonotidae). Biological Journal of the Linnean Society. ISSN 0024-4066. 117(2), p. 337–349. doi: 10.1111/bij.12663. Full text in Research Archive Show summary

  Sperm morphology is highly diversified across the animal kingdom and recent comparative evidence from passerine birds suggests that postcopulatory sexual selection is a significant driver of sperm evolution. In the present study, we describe sperm size variation among 20 species of African greenbuls and one bulbul (Passeriformes: Pycnonotidae) and analyze the evolutionary differentiation of sperm size within a phylogenetic framework. We found significant interspecific variation in sperm size; with some genera exhibiting relatively long sperm (e.g. Eurillas) and others exhibiting short sperm head lengths (e.g. Phyllastrephus). However, our results suggest that contemporary levels of sperm competition are unlikely to explain sperm diversification within this clade: the coefficients of inter-male variation (CVbm) in sperm length were generally high, suggesting relatively low and homogeneous rates of extra-pair paternity. Finally, in a comparison of six evolutionary or tree transformation models, we found support for both the Kappa (evolutionary change primarily at nodes) and Lambda (lineage-specific evolutionary rates along branches) models in the evolutionary trajectories of sperm size among species. We therefore conclude that African greenbuls have more variable rates of sperm size evolution than expected from a neutral model of genetic drift. Understanding the evolutionary dynamics of sperm diversification remains a future challenge. The final version of this research has been published in Biological Journal of the Linnean Society. © 2016 Oxford University Press

• Cramer, Emily Rebecca A; Stensrud, Even; Marthinsen, Gunnhild; Hogner, Silje; Johannessen, Lars Erik & Laskemoen, Terje [Show all 10 contributors for this article] (2016). Sperm performance in conspecific and heterospecific female fluid. Ecology and Evolution. doi: 10.1002/ece3.1977. Full text in Research Archive Show summary

  Divergent sexual selection within allopatric populations may result in divergent sexual phenotypes, which can act as reproductive barriers between populations upon secondary contact. This hypothesis has been most tested on traits involved in precopulatory sexual selection, with less work focusing on traits that act after copulation and before fertilization (i.e., postcopulatory prezygotic traits), particularly in internally fertilizing vertebrates. However, postcopulatory sexual selection within species can also drive trait divergence, resulting in reduced performance of heterospecific sperm within the female reproductive tract. Such incompatibilities, arising as a by-product of divergent postcopulatory sexual selection in allopatry, can represent reproductive barriers, analogous to species-assortative mating preferences. Here, we tested for postcopulatory prezygotic reproductive barriers between three pairs of taxa with diverged sperm phenotypes and moderate-to-high opportunity for postcopulatory sexual selection (barn swallows Hirundo rustica versus sand martins Riparia riparia, two subspecies of bluethroats, Luscinia svecica svecica versus L. s. namnetum, and great tits Parus major versus blue tits Cyanistes caeruleus). We tested sperm swimming performance in fluid from the outer reproductive tract of females, because the greatest reduction in sperm number in birds occurs as sperm swim across the vagina. Contrary to our expectations, sperm swam equally well in fluid from conspecific and heterospecific females, suggesting that postcopulatory prezygotic barriers do not act between these taxon pairs, at this stage between copulation and fertilization. We therefore suggest that divergence in sperm phenotypes in allopatry is insufficient to cause widespread postcopulatory prezygotic barriers in the form of impaired sperm swimming performance in passerine birds.

• Lifjeld, Jan Terje; Anmarkrud, Jarl Andreas; Calabuig, Pascual; Cooper, Joseph E.J.; Johannessen, Lars Erik & Johnsen, Arild [Show all 12 contributors for this article] (2016). Species-level divergences in multiple functional traits between the two endemic subspecies of Blue Chaffinches Fringilla teydea in Canary Islands. BMC Zoology. doi: 10.1186/s40850-016-0008-4. Full text in Research Archive Show summary

  Background One of the biggest challenges in avian taxonomy is the delimitation of allopatric species because their reproductive incompatibility cannot be directly studied in the wild. Instead, reproductive incompatibility has to be inferred from multiple, divergent character sets that indicate a low likelihood of allopatric populations amalgamating upon secondary contact. A set of quantitative criteria for species delimitation has been developed for avian taxonomy. Results Here, we report a broad multi-trait comparison of the two insular subspecies of the Blue Chaffinch Fringilla teydea, endemic to the pine forests of Tenerife (ssp. teydea) and Gran Canaria (ssp. polatzeki) in the Canary Islands. We found that the two taxa were reciprocally monophyletic in their whole mitogenomes and two Z chromosome introns. The genetic distance in mitogenomes indicates around 1 Mya of allopatric evolution. There were diagnostic differences in body morphometrics, song and plumage reflectance spectra, whose combined divergence score (=11) exceeds the threshold level (=7) set for species delimitation by Tobias et al. (Ibis 152:724–746, 2010). Moreover, we found a marked divergence in sperm lengths with little range overlap. Relatively long sperm with low intra- and intermale CV compared to other passerines suggest a mating system with high levels of sperm competition (extrapair paternity) in these taxa. Conclusion The large and diagnostic divergences in multiple functional traits qualify for species rank, i.e., Tenerife Blue Chaffinch (Fringilla teydea) and Gran Canaria Blue Chaffinch (Fringilla polatzeki). We encourage a wider use of sperm traits in avian taxonomy because sperm divergences might signal reproductive incompatibility at the postcopulatory prezygotic stage, especially in species with sperm competition.

• Støstad, Hanna Nyborg; Rekdal, Silje Larsen; Kleven, Oddmund; Laskemoen, Terje; Marthinsen, Gunnhild & Johnsen, Arild [Show all 7 contributors for this article] (2016). Weak geographical structure in sperm morphology across the range of two willow warbler Phylloscopus trochilus subspecies in Scandinavia. Journal of Avian Biology. ISSN 0908-8857. 47(5), p. 731–741. doi: 10.1111/jav.00981. Full text in Research Archive Show summary

  Sperm morphology is highly diversified among species and at higher taxonomic levels. In birds, there is also increasing evidence of geographical differentiation in sperm traits within species, especially in those with strong sperm competition. Geographical divergences in sperm traits might imply the formation of a reproductive barrier in a speciation process. Here we study sperm morphology variation of willow warblers Phylloscopus trochilus in a geographical context in Scandinavia, across the range of two subspecies that are differentiated in certain genetic markers, morphology and migratory routes. We describe geographical patterns in genotypes (two previously described single-nucleotide polymorphism (SNP) markers and four polymorphic microsatellites); in wing length, tarsus length and body mass; and in sperm traits of 330 male willow warblers sampled at 33 localities across Norway (58°N–69°N). Birds were on average larger and longer-winged in the north (spp. acredula) than in the south (spp. trochilus), and showed a sigmoid change in the SNP allele frequencies and body morphology around 65°N. We found no evidence of genetic structuring in the microsatellites. There was no geographical variation in sperm traits across Norway, except that sperm heads were on average longer in the south. Sperm head length was also associated with the two SNP markers, with longer sperm heads for the southern alleles, and midpiece length was weakly associated with one of the SNP markers. Similar among-male variances in total sperm length among the 33 sampling sites indicate uniform levels of sperm competition across Norway. We conclude that sperm morphology remains a rather undifferentiated trait between the two willow warbler subspecies in Scandinavia, which is consistent with a pattern of a shallow genetic divergence. This indicates that sperm morphology is not a reproductive barrier maintaining the narrow hybrid zone.

• Gohli, Jostein; Lifjeld, Jan Terje & Albrecht, Tomas (2016). Migration distance is positively associated with sex-linked genetic diversity in passerine birds. Ethology Ecology & Evolution (Testo stampato). ISSN 0394-9370. 28(1), p. 42–52. doi: 10.1080/03949370.2015.1018954. Full text in Research Archive Show summary

  A recent comparative analysis of passerine birds found that the frequency of extra-pair paternity was positively associated with neutral genetic diversity. It has been hypothesised that migratory species have more extra-pair paternity than resident species, so we reanalysed the same comparative data set to test whether migration distance could also explain variation in genetic diversity. In this comparative analysis of 19 local populations from 18 passerine species, spanning year-round residents to long-distant migrants, we found that migration distance was positively correlated with sequence diversity at homologous nuclear Z-linked (sex) introns, but not with autosomal intronic diversity. This pattern contrasts with that of extra-pair paternity in the previous study, where extra-pair paternity was positively associated with autosomal diversity and not Z-linked diversity. We discuss several possible explanations for the correlation between Z-linked diversity and migration distance; effects that are specific to sex-linked genetic diversity may occur if migratory behaviour implies different selective regimes in the sexes or affects population structuring. These results suggest that significant amounts of genetic diversity at sex chromosomes and autosomes are shaped by different processes, and should therefore be analysed and interpreted independently in population genetics studies. The final version of this research has been published in Ethology Ecology and Evolution. © 2016 Taylor & Francis

• Lifjeld, Jan Terje (2015). When taxonomy meets genomics: Lessons from a common songbird. Molecular Ecology. ISSN 0962-1083. 24(12), p. 2901–2903. doi: 10.1111/mec.13244. Full text in Research Archive Show summary

  Taxonomy is being increasingly informed by genomics. Traditionally, taxonomy has relied extensively on phenotypic traits for the identification and delimitation of species, though with a growing influence from molecular phylogenetics in recent decades. Now, genomics opens up new and more powerful tools for analysing the evolutionary history and relatedness among species, as well as understanding the genetic basis for phenotypic traits and their role in reproductive isolation. New insights gained from genomics will therefore have major effects on taxonomic classifications and species delimitation. How a genomics approach can inform a flawed taxonomy is nicely exemplified by Mason & Taylor (2015) in this issue of Molecular Ecology. They studied redpolls, which comprise a genus (Acanthis) of fringillid finches with a wide distribution in the Holarctic region, and whose species taxonomy has been a matter of much controversy for decades (Fig. 1). Current authoritative checklists classify them into one, two or three species, and five or six subspecies, based largely on geographical differences in phenotypic traits. Previous studies, including a recent one of the subspecies on Iceland (Amouret et al. 2015), have found no evidence of differentiation between these taxa in conventional molecular markers. The lack of genetic structure has been interpreted as incomplete lineage sorting among rapidly evolving lineages. Now Mason & Taylor (2015), using a large data set of genomewide SNPs, verify that they all belong to a single gene pool with a common evolutionary history, and with little or no geographical structuring. They also show that phenotypic traits used in taxonomic classifications (plumage and bill morphology) are closely associated with polygenic patterns of gene expression, presumably driven by ecological selection on a few regulatory genes. Several lessons can be learned from this study. Perhaps the most important one for taxonomy is the risk of taxonomic inflation resulting from overemphasizing phenotypic traits under local adaptation and ignoring a lack of phylogenetic signal in molecular markers. The final version of this research has been published in Molecular Ecology. © 2015 Wiley

• Rowe, Melissah; Griffith, Simon C.; Hofgaard, Antje & Lifjeld, Jan Terje (2015). Subspecific variation in sperm morphology and performance in the Long-tailed Finch (Poephila acuticauda). Avian Research. 6:23, p. 1–10. doi: 10.1186/s40657-015-0032-z. Full text in Research Archive Show summary

  Background Evolutionary biology endeavours to explain biological diversity, and as such it is critical to develop an understanding of the adaptive and functional significance of trait variation. Spermatozoa exhibit remarkable levels of morphological diversification. However, our understanding of the evolutionary causes and functional significance of this variation is limited, especially at the intraspecific level. Methods We quantified variation in sperm morphology and performance between two subspecies of Long-tailed Finch (Poephila acuticauda acuticauda and P. a. hecki), a small grassfinch found in tropical northern Australia. Despite a zone of secondary contact, these subspecies are maintained as two distinct forms: P. a. acuticauda occurs in the western part of the species’ range and has a yellow bill, while P. a. hecki exhibits a red bill and is found in the eastern part of the range. Results We found small, but significant differences in sperm size between these subspecies (P. a. acuticauda had longer and narrower sperm than P. a. hecki), which was surprising given the recent evolutionary origins of these two taxa (i.e. 0.3 million years ago). Additionally, both subspecies exhibited high values of between- and within-male variation in sperm morphology, though in the case of sperm midpiece length this variation was significantly lower in P. a. acuticauda relative to P. a. hecki. Conclusions We suggest these observed differences in sperm morphology are the result of genetic drift and reflect historical processes associated with divergence between the eastern and western populations of these two subspecies. Finally, we discuss the potential implications of our findings for the process of population divergence and reproductive isolation.

• Cramer, Emily Rebecca A; Laskemoen, Terje; Stensrud, Even; Rowe, Melissah; Haas, Fredrik & Lifjeld, Jan Terje [Show all 8 contributors for this article] (2015). Morphology-function relationships and repeatability in the sperm of Passer sparrows. Journal of morphology. ISSN 0362-2525. 276(4), p. 370–377. doi: 10.1002/jmor.20346. Full text in Research Archive
• Hogner, Silje; Riera, Albert Burgas; Wold, Margrethe; Lifjeld, Jan Terje & Johnsen, Arild (2015). Intergeneric hybridization between Common Redstart Phoenicurus phoenicurus and Whinchat Saxicola rubetra revealed by molecular analyses. Journal of Ornithology. ISSN 2193-7192. 156, p. 829–836. doi: 10.1007/s10336-015-1165-x. Full text in Research Archive
• Rowe, Melissah; Albrecht, Tomas; Cramer, Emily Rebecca A; Johnsen, Arild; Laskemoen, Terje & Weir, Jason T [Show all 7 contributors for this article] (2015). Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology. Evolution. ISSN 0014-3820. 69(4), p. 1044–1052. doi: 10.1111/evo.12620. Full text in Research Archive
• Dale, Svein; Lifjeld, Jan Terje & Rowe, Melissah (2015). Commonness and ecology, but not bigger brains, predict urban living in birds. BMC Ecology. 15. doi: 10.1186/s12898-015-0044-x. Full text in Research Archive Show summary

  Background Several life history and ecological variables have been reported to affect the likelihood of species becoming urbanized. Recently, studies have also focused on the role of brain size in explaining ability to adapt to urban environments. In contrast, however, little is known about the effect of colonization pressure from surrounding areas, which may confound conclusions about what makes a species urban. We recorded presence/absence data for birds in 93 urban sites in Oslo (Norway) and compared these with species lists generated from 137 forest and 51 farmland sites surrounding Oslo which may represent source populations for colonization. Results We found that the frequency (proportion of sites where present) of a species within the city was strongly and positively associated with its frequency in sites surrounding the city, as were both species breeding habitat and nest site location. In contrast, there were generally no significant effects of relative brain mass or migration on urban occupancy. Furthermore, analyses of previously published data showed that urban density of birds in six other European cities was also positively and significantly associated with density in areas outside cities, whereas relative brain mass showed no such relationship. Conclusions These results suggest that urban bird communities are primarily determined by how frequently species occurred in the surrounding landscapes and by features of ecology (i.e. breeding habitat and nest site location), whereas species’ relative brain mass had no significant effects.

• Svoboda, Ales; Marthinsen, Gunnhild; Pavel, Václav; Chutný, Bohumír; Turčoková, Lucia & Lifjeld, Jan Terje [Show all 7 contributors for this article] (2014). Die prävalenz von blutparasiten beim blaukehlchen ist verknüpft mit der unterart und dem bruthabitat. Journal of Ornithology = Journal fur Ornithologie. ISSN 0021-8375. 156(2), p. 371–380. doi: 10.1007/s10336-014-1134-9. Full text in Research Archive
• Cramer, Emily Rebecca A; Laskemoen, Terje; Eroukhmanoff, Fabrice; Haas, Fredrik; Hermansen, Jo Skeie & Lifjeld, Jan Terje [Show all 9 contributors for this article] (2014). Testing a post-copulatory pre-zygotic reproductive barrier in a passerine species pair. Behavioral Ecology and Sociobiology. ISSN 0340-5443. 68(7), p. 1133–1144. doi: 10.1007/s00265-014-1724-9. Full text in Research Archive
• Larson, Keith W; Liedvogel, Miriam; Addison, BriAnne; Kleven, Oddmund; Laskemoen, Terje & Lifjeld, Jan Terje [Show all 9 contributors for this article] (2014). Allelic Variation in a Willow Warbler Genomic Region Is Associated with Climate Clines. PLOS ONE. 9(5). doi: 10.1371/journal.pone.0095252. Full text in Research Archive Show summary

  Local adaptation is an important process contributing to population differentiation which can occur in continuous or isolated populations connected by various amounts of gene flow. The willow warbler (Phylloscopus trochilus) is one of the most common songbirds in Fennoscandia. It has a continuous breeding distribution where it is found in all forested habitats from sea level to the tree line and therefore constitutes an ideal species for the study of locally adapted genes associated with environmental gradients. Previous studies in this species identified a genetic marker (AFLP-WW1) that showed a steep north-south cline in central Sweden with one allele associated with coastal lowland habitats and the other with mountainous habitats. It was further demonstrated that this marker is embedded in a highly differentiated chromosome region that spans several megabases. In the present study, we sampled 2,355 individuals at 128 sites across all of Fennoscandia to study the geographic and climatic variables associated with the allele frequency distributions of WW1. Our results demonstrate that 1) allele frequency patterns significantly differ between mountain and lowland populations, 2) these allele differences coincide with extreme temperature conditions and the short growing season in the mountains, and milder conditions in coastal areas, and 3) the northern-allele or ‘‘altitude variant’’ of WW1 occurs in willow warblers that occupy mountainous habitat regardless of subspecies. Finally these results suggest that climate may exert selection on the genomic region associated with these alleles and would allow us to develop testable predictions for the distribution of the genetic marker based on climate change scenarios.

• Wojczulanis-Jakubas, K; Kilikowska, A; Harding, AMA; Jakubas, D; Karnovsky, NJ & Steen, Harald [Show all 11 contributors for this article] (2014). Weak population genetic differentiation in the most numerous Arctic seabird, the little auk. Polar Biology. ISSN 0722-4060. 37(5), p. 621–630. doi: 10.1007/s00300-014-1462-5. Full text in Research Archive
• Gohli, Jostein; Leder, Erica H.; Garcia-Del-Rey, Eduardo; Johannessen, Lars Erik; Johnsen, Arild & Laskemoen, Terje [Show all 8 contributors for this article] (2014). The evolutionary history of Afrocanarian blue tits inferred from genomewide SNPs. Molecular Ecology. ISSN 0962-1083. 24(1), p. 180–191. doi: 10.1111/mec.13008. Full text in Research Archive
• Cramer, Emily Rebecca A; Laskemoen, Terje; Kleven, Oddmund; LaBarbera, Katie; Lovette, Irby J. & Lifjeld, Jan Terje (2013). No evidence that sperm morphology predicts paternity success in wild hous wrens. Behavioral Ecology and Sociobiology. ISSN 0340-5443. 67(11), p. 1845–1853. doi: 10.1007/s00265-013-1594-6. Full text in Research Archive
• Hermosell, Ignacio Garcia; Laskemoen, Terje; Rowe, Melissah; Møller, Anders P.; Mousseau, Timothy A. & Albrecht, Tomas [Show all 7 contributors for this article] (2013). Patterns of sperm damage in Chernobyl passerine birds suggest a trade-off between sperm length and integrity. Biology Letters. ISSN 1744-9561. 9(5:20130530 ). doi: 10.1098/rsbl.2013.0530. Full text in Research Archive
• Gohli, Jostein; Anmarkrud, Jarl Andreas; Johnsen, Arild; Kleven, Oddmund; Borge, Thomas & Lifjeld, Jan Terje (2013). Female promicuity is positively associated with neutral and selected genetic diversity in passerine birds. Evolution. ISSN 0014-3820. 67(5), p. 1406–1419. doi: 10.1111/evo.12045. Full text in Research Archive
• Hogner, Silje; Laskemoen, Terje; Lifjeld, Jan Terje; Pavel, Vaclav; Chutny, Bob & Garcia, J [Show all 9 contributors for this article] (2013). Rapid sperm evolution in the bluethroat (Luscinia svecica) subspecies complex. Behavioral Ecology and Sociobiology. ISSN 0340-5443. 67(8), p. 1205–1217. doi: 10.1007/s00265-013-1548-z. Full text in Research Archive

View all works in NVA

• Lifjeld, Jan Terje & Leder, Erica Helen (2026). Sperm size diversification and acrosin gene expansion in passerine birds. Full text in Research Archive Show summary

  Why are sperm cells so variable in size and shape among species? In this talk we examine the evolutionary diversification of sperm size among passerine birds and the possible selective factors that shape them. We find that multiple mating by females is the most important factor driving the evolution of long sperm in this group. We will discuss some possible mechanisms for this evolutionary response. Another approach we are using is to identify the genomic basis for sperm morphological variation. While examining various spermatogenesis genes, we identified a copy-number expansion of the acrosin gene which is a serine protease contained in the acrosome thus playing an important role for sperm-egg interaction. Although not a structural gene, and not contributing to morphological variation, it may provide a mechanism for ensuring gamete compatibility for rapidly diverging taxa.

• Lifjeld, Jan Terje (2025). Sperm as a link between mating system and speciation in songbirds. Full text in Research Archive Show summary

  The current paradigm of speciation combines elements of drift, ecology and sexual selection in explaining how reproductive isolation evolves between diverging populations. Recent research on sperm biology adds another dimension to the speciation process, the role of sexual conflict and co-evolution at the level of gametes. In this talk I present some ideas about the evolutionary diversification of sperm in songbirds. Why are sperm so variable among species? Can strong selection on sperm traits lead to rapid speciation and high diversification rates in songbirds?

• Lifjeld, Jan Terje (2025). Sperm-mediated diversification rates in Passerides songbirds. Full text in Research Archive
• Lifjeld, Jan Terje (2025). A resolution of the "big sperm paradox" in songbirds? Full text in Research Archive
• Lifjeld, Jan Terje (2025). Sperm and speciation in songbirds. Full text in Research Archive Show summary

  Speciation phenotypes are traits whose divergence contributes to reduced gene flow during speciation. In this talk I explore the hypothesis that rapid sperm length divergence promotes speciation in songbirds. Songbirds are characterized by a wide range of species-specific sperm lengths. There are also multiple examples of geographic variation in sperm length within species, which suggests that this trait evolves rapidly. Furthermore, songbirds vary in the extent of female promiscuity, which exerts selection for reduced sperm length variance in a population. In a comparative study of 20 populations pairs at different stages of the speciation continuum, I found that sperm length diverged faster, in terms of reduced overlap between populations, in more promiscuous species. Reduced overlap was caused by smaller variance in sperm length in promiscuous species, and not by different evolutionary rates of the mean sperm length. Since female promiscuity promotes non-overlapping sperm length distributions in allopatry and exerts selection on the trait, conspecific sperm precedence based on sperm length can be expected when populations make secondary contact. Thus, female promiscuity can drive prezygotic isolation through sperm length as a speciation phenotype. This “gametic speciation” hypothesis predicts that promiscuous lineages have elevated speciation rates. I will present some indirect support for this prediction in songbirds.

• Lifjeld, Jan Terje (2024). Sperm as a speciation phenotype in songbirds. Full text in Research Archive Show summary

  Speciation phenotypes are traits whose divergence contributes to reduced gene flow during speciation. In this talk I explore the hypothesis that rapid sperm length divergence promotes speciation in songbirds. Songbirds are characterized by a wide range of species-specific sperm lengths. There are also multiple examples of geographic variation in sperm length within species, which suggests that this trait evolves rapidly. Furthermore, songbirds vary in the extent of female promiscuity, which exerts selection on sperm length variance in a population. In a comparative study of 20 populations pairs at different stages of the speciation continuum, I found that sperm length diverged faster, in terms of reduced overlap between populations, in more promiscuous species. Reduced overlap was caused by smaller variance in sperm length in promiscuous species, and not by different evolutionary rates of the mean sperm length. Since female promiscuity promotes non-overlapping sperm length distributions in allopatry and exerts selection on the trait, conspecific sperm precedence based on sperm length can be expected if and when populations make secondary contact. Thus, female promiscuity may promote prezygotic isolation through sperm length as a speciation phenotype. I encourage more studies of sperm length variation and conspecific sperm precedence in hybrid zones of songbirds with a promiscuous mating system.

• Lifjeld, Jan Terje (2024). Hva kan spermiene fortelle oss om spurvefuglenes evolusjon? Full text in Research Archive
• Lifjeld, Jan Terje; Cramer, Becky; Leder, Erica Helen & Voje, Kjetil L. (2024). Sperm as a speciation phenotype in songbirds. Full text in Research Archive
• Lifjeld, Jan Terje (2023). Sjarm, sjalusi og sjekketriks i fjellheimen: et innblikk i blåstrupens hekkebiologi. Full text in Research Archive
• Melteig, Elina; Johnsen, Arild & Lifjeld, Jan Terje (2023). Far er utro for å få flere barn. Mor er utro for å få bedre barn. [Journal]. Titan.no. Full text in Research Archive
• Lifjeld, Jan Terje (2023). Kan utroskap hos fugler føre til raskere evolusjon? [Radio]. Ekko i P2 - NRK radio. Full text in Research Archive
• Leder, Erica Helen & Lifjeld, Jan Terje (2023). Spermatogenesis in birds: a comparative, molecular approach. Full text in Research Archive
• Lifjeld, Jan Terje (2023). Sjarm, sjalusi og sjekketriks i fjellheimen: et innblikk i blåstrupens hekkebiologi. Full text in Research Archive
• Whittington, Emma; Cramer, Becky; Grønstøl, Gaute; Leder, Erica Helen; Johnsen, Arild & Lifjeld, Jan Terje (2023). Comparative proteomics of the perivitelline layer - the site of avian sperm-egg interaction. Full text in Research Archive
• Lifjeld, Jan Terje (2023). Sperm as speciation phenotypes. Full text in Research Archive
• Cramer, Becky; Yilma, Zelealem Belaineh & Lifjeld, Jan Terje (2022). Selection on sperm size in response to promiscuity and variation in female sperm storage organs. Full text in Research Archive
• Lifjeld, Jan Terje & Slagsvold, Tore (2022). Female extra-pair copulation for direct fitness benefits: A cautionary note. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 119(30). doi: 10.1073/pnas.2207108119. Full text in Research Archive
• Whittington, Emma; Cramer, Becky; Leder, Erica Helen; Johnsen, Arild & Lifjeld, Jan Terje (2022). Comparative proteomics of the perivitelline layer – the site of avian sperm-egg interaction. Full text in Research Archive Show summary

  Fertilisation requires interaction between the egg and sperm. Miscommunication between the two gametes can lead to incompatibilities, loss of fertility, and failure to produce a viable zygote. Sperm and egg proteins underlying this interaction are a potential post-mating pre-zygotic mechanism of female choice and species recognition. To understand sperm-egg interactions, and how they may contribute to reproductive barriers, we need to identify and explore these proteins across species. Thus far, research in model systems has identified some interacting sperm and egg proteins, including lysin and VERL in abalone, and Izumo1 and Juno in mice. In birds, sperm interact with the membrane surrounding the yolk, the inner perivitelline layer (IPVL). We used a proteomics approach to characterise the IPVL across eight passerine species to identify candidate proteins involved in sperm-egg interactions. Immediately before fertilisation, the sperm membrane weakens, and the contents of the acrosome (a vesicle in the sperm head) are released. Proteins released in this acrosome reaction are in a position to interact with the egg. Two phenomena distinguish avian fertilisation. Firstly, the IPVL is required to induce the acrosome reaction. Secondly, polyspermy is essential for fertilisation. In addition, in passerine birds, sperm length (40-300μm) and acrosome volume (0.48-7.18μm3) varies considerably across species. We take advantage of this natural variation to explore how IPVL protein composition and structure differ across species and how variation in sperm morphology may influence the IPVL and sperm-egg interactions. Our results give insights into avian sperm-egg interaction and proteins that may underlie barriers to hybridisation.

• Leder, Erica Helen; Viitaniemi, Heidi; Stopka, Pavel; Lifjeld, Jan Terje & Albrecht, Tomáš (2022). A sex chromosome inversion impacts gene expression in testes and liver in the zebra finch. Full text in Research Archive
• Cramer, Becky; Yilma, Zelealem Belaineh & Lifjeld, Jan Terje (2022). Selection on sperm size in response to promiscuity and variation in female sperm storage organs. Full text in Research Archive
• Bagdonaite, Laima; Lifjeld, Jan Terje; Johnsen, Arild & Leder, Erica Helen (2022). Mitochondrial DNA copy number variation in birds and its relationship to sperm midpiece length. Full text in Research Archive Show summary

  Tremendous variation in sperm morphology is observed across the animal kingdom. In birds, the length of the midpiece, which is formed when several mitochondria fuse during spermatogenesis, explains a considerable amount of this variation. The midpiece is one of the main structural components of the sperm cell and is an important source of energy used for the propulsion of the sperm, mainly generated by oxidative phosphorylation reactions in the fused mitochondria. However, there is still little known about the total number of mitochondria that form the midpiece. In this project, we aim to identify the relationship between the length of the midpiece and the copy number of mitochondrial DNA (mtDNA). To test the hypothesis that sperm with longer midpiece have a higher number of mtDNA, we analyse a range of wild passerine species representing extreme ends of sperm phenotypic variation and major phylogenetic clades within Passerides. We use universal primers to amplify a region of the mitochondrial cytochrome c oxidase subunit I (COI) gene and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as a single reference gene for nuclear DNA, and employ quantitative real-time PCR on rinsed sperm samples to measure the relative number of mtDNA copies across species. Our findings will be important for better understanding sperm motility and uncovering the association between sperm midpiece length and mitochondrial loading in wild passerines.

• Lifjeld, Jan Terje (2022). A museum collection of bird sperm. Full text in Research Archive
• Lifjeld, Jan Terje (2022). The ecology and evolution of female promiscuity. Full text in Research Archive
• Lifjeld, Jan Terje (2022). Sperm Evolution in Birds. Full text in Research Archive
• Rekdal, Silje Larsen; Anmarkrud, Jarl Andreas; Lifjeld, Jan Terje & Johnsen, Arild (2022). MHC-based mate choice in bluethroats. Full text in Research Archive
• Cramer, Becky; Grønstøl, Gaute; Maxwell, Logan; Kovach, Adrienne & Lifjeld, Jan Terje (2021). Sperm divergence as a potential reproductive barrier in Ammospiza sparrows (and other passerines). Full text in Research Archive
• Cramer, Emily Rebecca Alison; Garcia-del-Rey, Eduardo; Johannessen, Lars Erik; Laskemoen, Terje; Marthinsen, Gunnhild & Johnsen, Arild [Show all 7 contributors for this article] (2021). Sperm evolution in the Canary Islands chiffchaff. Full text in Research Archive
• Lifjeld, Jan Terje (2021). A museum collection of bird sperm. Full text in Research Archive
• Lifjeld, Jan Terje (2017). Sperm size evolution in passerine birds. Full text in Research Archive
• Lifjeld, Jan Terje (2017). Alternative fakta om artsutryddelser. Full text in Research Archive
• Lifjeld, Jan Terje (2017). Why is sperm competition so variable among passerine birds? Full text in Research Archive
• Lifjeld, Jan Terje (2017). Genotyping the hypervariable MHC: a comparison of HTS methods. Full text in Research Archive
• Lifjeld, Jan Terje (2015). Sperm traits and the evolution of reproductive barriers. Full text in Research Archive
• Rowe, Melissah; Albrecht, Tomas; Cramer, Emily Rebecca A; Johnsen, Arild; Laskemoen, Terje & Weir, Jason T [Show all 7 contributors for this article] (2014). Postcopulatory sexual selection accelerates rates of evolutionary divergence in sperm morphology. Full text in Research Archive
• Cramer, Emily Rebecca A; Ålund, Murielle; Laskemoen, Terje; Eroukhmanoff, Fabrice; Eybert, Marie-Christine & Haas, Fredrik [Show all 12 contributors for this article] (2014). Post-copulatory, pre-zygotic isolating barriers in passerine birds? Full text in Research Archive
• Dale, Svein; Rowe, Melissah & Lifjeld, Jan Terje (2014). What makes an urban bird? Bird species in the city are not big-brained but simply more common in the surroundings. Full text in Research Archive
• K., Supriya; Rowe, Melissah; Laskemoen, Terje; Mohan, D; Price, Trevor & Lifjeld, Jan Terje (2014). Correlated evolution of sperm and song in the Old World warblers. Full text in Research Archive
• Finstad, Anders Gravbrøt; Boer, Hugo de; Brurberg, May Bente; Dahle, Geir; Edgar, Kristin Skarsfjord & Eiler, Alexander [Show all 25 contributors for this article] (2020). Kriterier for lagring av miljø-DNA prøver og data, herunder henvisning til referansemateriale. Criteria for depositing eDNA samples and data, including vouchered specimens. Miljødirektoratet. Full text in Research Archive

View all works in NVA