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The doctor shares: Pros and cons with spiral shaped sperm

Hanna Støstad recently defended her PhD thesis at the Natural History Museum. She has spend the last few years studying bird sperm, has participated in the Researcher Grand Prix and is currently writing a book on animal behavior.


"I'm studying bird sperm!" is the opening line on your research page. What is your approach when explaining you research to family and friends?

– It was a lot harder in the beginning when I didn't yet have a good grasp on what I was studying myself. Now that I'm done and have a coherent story, it goes as follows. 

– Birds have weird sperm cells. They look like small ice drills or screws, and I am trying to figure out why. Some species have spiral shaped ones while others have straighter shaped sperm. From my multiple species comparisons it looks like spiral shaped sperm swims faster than the straighter ones. That's probably why these species have evolved these spiral shapes. 

Why is the spiral shape faster?

– It is similar to when you're mounting a screw in a wall. Since sperm cells are tiny and the fluid they're suspended in is a lot heavier than themselves, they struggle to move. Even water is like moving in syrup for them. Additionally, the fluid sperm move through is already quite viscous, so they really have to work in order to move. Like a screw in a wall, a screw-shaped sperm cell moves ahead when it twists around. The spiral shape likely contributes to rotation, which in turn results in speed. 

[Støstad recommends the following video explaining the basics of sperm swimming]

If ice drill shaped sperm swim faster, why do no all sperm have this spiral shape? 

– In my work I also discovered that ice drill shaped sperm are more prone to being damaged compared to straighter sperm cells. It is like a trade-off with pros and cons with both types. 

Why is there so much variation?

– I don't know for sure why some species have evolved ice drill shaped sperm while others have not. At first I suspected it being related to the amount of sperm competition.In some bird species females mate with several males, resulting in competition between the sperm cells, whereas other species are monogamous. However, my results did not indicate an association between sperm competition and shape. Maybe it is related to the females' physiology? This remains to be investigated. 

You have also found time communicating your work to the general public, for instance by participating in Forsker Grand Prix. Why have you prioritized doing outreach?

– First of all it is fun, I enjoy sharing the exciting stuff we find in nature. I also believe it is important that we reach the general public with our science, sharing what we learn. It is nice to inform people, and after all it is made possible through tax payers. Forsker Grand Prix was a lot of fun. A little intimidating with all the audience, but we got great coaching in outreach that I've used extensively since. I recommend all you researchers to signing up, or doing outreach in other ways. 

What does the future bring?

– I hope to continue doing research, but first out is a book project. It is a popular science book on the evolution of animal behavior in which I write about weird animal behaviors and why they do it. The aim is to publish it by fall next year. I hope it will be as fun reading as it is writing it. 

Tags: Sperm competition, Sperm cells, Birds, PhD, Science Outreach
Published Dec. 3, 2018 1:20 PM - Last modified Dec. 3, 2018 1:24 PM