Sperm trains and promiscous rodents

I was browsing Science Daily today when I came across the headline “Rodent Sperm Work Together For Better Results”. I had thought that sperm were mindless little swimmers just looking for an egg so I had to take a look at the article (available online and free).

Hooked heads of several species of rodent's sperm

I had always pictured sperm as little tadpoles but it turns out that rodent sperm actually have some rather nasty looking hooks on their heads. Scientists were not sure what the purpose of these hooks was. Some guessed they were just for aerodynamics while others had observed sperm using their hooks to link up and form cooperative groups.

This paper theorized that sperm cooperation would occur more in mice and rats where the sperm of different fathers had to compete against each other. In rodents where females were promiscuous, sperm from the same father, since they are on average 50% genetically identical (the same as brothers), would be evolutionarily selected to cooperate with each other. Promiscuous females present many problems to males trying to pass along their genetic material. One other ways that males compete is by volume of semen and number of sperm (the more players in the game, the better the chance of winning). This means that males in species where many males mate with one female will tend to be selected for big testicles. The authors of this study used testicle size as an index of female promiscuity and found that in more sexed-up species sperm are in fact hookier and more cooperative.

To back up their results, the scientists measured the speed of sperm trains vs. individual sperms. They found that in rats sperm trains are about 30% faster but in mice the sperm trains are slower. They guess that sperm trains also provide greater thrust to help move through viscous fluid so mice sperm may still perform better in trains.

A pretty cool study and a nice quick read at only 3 pages. Before today, I had no idea sperm cooperated. That’s why I like biology, there’s always something surprising and interesting to learn yet it’s all still tied together by evolution. Now if you’re anything like me, you probably want to see one of these sperm conga lines. I know I would have been pretty disappointed if the article didn’t have any pictures. So here’s the money shot:

Cooperative trains of rodent sperm

If pictures aren’t enough, there’s also a video.


By Hook or by Crook? Morphometry, Competition and Cooperation in Rodent Sperm. S. Immler, H.D.M. Moore, W.G. Breed, T.R. Birkhead. PLoS. 2007.