Cooperation in sperm

January 25th, 2007 · No Comments

By Hook or by Crook? Morphometry, Competition and Cooperation in Rodent Sperm

Sperm design varies enormously across species and sperm competition is thought to be a major factor influencing this variation. However, the functional significance of many sperm traits is still poorly understood. The sperm of most murid rodents are characterised by an apical hook of the sperm head that varies markedly in extent across species. In the European woodmouse Apodemus sylvaticus (Muridae), the highly reflected apical hook of sperm is used to form sperm groups, or “trains,” which exhibited increased swimming velocity and thrusting force compared to individual sperm.

In short, the authors find that as sperm competition increases (as measured by testes size) , so does the angle of this apical hook, which they propose is involved in sperm cooperation- i.e., forming long sperm chains that enhances competitive ability. I am quite sure that this is a new observation, the generality of which is still to be determined.

The most interesting idea contained within this paper however (in my estimation) is stemming for the observation that only the leading sperm in the “chain” has any chance of being the one to do the fertilization! Following this, they remind us that sperm are related to each other by a factor of ½ and as such, Hamilton’s Rule applies, and competition for that leading position might occur. Now this is a cool idea!

In ending, I was really hoping that instead of using testes size as an indirect measure of sperm competition, as is standard in all these sperm competition papers, that they used a direct measure like Clutton-Brock’s extra group paternity or use mating system data. I wonder if the results change?? I doubt it, but it would be nice to capitalize on all the existing information.

Please go here and see the sperm video and to GNXP to see another bloggers take.

Here are some photos from the paper of sperm morhology- note the hooks. Also, B-E show the types of sperm collections that are possible.