Sperm Cooperation: Sometimes Sperm Swim Together To Enhance Performance

Most of us think of sperm as being competitive by nature, with each sperm cell trying to “swim” just a little faster than the others in order to reach the egg first. However, new research out of Harvard suggests that, at least in some species, there isn’t a constant rivalry en route to the egg. In fact, sometimes sperm actually appear to help each other by cooperating and forming groups while they travel.

Why might sperm want to help one another? In environments where sperm from multiple males are competing intra-vaginally for fertilization, cooperation may actually increase the odds of reproductive success. Think of it this way: in species that are promiscuous by nature, sperm from the same male might have a tendency to cooperate instead of compete in order to reduce the odds that sperm from another male will reach the egg first.

Indeed, this is what the Harvard study found: the researchers studied two species of mice, one that is promiscuous by nature (Peromyscus maniculatus) and one that is monogamous (Peromyscus polionotus). In the promiscuous mice (where sperm from multiple males are often competing in the female reproductive tract), their sperm had a tendency to clump together in optimally sized packs that ended up reaching the egg faster. In contrast, in the monogamous mice (where sperm don’t have any external competition), their sperm were less likely to form these optimally sized packs.

The researchers ran an experimental study and a mathematical simulation and found that the optimal size of a sperm group is seven. Groups larger and smaller than this did not make it to the egg quite as quickly. However, it is not because groups of seven sperm were actually swimming at a faster speed—it was because they took a more direct route to the egg. In other words, they traveled a more linear path. And when you follow a straight line, it’s faster to get from Point A to Point B.

Fascinating, right? Perhaps there is more to sperm dynamics than we ever realized.

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To learn more about this research, see: Fisher, H. S., Giomi, L., Hoekstra, H. E., & Mahadevan, L. (2014). The dynamics of sperm cooperation in a competitive environment. Proceedings of the Royal Society B: Biological Sciences, 281(1790), 20140296.

Image Source: iStockphoto.com

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