In 2002, biologists were presented with a vivid account of how sperm of the common wood mouse hook up together in 'trains'. Such trains, it was shown, form a fast vehicle in the race for the great prize — fertilization of an egg. But only one sperm can be successful in that goal. Simone Immler and colleagues have now revisited the question of what prompts the selfless behaviour of the others (PloS One doi: 10.1371/journal.pone.0000170; 2007).
A feature of the sperm of wood mice and many of their relatives among the murine rodents is that their heads carry a hook structure, which varies in shape and size between species, as shown in the picture.
Immler et al. carried out a survey of the sperm of 37 species of murine rodent. They find that hook shape and curvature are more pronounced in species in which the female is more likely to mate with different males. The principle of 'together we succeed, divided we fail' makes sense in this situation. These sperm are better equipped to cooperate: so those from any one male are better able to see off the competition from another male.
The authors also looked at the behaviour of sperm in two of the species, the Norway rat and the house mouse. In both, the sperm formed groups. But in the house mouse, individual sperm outperformed the group in sheer speed. Immler et al. speculate that maybe speed isn't everything: perhaps in this case the group can make surer progress in the journey up the female reproductive tract.
And there remains the issue of competition among the collaborators: who gets to claim the envied job of fertilization?
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Lincoln, T. Sperm alliance. Nature 445, 499 (2007). https://doi.org/10.1038/445499a