A fascinating new article in PLoS One follows up on a previous paper documenting a possible evolutionary response to sperm competition— sperm cooperation.
The idea here is simple– if a number of males mate with a female, there are a huge number of sperm competing to be the lucky one who ends up fertilizing the egg. Selection on swimming speed and efficiency must be intense– individuals with slow sperm just don’t have any offspring. However, many of those single sperm are actually from the same individual, and are thus related; if those related sperm could somehow work together to outcompete the sperm from other individuals, they might increase their fitness. The logic here is an application of Hamilton’s rule (altruism can evolve if r*B>C, where r is relatendess between two organisms, B is the benefit to cooperation, and C is the cost), except here we’re talking about haploid germ cells, not diploid organisms.
Two individual sperm share, on average, 50% of their genome, giving them an r of 0.5. So altrism can evolve if the benefit of cooperation to any individual sperm is more than half the cost; if sperm competition is strong, this might not be a bad proposition. And indeed, sperm in some rodent species band together in “trains”. The video they include is pretty sweet:
The puzzling thing for me about this is that, in some species, there can be 50-100 sperm in these “trains”, while only those sperm at the head of the train have the opportunity to fertilize the egg. This seems like a lot, and certainly suggests a very strong benefit to forming these trains, as the cost to each sperm is likely proportional to N, the number of sperm in the train.
Another possibility is suggested by noting that the relatedness of two sperm has an expectation of 0.5; some sperm will be more or less related to each other. If there were some mechanism by which more closely-related sperm could preferentially group together, the necessary benefit for Hamilton’s rule to apply would be greatly decreased, and the number of sperm willing to act altruistically greatly increased.