I recently listened to an episode of RadioLab concerned with the subject of sperm. It was highly enlightening, as most of their programming is, in my opinion, and it turned me on to one concept in particular that I found of particular interest. In general amongst our close animal relatives, promiscuity is the rule; approximately 3 percent of mammalian species are considered monogamous. One predicted result of this behavioral ubiquity is the specific evolution of sperm, for if male genes are to be carried on, an individual’s sperm must compete with the sperm of others inside the female for the right to fertilize her egg(s). In fact, it has been known for some time that evolutionary selection will operate on sperm whenever access to a female’s eggs is contested by sperm from more than one male1. Furthermore, those who speculate about the subject speculate that such competition should yield larger sperm, based on the paired assumptions that larger sperm are faster, and faster sperm are more likely to fertilize an egg.
A study published in the Proceedings of the National Academy of Sciences has shown that female promiscuity does in fact, lead to the evolution of faster sperm in 29 closely related species of cichlid fishes of Lake Tanganyika, Africa. These fish in this lake are of particular interest to evolutionary researchers and theorists because the lake is large enough to constitute several environments – thus it harbors several closely related but distinct species of cichlids – and because of certain “explosive speciation events2,” the relationships amongst these species is very well documented.
These researchers scored each species, assigning them a number according to their “sperm competition rank” (see table above). Which strongly predicted the speed of those species sperm (see table, below).
This research is quite intriguing because it represents an example of behavior feeding back on evolution. The effects of behavior on evolution are fascinating because such phenomena must have played a significant role in our own evolution, and continue to be perhaps the most important determinant of our biological fate.
1. Parker GA. Sperm competition and its evolutionary consequences in the insects. Biol Rev 45: 525–567, 1970.
2. Fitzpatrick JL, Montgomerie R, Desjardins JK, Stiver KA, Kolm N, Balshine S. Female promiscuity promotes the evolution of faster sperm in cichlid fishes. Proc Natl Acad Sci U S A 106: 1128-32, 2009.