Seminar Two
Evolution of sexuality
Sperm competition

In the November 6 issue of Nature in 2003, a study reports that roosters progressively reduce their sperm count in repeated copulations with any one female, but instantly produce a larger ejaculate with more sperm each time they encounter a new female. Roosters are thus saving larger ejaculates for new opportunities, which should improve their chances in the arena of sperm competition with other males. Here is a clear example of male-male competition.

Why are chimpanzee testicles four times the size of gorilla testicles, even though chimpanzees are one-fourth the size of gorillas? Our best answer is sperm competition. Chimpanzee females are highly promiscuous, an unfortunate anthropomorphic term, whereas gorillas live in harems and a harem-owner has relatively little competition from other males.

Do you think sperm competition plays a role in seahorses? Think about it: the pregnant male seahorse below is carrying babies in his external pouch, which has a special blood supply for the brood. The female injects her eggs into his pouch, where he fertilizes them. Should he have a small or a large sperm supply? The supply will be small, as there is no competition at all after egg acquisition.