The evolution of ape ejaculate

Yup! That's the headline!

An essay on The Panda's Thumb discusses the effect of different types of selection on apes. Different types of mating structure turn out to result in different selective pressures, which favor different adaptations.

There are two realms in which competition can occur: outside the female (competition among rivals), and inside the female (which sperm is the fastest).

Gorillas accumulate a harem, creating a shortage of eligible females, so selection favors large males who can fight off rivals. Once a harem is obtained, only that one male will mate with the females, so it doesn't matter if the sperm is less aggressive.

Male gorillas are twice the size of female gorillas, but have the smallest testicles, as a fraction of body weight, among the great apes.

Female chimpanzees are very promiscuous, and are available, theoretically, to any male. Since a female may be inseminated by any number of males, the one with the most (and fastest) sperm will have an advantage on the "inside track". Male chimps and female chimps are close to the same size, but males have very large testicles, as a fraction of their body weight.

Gibbons are exclusively monogamous. The males are very little larger than the females, and they have small testicles as a fraction of their body weight – about what we'd expect.

Humans show up as somewhat less than perfectly monogamous, which should not surprise any who observe the human condition. (Though we're not nearly as libertine as chimpanzees.)

Now it seems there's also a correlation with the composition of a protein that causes semen to coagulate. So far, all we know is that selection is occurring, and it's stronger with more polygamous species.