Battle of the Chimpanzee Sexes

Chimps have experimented with sex more than humans - life - 18 July 2013 - New Scientist

Science is moving a little close to working out what our common ape/human ancestors got up to in the bedroom, or whatever their forest equivalent was. And in doing so have discovered a fascinating example of a battle of the sexes in the common chimpanzee (Pan troglodytes)

Unlike the other great apes, us included, male chimpanzees use the barrier method of contraception, not to prevent their sperm from finding the female's ovum, mind you, but to prevent those of rival males finding it. The sperm of the common chimpanzee coagulates and forms a plug in the vagina because it contains a coagulating enzyme. The normal method of choosing a mate for the common chimpanzee is to simply offer herself to any available male when she is oestrus. In this way she ensures that she finds at least one fertile and effective male to father her offspring.

With gorillas, the alpha male is the mate of choice for the females, not that she has much choice because he vigorously enforces his right over other males, so is the only male available. She can have any male she wants so long as it's the silver-backed male. This method ensures the female's offspring have the strongest, fittest father. Male gorilla genes have ensured they produce the most descendents by evolving this particular strategy which works because of the slow reproductive cycle of the gorilla, even though it leaves many males redundant. So long as the females reproduce at close to maximum capacity, it matters not to male genes if they all come from the alpha male and leave the other males redundant.

With the common chimpanzee however, there is no particular alpha male so far as the females are concerned. How then do male chimpanzee genes ensure they maximise their chance of success? With very great difficulty, unless they use subterfuge. So, at some time in the past an enzyme which causes the sperm to coagulate in the vagina, so acting as a barrier to other male's sperms, gave those males who carried it an enormous advantage because, if they were selected first, they had a much better chance of success. These genes would have quickly spread throughout the population.

This illustrates perfectly how an arms race (in this case between a female wanting a spread bet and a male wanting to exclude competitors) can lead to no particular advantage to anyone in the long run but mindless, undirected, unplanned and unintelligent evolution ensures it happens anyway.

However, as with many things in evolution, it's not quite that straightforward. Researchers have found that male humans and the other apes all have an enzyme in their sperm which prevents this coagulation occurring. In the common chimp this enzyme is broken, so effectively turning it off. We all have the same gene. Humans have four times as much of this enzyme as chimpanzees do.

Michael Plavcan at the University of Arkansas agrees that the study is consistent with the idea that chimps evolved a unique mating system since their lineage split from ours. "People often forget that chimps, like humans, have evolved from a common ancestor and are not some relict species frozen in time," he says.

Chimps have experimented with sex more than humans, Colin Barras, New Scientist magazine issue 2926, 18 July 2013

What seems to have happened is that an even more remote common ancestor had a similar arms race but that females hit back with a gene to prevent coagulation, which countered that of the male. If this gave them more descendants, this would have come to dominate the genepool. At some point after divergence, the common chimpanzee switched off this gene and a new arms race ensued with the result we see today.

From an evolutionary point of view since this method is absent in all other apes we can be fairly certain that it evolved in the common chimpanzee after they diverged from gorilla and then from us and the bonobo where the female counter enzyme is still active. In the gorilla, male genes have adopted a different strategy in the form of a patriarchal social structure and in humans a more-or-less monogamous relationship, at least for most people most of the time but with cuckolding frequent, and a gorilla-like hareem system in a few, especially for high-caste males.

And this in turn suggests that the chimpanzees have evolved their promiscuous female mating strategy after divergence too.

One intriguing question remains: why hasn't this system evolved in the bonobo, which is notorious promiscuous in it's mating habits, using sex as a social bonding mechanism and for pleasure as well as for reproduction.

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