IT IS a question not just of prurient but also evolutionary interest: what was the mating behaviour of our extinct relatives?
With living apes so variable in their sexual preferences, it's tough to work out the sex habits of the ancestor common to humans and chimps. Gorilla groups contain just one sexually active male and several females, for example, while among chimps, several sexually active males breed with the group's active females ? and vice versa. Humans, meanwhile, show a variety of mating behaviours but often form monogamous couples.
Michael Jensen-Seaman and Scott Hergenrother at Duquesne University in Pennsylvania think that it is the chimps ? not humans ? that have experimented with new sexual behaviours since our lineages diverged. Uniquely among apes, male chimps produce thick semen that coagulates into a plug in the female's genital tract. In a society where females mate freely with several males the strategy increases the chances that a male will fertilise the female's eggs. But did male chimps inherit their mating plugs from the last common ancestor they shared with us or did they evolve it later?
To try and resolve the question, Jensen-Seaman's team looked at ACPP, an enzyme in semen that prevents it coagulating into a plug. They found that the enzyme is four times as abundant in human semen as it is in chimp semen. The change is related to the way the ACPP gene is turned on and off.
For clues about whether the human-chimp ancestor had similar levels of the enzyme to humans or chimps, the team turned to gorillas. The gorilla lineage separated from the human-chimp ancestor a few million years earlier, so offers a perspective on mating habits in the human-chimp ancestor. The analysis showed gorillas regulate ACPP in the same way as humans, suggesting that the human-chimp ancestor did as well.
"The chimp mating behaviour does appear to be the derived state," says Jensen-Seaman. Hergenrother presented the work at last week's meeting of the Society for Molecular Biology and Evolution in Chicago.
However, we can't be sure that chimps made the change after the split from humans. It is possible that the human-chimp ancestor had chimp-like behaviour and that our lineage has since reverted to a gorilla-like condition. Only when similar patterns are seen for more genes can we be sure when the changes occurred.
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.
This article appeared in print under the headline "Chimps have more experimental sex"
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