Ryne Palombit studies the behavior of baboons, nonhuman primates with whom we shared a common ancestor more than 20 million years ago. They are much more distantly related to us than chimps or gorillas but their complex social interactions can help us understand the evolution of behavior. A change in behavior can be an adaptation to a change in environmental conditions – and this is the stuff of evolution.
Primatologists have been studying baboons in the wild for a half century, a length of time rivaled only by Jane Goodall’s work with chimpanzees. Project Papio is Palombit’s long-term comparative study of two populations: chacma baboons (Papio hamadryas ursinus) in Botswana; and olive baboons (Papio hamadryas anubis) in central Kenya. These two are either the same species or very closely related species. Either way, they are known to have diverged from each other recently.
While similar in many ways – large groups with multiple males and females and social relationships – there are some striking differences between them. Physically, the chacma is taller and leaner, but the real differences are in how the males and female behave, and not simply with respect to mating. Palombit sees the behavior differences as adaptations that occurred after the divergence.
Infanticide figures prominently in baboon life. Among the chacma it is the major source of infant mortality. “Pretty clearly it seems to be a male reproductive strategy,” Palombit said.
A male who immigrates into a group and quickly reaches alpha status often kills baboon babies that are still nursing. Without a baby to feed, the mother will stop lactating and once again become sexually available. The alpha male then seizes the mating opportunity. When the offspring arrive, the mother often bonds with that male for protection as a temporary “monogamous” pair, a relationship called a “friendship.”
The olive baboons, essentially the same animal as the chacma, behave quite differently. There is little or no infanticide and, as opposed to the chacma, two or three males form a nonsexual “friendship” with a nursing female, providing companionship as well as protection. Since she is lactating, there is no sex. But is there a real need for protection? Without the threat of infanticide, the question here is why the female feels the need to form friendships at all. Interestingly, female olive baboons seem less committed than female chacma baboons to friendships with males.
“The bigger question is why male chacmas attempt to kill infants while the olives do not,” Palombit said. “By studying them, we are trying to learn what conditions have made infanticide a male reproductive strategy, not just for baboons but in general.”
Because they are so closely related, Palombit suggests the differences are more likely to reflect the action of natural selection.