If she were a chimp, Mrs. Robinson still would have brought Benjamin Braddock to his knees, researchers say.
A team of researchers led by Boston University anthropology professor Martin Muller is studying the behavior of chimpanzees who, like The Graduate’s Braddock, prefer older female mates. Their findings are challenging the assumption that all males, regardless of species, like them young.
For the past eight years, Muller’s team of field researchers has collected data on the mating preferences of the Kanyawara chimpanzee troupe in Uganda’s Kibale National Park. Muller, who has been working with chimpanzees for almost 20 years, said a recent published study was part of a larger inquiry into the general mating strategies of male and female chimps. The study was published Nov. 21 in the journal Current Biology.
“People have noted before that males are less interested in the youngest adult females,” he said. “This is new in that we show they prefer not only middle-aged mothers, but older mothers.”
According to Harvard University biological anthropology professor Richard Wrangham, a co-author of the study, male chimpanzees’ tendency toward older female mates shows the human preference for youth is somewhat unusual among the species.
“People sometimes assume it is obvious that males should prefer younger females,” he said in an email. “This study reminds us that the human male interest in youth is specific to our species. It now seems likely that the human male preference for a youthful mating partner is an evolutionarily novel tendency that may turn out to be a relatively rare phenomenon among primates.”
Muller said human mating habits may have evolved after the separation of humans and chimpanzees.
“The human preference for young mates is a fairly recently derived trait,” he said, “if we assume that humans evolved from a chimp-like ancestor.”
Muller said two main differences between humans and chimps that may account for the variations in mating preferences are menopause and long-term pair bonding. Both of these aspects of human biology make it difficult for males to procreate with older females. Chimps, however, have multiple partners and do not go through menopause, Muller said.
According to Harvard professor Melissa Thompson, a co-author of the study, other primates with long-term pair bonding also seek younger females. She said male chimpanzees are attracted to older females because they do not need to have the same partner for life.
“Chimps are looking for who’s most likely to conceive right now versus something long-term,” she said.
Wrangham said the greatest difference between chimpanzee and human mating habits is “the attitude of females to mating.”
“Among chimps, females are very active in seeking multiple mates,” he said. “A female who is sexually attractive might join a group of males and proceed to mate with each of them, in turn, in public view, in the space of an hour or two. No human culture has behavior that is remotely similar.”
The increase of age might mean older female chimps are stronger because they are able to survive longer than younger females.
“The ones who are still alive are the fittest individuals,” Thompson said.
Wrangham said he made the initial observation that male chimpanzees prefer older females.
“I drew attention a few years ago to growing evidence from our observations that males preferred mature females compared to adolescents,” he said. “That finding preceded the detailed and much more elaborate analysis made by Muller and [Melissa] Emery Thompson in this study.”
Thompson said the process of gathering data for the study was not as difficult as sifting through the information.
“Field assistants collect data all day, every day,” she said. “The difficulty lies in analyzing what attractiveness means. A female may copulate rarely because she’s unattractive or because she is attractive and a dominant male keeps guard over her.”
She said a possible follow-up on the chimpanzee study might be an inquiry about females’ preferences in their mating habits.
“It might not be appealing to be an attractive female,” she said. “Males hit and chase the females they like the best. The females don’t get to eat much because they’re constantly on their guard, mating or running from males.”