Female chimps don’t stray in mate search

Wild chimpanzees’ mating habits aren’t nearly as wild as scientists had suspected, at least not according to the latest genetic analysis of chimps living in three western African communities.

Chimps heed homegrown mating call. Corbis

Females don’t have frequent mating flings outside those groups, as earlier genetic data suggested, contends a team led by geneticist Linda Vigilant and anthropologist Christophe Boesch, both of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Moreover, although adult males live their whole lives in the same community and form strong alliances with one another, they’re no more closely related genetically than are their adult-female peers, which emigrate into new communities upon reaching maturity, the researchers say.

“This suggests that, rather than a primarily male-bonded social structure, the group is [held together] through relationships between males and females,” Vigilant and Boesch’s team says. Its findings are due to appear in an upcoming Proceedings of the National Academy of Sciences.

Two previous genetic studies–both coauthored by Boesch–indicated that female chimps have frequent liaisons with males from other communities. Half of all children had outside fathers, according to those DNA analyses. Adult females mated on the side to boost genetic diversity within their home groups, the researchers suggested.

Other DNA evidence has suggested that adult males living in the same community share about as many genes as half-siblings do, thus promoting cooperative hunting and meat sharing.

However, technical problems in those studies led to an underestimation of genetic ties throughout entire chimp communities, Boesch argues. The fundamental snag involved the method used to analyze DNA obtained from feces, hair, bones, and teeth. Laboratory tests often copied only one of two versions of the same gene from these sources, thus missing a substantial amount of common DNA within a group, he says.

Improved techniques for retrieving DNA from the same sources enabled the new study to avoid that pitfall, according to Boesch.

The researchers determined the nucleotide arrangements in nine DNA segments–each known to undergo rapid change–for 108 individuals from three communities in western Africa. This sample included 21 adult males.

An initial paternity analysis for 41 offspring found that 34 of them probably had a father in their home community. Among 14 offspring for which all potential fathers within their communities had been tested, only one likely case of paternity outside the home group turned up.

On average, males in the same community were related to each other only to about the same extent as were females. The researchers explain the dilution of DNA links among males in part by noting that various males, rather than just an alpha male, in each group produced offspring and outsiders occasionally mated with females.

The new findings underscore the diversity of chimp social structure, says anthropologist Richard W. Wrangham of Harvard University. While male-female relationships prove crucial in western African groups, male affiliations loom larger in eastern Africa, he argues.

Researchers have yet to pin down paternity lines for chimps in eastern Africa, counters anthropologist William C. McGrew of Miami (Ohio) University in Oxford. Preliminary data suggest that females there also mate mainly in their home groups.

Although male and female chimps in a group often split up during the day, the new study indicates that “both sexes recognize an ongoing community structure, at least for mating,” McGrew says.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.