HOLD FOR NATURE EMBARGO:
Wednesday, May 21, 1997
Media Contact: Warren R. Froelich, (619) 534-8564, [email protected]
SECRET LIAISONS OF FEMALE CHIMPANZEES DOCUMENTED BY PRIMATOLOGISTS
Contrary to popular beliefs, female chimpanzees routinely sneak away for what could be dangerous liaisons with males from neighboring communities. If their secret activities are discovered by the males of her community, they could be beaten, and any offspring sired outside the community killed.
These findings, published in the current issue of the journal Nature, undermine three decades of painstaking observations in Africa suggesting that after they've shopped around and settled into a social group, females mate exclusively with males in their adopted community.
Don't confuse this behavior with cheating or adultery, cautions Pascal Gagneux, the study's lead author and biology graduate student working at the University of California, San Diego.
"You can't call something cheating if there are no rules, no standards, no laws," he said. "Females have their own agenda for mating, just like the males. They follow their own interests as far as they can.
"But they're very constrained because the big male chimps tend to beat them up. So the females have to be real smart."
To better understand the mating habits of chimpanzees, for about five years Gagneux and a group of primatologists observed and conducted non-invasive genetic testing on a community of West African chimpanzees living in the Tai Forest International Park, Ivory Coast. This community has been observed and studied for a total of 17 years by Swiss primatologist Christophe Boesch and his wife, Hedwige Boesch.
In chimp societies ranging in size from 20 to 100 animals, males stay in the same community their entire lives, defending and protecting a territory of about 1.5 to 3.5 square miles. In contrast, females typically leave their birth homes for new communities when they reach sexual maturity, at about age 12. Once in their new communities, the females--who generally outnumber their male counterparts by four-to-one--adopt a variety of mating strategies.
"Most matings are promiscuous and opportunistic," said Gagneux, "but some are restricted by a dominant male and others may be exclusive, with couples leaving on a consortship' for several days or weeks."
To study the mating behavior of chimps in more detail, the researchers conducted observational studies and non-invasive genetic testing involving the analysis of chimpanzee hair.
Each evening in the Tai Forest community, Gagneux and Boesch would watch individual chimps build their tree-top nests of folded branches and leaves. Chimps build new nests every night, and being hairy, leave behind a few hairs once they leave. The following dawn, the researchers would return and climb up the tree, generally more than 120 feet above the ground, to look for hair. The researchers collected samples from all 55 community members--including 13 infants. The samples were subsequently packed up and brought by Gagneux for genetic testing to the laboratory of David Woodruff, professor of biology at UCSD.
Here, the hairs were analyzed using an adaptation of the polymerase chain reaction (PCR), a technique used by researchers to make multiple copies of a gene, almost like a genetic "photocopier machine." Since its invention in 1983 by Nobelist Kary B. Mullis, PCR has been applied to a variety of fields. Not only has it been used to diagnose genetic diseases, it has also been valuable in forensics by permitting analysis of small bits of blood or tissue left at a crime scene.
Gagneux and Woodruff isolated from the chimp hair samples specific gene segments called simple sequence repeats or microsatellites which are highly variable and routinely used for mapping important disease genes in humans. The results showed that seven of the 13 offspring in the community could not have been sired by any of the community males--they must have been sired from outside the community. In fact, the researchers documented the wanderings of females from their communities at just the right time--about eight months before the birth--the approximate gestation period for chimps.
All seven females who bore infants sired outside the group left the community during the most probable times of conception. Some left the community for as little as a day or two; others for as long as 15 days.
Since it usually takes only an hour-an-a-half to reach a neighboring community, Gagneux said the females could slip back and forth for secret matings with their neighbors several times a week, and return periodically to interact with males from their community. In that way, they wouldn't be missed.
Gagneux likens the behavior of the females to the principle of "not putting all your eggs in one basket."
"What you have here is two worlds, one for the male and the other for the female," he said. "It's in the male's interest to hold a big territory, and that means providing resources to females who move into that territory. For the female, it's in her genetic interests to choose from the widest possible array of fathers, which is an important part of evolution, as well as to keeping access to the good resources of the male group she has joined. It's like a backup system. In a few words, what these females are doing is shopping from a wider gene pool."
Gagneux said that this study has important implications for conservation management of this threatened species in the zoo and in the wild.
"When people talk about creating wildlife reserves, they need to take into consideration that chimpanzee populations should be protected as several contiguous communities," he said. "This will guarantee the opportunity for the continuation of their natural mating system which relies on more than one isolated community."
As neighboring communities come under the watchful eye of other human observers, studies are under way to choose what criteria females use to determine their mates. One possibility is that they are drawn to the dominant males of their neighboring community.
"Dominant males play important roles during territorial fights, sometimes raiding deep into their neighbors' territory," said Gagneux. "Females could remember such individuals and look for them when they sneak off during receptive periods."
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