September 2009 by
Female gorillas use sex as a tactic to thwart their rivals, new research suggests. Pregnant apes court their silverback male to stop other females conceiving.
"It seems to us that mating is another tactic that females use to compete with each other – in this case to gain favour with another male," says Diane Doran-Sheehy, a primatologist at Stony Brook University in New York.
Her team chronicled the sex lives of five female western lowland gorillas and one silverback almost every day for more than three years. "We wondered if, basically, [pregnant] females can mimic [ovulating] females and dupe the male into mating with them and distract him from what those other girls are doing," Doran-Sheehy says.
This kind of competitive behaviour may even help explain how humans evolved into a mostly monogamous species, she says.
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However, Homo sapiens and Gorilla gorilla aren't the only apes who engage in recreational sex. Bonobos treat coitus like a handshake, while female chimpanzees mate during pregnancy and outside fertile periods, or oestrus, to gain support from males and to protect against infanticide. "All of the males think they could be the father of your offspring," Doran-Sheehy says.
Yet paternity isn't an issue for silverback gorillas, which usually enjoy exclusive access to a harem of females. "It doesn't have all that much to do with the males," Doran-Sheehan says. "It's what's going on with the females in the group."
Her team recorded most copulations and all births among a human-habituated group of gorillas at the Mondika research centre in the Republic of Congo for 1147 days between September 2003 and January 2007.
All five females gave birth to one infant during the study and all engaged in sex after pregnancy, the researchers found. However, females seemed to time such post-conceptive romps with the fleeting fertility of another female.
Strategic sex
For instance, after one female, MK, became pregnant she mated with the silverback during three consecutive oestrus cycles of another gorilla, EB, who left the group afterwards. With only lactating – and therefore sexually inactive – females remaining, MK ceased offering herself to the silverback. Another female named UG mated throughout her pregnancy, almost always when another female was trying to become pregnant.
The silverback seemed none the wiser. Unlike male mountain gorillas, which prefer to mate with fertile females, Doran-Sheehy's silverback went for higher-ranking females, fertile or not.
Such prestige could be one reason why pregnant females fake oestrus: group sizes are limited, and females must curry favour with a male to stick around.
By delaying the pregnancy of others, females could also gain a reproductive advantage over competitors, says Tara Stoinski, a primatologist at Zoo Atlanta in Georgia, who found that pregnant female gorillas in captivity also time their sexual advances to coincide with those of other females. "I agree with Diane's assertion that females are competing with each other."
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