It's also suspected the same holds true for hunter-gatherer humans, since earlier studies show better hunters tend to have more sexual partners. Females of both primate species benefit by receiving better nutrition, especially during depleting fertile periods, so they may be driving the relationship between reproductive success and good male hunting skills.
It is not always easy, however, for a female chimp to obtain her extra protein.
"Females who are bolder or are more relaxed around males will approach the male who is in possession of meat and try to take a piece of meat or immediately start eating from the carcass," lead author Cristina Gomes told Discovery News.
"The male will either not react to this and allow the female to eat, or will pull the meat away from the female, to which females usually react by screaming, crying or throwing a temper tantrum," added Gomes, a researcher in the Department of Primatology at the Max Planck Institute for Evolutionary Biology.
Female chimps that are unsuccessful in the attempt, or less bold in general, may resort to just sitting next to the male and whimpering, "occasionally touching the carcass or extending their cupped hand towards the male."
Gomes and co-author Christophe Boesch observed all of this while studying wild chimpanzees in the Tai National Park at the Cote d'Ivoire, West Africa. The chimp group — consisting of 49 individuals total — included five adult males and 14 adult females, which were the focus of the study published in the latest PLoS One.
The researchers recorded 262 male to female meat transfers, with the meat mostly coming from red colobus monkeys. Chimps also kill other types of monkeys, duikers and small mammals.
10 peeks at sex in the wildGomes and Boesch collected data on matings, observing the same number — 262 — during times when females were in estrous. The scientists noted that males would share with all types of females, whether in estrous or not, although the former received preference.
"After all," Gomes said, "males double their mating success by sharing meat with females, and this is a potentially enormous benefit."
The researchers determined that when chimps groomed each other or shared other types of foods, such as fruit and nuts, males didn't improve their mating records. In fact, "grooming is mainly exchanged for grooming," Gomes said.
She and her colleague also controlled for other possible influencing factors, such as a chimp's social rank.
Craig Stanford, a professor in the departments of anthropology and biological sciences at the University of Southern California, has conducted related studies on chimps. He told Discovery News that this latest research presents "a very insightful analysis."
Anthropologist Michael Gurven at the University of California at Santa Barbara believes "the study results are convincing." But, he added, they raise many important questions.
"If hunting and meat-sharing improve male mating success, why don't more males do it more frequently? If no additional benefits accrue by sharing minimal amounts to females, why do males share more than the minimum?" Gurven asked.
He added that "one powerful result suggesting that meat-for-sex in humans is reflective of long-term pair bonds rather than just market exchange is that older men continue to hunt and share meat with their post-menopausal wives who are incapable of further reproduction."
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