Prostitution might be the world鈥檚 oldest profession, but it鈥檚 not nearly as ancient as some had suggested. It turns out that there is no support for the widespread belief that male chimpanzees trade meat for sex, suggesting that sexual bartering among humans may be an evolutionarily recent phenomenon.
鈥淚 kept finding references to 鈥榤eat for sex鈥� all over the place, saying this is what chimpanzees do,鈥� says , a primatologist at Duke University in Durham, North Carolina. 鈥淜nowing from observation and reading the evidence, they really don鈥檛.鈥�
Most reports of male chimps trading meat for sex are anecdotal, Gilby says. has found statistically meaningful, if indirect, support for such swaps, showing that male chimpanzees are more likely to hunt for monkeys when oestrous females are around.
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Yet when Gilby鈥檚 team examined observations from four chimpanzee communities in Uganda and Tanzania spanning 28 years, they found no evidence that female fertility affected whether males hunted or not.
Other evidence also questions the idea of meat for sex, Gilby says. Males with access to meat were no likelier to share it with oestrous females 鈥� who can become pregnant 鈥� than with non-oestrous females. Nor do they preferentially give meat to older females, who tend to be more likely to conceive than younger females.
Why share?
What鈥檚 more, sex rarely occurs right after meat-sharing, and males who share meat are no likelier to have sex than males who don鈥檛 share, Gilby鈥檚 team report.
Why, then, do chimpanzees share meat, if not for sex?
One possibility is that chimpanzees use meat to gain coalitional support and even grooming services from other males and females, Gilby says. The extent of meat sharing tends to correlate with grooming, but no one has yet demonstrated any sort of quid pro quo.
A more likely explanation is that chimps share meat because others beg, Gilby says. He has found that the more a chimp eating meat is harassed, the more he shares; that begging slows the speed at which chimps eat meat; and that begging tends to stop one meat is shared.
鈥淚t doesn鈥檛 involve males thinking about who owes them favours,鈥� Gilby says. 鈥淚t is essentially 鈥榶ou鈥檙e in my face bugging me, because you鈥檙e there harassing me for meat I can鈥檛 eat鈥�.鈥�
Dead and buried?
The meat-for-sex hypothesis may not be ready for burial just yet, however.
Last year, Cristina Gomes and at the Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany, found that, over a 22-month period, female chimpanzees were more likely to mate with males who offered them meat than with males who did not share. However, they found no evidence for immediate meat-for-sex trades, nor did they find that males who shared more meat got more sex.
One explanation for this discrepancy could be the cost of short-term exchanges, Gomes says. Males who hunt risk ceding their access to oestrous females to other chimpanzees. Long-term exchanges could be a way of gaining the benefits of meat for sex, without taking such risks.
Gomes also notes that Gilby鈥檚 team examined only east African chimpanzees, and points out that the west African animals she studied in the Ivory Coast鈥檚 Ta茂 National Park behave differently. Compared to their eastern brethren, western chimps share meat more often but have less sex.
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