Many men think of little else besides sex and meat, but male chimpanzees will sometimes exchange one for the other. Chimps are mostly vegetarian but they will occasionally supplement their diet by hunting other animals, especially monkeys. Males do most of the hunting, but they don't eat their spoils alone - often, they will share the fresh meat with females, even those who are unrelated to them. Some scientists have suggested that this apparently selfless act is a trade - the males are giving up their nutritious catch in exchange for sex.
Cristina Gomes and Christophe Boesch from the Max Planck Institute for Evolutionary Anthropology have found new evidence to support this idea. They spent four years in the Tai National Park in Cote d'Ivoire watching a group of 49 chimps, including 5 adult males and 14 females. They recorded a huge amount of data on the group's behaviour, and across 3,000 hours of observation, they were privy to 262 bouts of chimp sex.
These years of voyeurism told them that meat was a big factor in separating the Casanovas from the sexually frustrated males. Females mated more frequently with males who gave them meat at least once, and meat-sharing was much more important than other shows of support such as grooming, sharing other types of food or taking their sides in fights. None of these other actions had much bearing on the male's sexual success.
Gomes and Boesch wonder if human hunter-gatherers rely on similar trades. That's certainly been suggested before, especially since better hunters tend to have more wives (or at least, more affairs). These results do nothing to confirm or deny that idea, but they certainly provide strong evidence that chimps, at least, are indeed exchanging meat for sex.
There are, of course, other possible explanations for the data. It's possible that dominant males get the lion's share of meat after a kill, and tend to monopolise the females. It could be that the most sociable females would be most likely to receive meat and to mate with males. It's even possible that females who mate most often with a male then harass him strongly for meat.
Gomes and Boesch ruled out all of these options by statistically adjusting their results for the social status of both partners, how often the partners interacted with each other, and how often the female begged for meat from the male. It was a challenging task but at the end of it, they found that the link between meat-sharing and sex was still significant - it was unlikely to be a statistical blip, and it couldn't be explained away by any other explanation they could come up with.
After all this mathematical fine-tuning, the duo estimated that males can doubled their chances of mating by sharing meat. After they do it once, it seems that there's little benefit in doing it more, for females don't make a distinction between males who give them small pieces, and those who serve up large slabs of monkey steak.
Males tended to slightly focus their attention on females during oestrus, the part of their sexual cycle when they are most fertile and sexually receptive. But even when Gomes and Boesch focused on exchanges between males and females outside of the oestrous phase, they still found a strong link between meat-sharing and mating frequency.
This suggests that the donation of meat doesn't just reward males with immediate sexual gratification; it has much longer-term benefits. As is becoming increasingly apparent, the chimpanzee is an animal that can think about, and plan for, the future. Certainly, if they can gather ammo to throw at tourists, they can donate some meat to barter for future matings.
Reference: Gomes, C., & Boesch, C. (2009). Wild Chimpanzees Exchange Meat for Sex on a Long-Term Basis PLoS ONE, 4 (4) DOI: 10.1371/journal.pone.0005116
More on chimps:
Congolese chimps modify fishing-sticks to make them even more effective tools
Chimps call during sex to confuse fathers, recruit defenders and avoid competitors