Male chimpanzees exchange meat for sex from females, according to a new study in PLoS One.

This study is bound to trigger lots of tittering and jokes about the “oldest profession.” However, what I found most interesting about it was this:

“We looked at chimps when they were not in oestrus, this means they don’t have sexual swellings and aren’t copulating.”

“The males still share with them – they might share meat with a female one day, and only copulate with her a day or two later.”

In other words, these aren’t just instantaneous you-give-me-this, I-give-you-that trades. Rather, these seem to be more like long-term contracts. Evolutionary psychologists like Leda Cosmides has argued that these kinds of long-term social exchanges played an important role in the evolution of human brains, because they require specialized and complex reasoning.

Of course, it would be a huge leap from modern-day chimps to human ancestors… but wouldn’t it be funny if it turned out that proto-prostitution is what made our brains so big?