A study published in Animal Behavior found that male long-tailed macaques in Indonesia traded grooming services for sex with females. The boys even paid upfront. And the lady macaques? Well, they held up their end of the deal, too. Female sexual activity more than doubled from an average of 1.5 times an hour to 3.5 times after a good grooming. The researchers also noted that the going rate, i.e., the amount of time males spent nit-picking, dwindled as the number of available females went up.