Monkeys Wash With Urine to Females

"During MRI tests, Phillips and colleagues exposed four female monkeys to urine previously collected from adult males and juvenile males. All of the monkeys used in the experiment were born in captivity.

The team found that parts of the females' brains associated with smell and sexual behavior were activated more by the adults' urine than by the juveniles', Phillips said.

Males and females of several monkey species pee into their hands and then vigorously rub the fluid into their fur. Scientists have posed various theories to explain the behavior, which range from regulating body temperature to communicating aggression. Now, brain images of female capuchin monkeys have revealed that male urine sends sexual signals.

Phillips doesn't know exactly what message urine-washing males are sending the females. But it's possible the urine—which contains the male sex hormone testosterone—is another way for females to assess a male's social status, she noted.

For instance, sexually receptive female capuchins will solicit alpha males—which have more testosterone in their pee—about 80 percent of the time, she said.

Phillips added that she hasn't fully explained the purpose of urine washing, especially since females and juveniles also engage in this behavior. In addition, she hasn't yet tested how male brains respond to female urine.