Why Do Capuchin Monkeys Wash Themselves with Urine?
Capuchin monkeys, found across Central and South America, routinely urinate in their hands and rub the liquid urine around their body. The reason for the strange habit has been a mystery to scientists for years. Still they have posed various theories to explain the behavior, which range from regulating body temperature to communicating aggression.
Both capuchin monkeys and squirrel monkeys perform urine washing, when they deposit a small quantity of urine onto the palm of a hand and then rub it on the sole of the opposite foot. It is thought to have multiple functions including hygiene, thermoregulation and response to irritation from biting ectoparasites (such as ticks and botfly). Some strepsirrhines and New World monkeys also self-anoint the body with urine to communicate.
White-faced capuchin monkeys sometimes anoint their bodies with mud and plant matter, a natural insect repellent. With their heads and faces slathered in this mixture, these highly social primates lose their ability to recognise each other and previously friendly monkeys can become fighting foes.
A new study, published in the American Journal of Primatology, has found the urine ‘turns on’ female monkeys. Researchers carried out brain scans of female tufted capuchins as they sniffed the urine. The urine of sexually mature males produced more activity than the urine of juveniles. Male urine sends sexual signals, according to study leader Kimberley A. Phillips, a psychologist at Trinity University in San Antonio, Texas. 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. ‘Urine washing’ is just one of many bizarre methods used in the animal kingdom to signal sexual availability. Female baboons, who flash their bottoms at males they are attracted to.
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. Sexual signaling, she added, is “certainly not the whole picture.”