It may seem strange, but many monkeys wash their hands and feet with urine. Researchers now think they know why.
Since this odd behaviour was first observed, explanatory theories have varied wildly from suggesting that it helps monkeys improve their grip when climbing to saying it is a method of cleaning. One widely supported theory argues that monkeys use urine washing to cool themselves down when temperatures get too high.
But new research hints that it's all about social communication.
The notion of animals using chemical scents to communicate with each other is hardly new. Dogs classically use urine to mark their territory, for example, as do many other creatures. But when it comes to peeing on oneself, researchers had thought physiological reasons might be as important as social ones. It seems they were wrong.
Primatologist Kimran Miller and her research colleagues at the National Institutes of Health Animal Center in Poolesville, Maryland, monitored capuchin monkeys for ten months in a captive environment. The researchers would record daily the enclosure temperature and humidity and then note rates of urine washing. Their report, to be published in the American Journal of Primatology, shows that urine washing behaviours did not change with either temperature or humidity.
Instead, Miller and her team noticed a link between urine washing and attention-seeking.
Alpha males, for example, doubled their urine washing rates when being solicited by females. The researchers think this might be how males encourage females to continue paying attention once they've started.
And in 87% of fights or aggressive incidents, the loser of the battle washed in urine. The team suspects that this is also an attention-seeking behaviour — in this case seeking sympathy. But more research is needed to be sure.
"This really challenges the dominant theory that urine washing in capuchins is related to thermoregulation," comments primatologist James Anderson at the University of Stirling in Scotland.
"We see antelope that pee on their throats, vultures that poop on their feet, and monkeys that wash their hands in urine," says behavioural endocrinologist Fred Bercovitch at the centre for Conservation and Research for Endangered Species in San Diego. "It's obvious that urination is about more than elimination and it is great to see research like this that is figuring out why."