By Matt Walker
Editor, Earth News
Capuchin monkeys have what at first glance appears to be an odd habit: they urinate onto their hands then rub their urine over their bodies into their fur.
Now scientists think they know why the monkeys "urine wash" in this way.
A new study shows that the brains of female tufted capuchins become more active when they smell the urine of sexually mature adult males.
That suggests males wash with their urine to signal their availability and attractiveness to females.
Details of the finding are published in the American Journal of Primatology.
A number of New World monkey species, including mantled howler monkeys, squirrel monkeys and the few species of capuchins, regularly "urine wash", urinating into the palm of the hand, then vigorously rubbing the urine into the feet and hindquarters.
Several hypotheses have been put forward as to why they do it, including that it may somehow help maintain body temperature or allow other monkeys to better identify an individual by smell.
Most studies into the behaviour have been inconclusive.
"But one study reported that when being solicited by a female, adult males increased their rate of urine-washing," said Dr Kimberley Phillips, a primatologist at Trinity University in San Antonio, Texas, US.
"Since female capuchins [when they are most fertile] actively solicit males, we reasoned that urine washing by males might provide chemical information to the females about their sexual or social status," she told BBC News.
Capuchins which helped with the "urine-washing" study
To investigate, Dr Phillips and her colleagues scanned the female monkeys' brains while the animals sniffed adult male and juvenile male urine.
These magnetic resonance imaging (MRI) scans revealed that female tufted monkeys' brains became significantly more active when they sniffed the scent of urine produced by adult males compared to that from juveniles.
Since adult males are sexually mature, they excrete higher concentrations of the male sex hormone testosterone in their urine.
The concentration of this testosterone is also linked to their social status; higher status males tend to produce more.
"Female capuchin monkey brains react differently to the urine of adult males than to urine of juvenile males," said Dr Phillips.
"We suggest that this is used as a form of communication to convey social and or sexual status."
She added that it was surprising that capuchin monkeys appeared to respond to these cues, because the species is not known for using communication based on smell.