Talk about "eew" de toilette—male monkeys that wash with their own urine may be putting out an irresistible scent to females, a new study suggests. (See monkey pictures.)

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, according to study leader Kimberley A. Phillips, a psychologist at Trinity University in San Antonio, Texas.

"Apparently, a male covered in urine is quite attractive," Phillips said.

Monkey Urine Reinforces Dominance?

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.

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.

Sexual signaling, she added, is "certainly not the whole picture."