For years, capuchin monkeys' "urine washing" behavior -- in which male monkeys rub pee into their fur -- had perplexed scientists. But an MRI study of female capuchin monkeys shed light on the mystery: Female capuchin monkey brains reacted to the urine of adult males (but not to the pee of young or other females). In light of this research, Kimberley Phillips, a primatologist at Trinity University in Texas, told The BBC she believes male pee communicates sexual status.
5. Female proboscis monkeys appreciate a big nose.