Outside of humans and a few other primates, oral sex is a rare phenomenon. But researchers have found male fruit bats do enjoy a little oral stimulation, and that the reasons females offer it go beyond simple pleasure.
Min Tan of China's Guangdong Entomological Institute recorded and carefully observed the mating habits of 60 fruit bats she captured in the wild. To her surprise, she found that, in 70 percent of the sexual encounters, the female bat would lick the shaft of her partner's penis. This makes fruit bats the only known species besides humans to engage in regular fellatio.
So why are these bats so batty for fellatio? Tan has a few theories. For one thing, sexual encounters that involved oral stimulation lasted, on average, 100 seconds longer than those that didn't, something that could be conducive to fertilization. Or the female bat could be occupying her mate for as long as possible so that a rival female doesn't snatch him away. The reasons could also be hygenic, as male bats lick their own penises after sex to clear away bacteria and fungi. Or, she could be looking for chemicals that indicate whether the male is a suitable mate, so that she knows whether to reject his sperm or look for a better partner next time around.
As an example, the researchers kindly provided a rather NSFW video that illustrates their findings, complete with frenetic mood music: