Many humans whinge about not getting oral sex often enough, but for most animals, it's completely non-existent. In fact, we know of only animal apart from humans to regularly engage in fellatio - the short-nosed fruit bat (Cynopterus sphinx).
The bat's sexual antics have only just been recorded by Min Tan of China's Guangdong Entomological Institute (who are either branching out, or are confused about entomology). Tan captured 60 wild bats from a nearby park, housed them in pairs of the opposite sex and voyeuristically filmed their liaisons using a night-time camera. Twenty of the bats got busy, and their exploits were all caught on video.
Male bats create tents by biting leaves until they fall into shape. These provide shelter and double as harems, each housing several females who the male mates with. Fruit bat sex goes like this: the female approaches and sniffs the male, and both partners start to lick one another. The male makes approaches with his thumbs (like the Fonz) and mounts the female (like the Fonz). Sex itself is the typical rhythmic thrusting that we're used to, and afterwards, the male licks his own penis for several seconds.
But Tan also found that female bat will often bend down to lick the shaft of her mate's penis during sex itself. This behaviour happened on 70% of the videos, making it the only known example of regular fellatio in a non-human animal. It also prolonged the sexual encounter - males never withdrew their penises when they were being licked and, on average, the behaviour bought the couple an extra 100 seconds of sex over and above the usual 2 minutes. The licking itself only lasted for 20 seconds on average, so each second of it buys six extra seconds of penetration.
NSFW - short-nosed fruit bats having sex. I will have you know that the music choice came with the video and has nothing to do with me.
Oral sex is rare in other animals. Bonobos do it (but really, what don't they do?) but it's more of a form of play among young males, and there's one anecdotal instance of an orang-utan doing the same. Some animals, such as ring-tailed lemurs, lick each other's genitals to judge whether they're ready for mating, but there's no evidence that they do so as an actual part of sex. As for other bats, it's entirely possible that they too engage in oral sex. However, given their inaccessible roosts and nocturnal habits, we're largely in their dark about their sex lives.
Nonetheless, Tan suggests a few possible reasons for the short-nosed fruit bat's penchant for fellatio, aside from the anthropocentric conclusion of 'pleasure-giving'. Bat penises contain erectile tissue much like our own. It gets stiffer if it's stimulated, so females could use oral sex to prolong their encounters with males, by maintain their erections or lubricating it for easier entry.
While many of us might nod sagely at the need for longer sex, Tan suggests that for the bats, it could mean easier transport of sperm to the oviduct, or more secretions from the female that are conducive to fertilisation. It could also be a way of hogging a mate, keeping him away from rival females.
Alternatively, the antiseptic properties of saliva might help to strip the male's penis of bacteria or fungi, and prevent the spread of sexually transmitted diseases. The fact that males lick their own penises after sex supports this idea.
And finally, oral sex might help females to pick up chemical traces on her mate that might suggest if he's a suitable mate. Obviously, they'd already be having sex, but female mammals often exert choice over their sexual partners after the fact, rejecting sperm from inferior males, or encouraging congress with superior ones to displace it. All of these explanations are just hypotheses for the moment, but they could all be tested in the future.
Reference: Tan, M., Jones, G., Zhu, G., Ye, J., Hong, T., Zhou, S., Zhang, S., & Zhang, L. (2009). Fellatio by Fruit Bats Prolongs Copulation Time PLoS ONE, 4 (10) DOI: 10.1371/journal.pone.0007595
More on animal sex:
Clock gene and moonlight help corals to co-ordinate a mass annual orgy
Frigid echidna sex - competition drives males to mate with hibernating females
Traumatic insemination - male spider pierces female's underside with needle-sharp penis
Horrific beetle sex - why the most successful males have the spikiest penises
