Humans aren't the only species that engage in oral sex.

In a 2009 study, researchers described fellatio in a species of fruit bat. Oral sex appears to be rare in the animal kingdom, so it was a surprising finding. As part of the study, they used nighttime cameras to film the bats' sexual escapades. Here's an example:

A recent study found that another species of bats (Indian flying foxes) engages in oral sex, only this time they observed cunnilingus. The bats' sexual experiences were characterized by longer periods of oral sex punctuated by shorter periods of intercourse. Here's a clip from their study:

National Geographic provided a nice review of the research. You can check it out here: link.

And a snippet of the review to help explain the observed behaviour:

In the summers of 2010 and 2011, Jayabalan Maruthupandian and Ganapathy Marimuthu clocked 1,170 hours watching a colony of flying foxes near a south Indian village. They saw the bats mate 57 times, most of which involved a brief amount of penetration bracketed by longer bouts of cunnilingus. The male would fluff up his penis and sidle over to a nearby female. He craned his neck over and licked her vagina for up to a minute, mounted her for around 15 seconds, and returned to 2.5 minutes of cunnilingus.
The actual sex isn’t exactly lengthy, but as in the short-nosed fruit bat, the flying foxes prolong their liaisons with oral sex. The males bought themselves an extra 2 seconds of penetration if they spent an extra 15 seconds of cunnilingus beforehand.
So: why? There’s the obvious explanation: it makes both partners more aroused, and the extra saliva keeps everything nice and lubricated.
In the case of the Indian flying fox, Maruthupandian and Marimuthu suggest that a male could remove the sperm of past partners by licking a female’s vagina. That doesn’t explain why he would continue after having mated himself, but Maruthupandian and Marimuthu did find that he spends less time on oral sex after penetration if he spent more time on it before. This might give him the best odds of removing a competitor’s sperm but not his own. Although, as they write, “Observation at close-range is needed to find out whether the male’s tongue enters the vagina or not.”