Min Tan and co-workers provide a seminal contributiuon to the scientific literature, with their new report "Fellatio by Fruit Bats Prolongs Copulation Time" which recently appeared in the journal PLoS One. The key detail here is that, for the short-nosed fruit-bat Cynopterus sphinx, the fellatio is not foreplay. It actually happens during intercourse as shown below:
The money quote: "Female bats often lick their mate's penis during dorsoventral copulation. The female lowers her head to lick the shaft or the base of the male's penis but does not lick the glans penis which has already penetrated the vagina. Males never withdrew their penis when it was licked by the mating partner. A positive relationship exists between the length of time that the female licked the male's penis during copulation and the duration of copulation. Furthermore, mating pairs spent significantly more time in copulation if the female licked her mate's penis than if fellatio was absent."
If you're like me, you won't be able to resist checking out the accompanying video, complete with gnarly techno soundtrack.
The authors go on to suggest some possible adaptive explanations for this behavior including 1) bat spit as lube, 2) bat spit an an antibacterial or antifungal solution, 3) licking as a way of sampling the genetic makeup of the male through detection of major histocompatibility class (MHC) proteins, all for the purpose of mate selection.
The latter seems a bit of a problem, however, as Batgirl doesn't get to smell/taste Batboy's shlong until they are already in the midst of copulation. At that point, the horse has already left the barn, so to speak.