Here's the summary for those of you who don't have access to the journal (but you should because it's PLoS):
Oral sex is widely used in human foreplay, but rarely documented in other animals. Fellatio has been recorded in bonobos Pan paniscus, but even then functions largely as play behaviour among juvenile males. The short-nosed fruit bat Cynopterus sphinx exhibits resource defence polygyny and one sexually active male often roosts with groups of females in tents made from leaves. 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. Males also show postcopulatory genital grooming after intromission. At present, we do not know why genital licking occurs, and we present four non-mutually exclusive hypotheses that may explain the function of fellatio in C. sphinx.
And they didn't just quantify the relationship; they included a HAND-DRAWN PICTURE of the act in process. The artist? Mei Wang, who didn't even make it as an author ... weird. Seems like a worthy contribution. I make powerpoints of mice having sex all the time.
And what peer-reviewed article about fellatio would be complete without a video ... and porn music to boot?
The only downside here is that even though we're talking about mammals, the overall duration of fellatio was pretty short. The authors write, "The average duration of penis licking was 19.14±3.45 s, representing about 8.7% of the average duration of copulation (220.29±26.19 s (N = 14))." But hey, almost 10% of total copulation was spent giving head ... that seems cool. Another interesting point is that the female only licks the shaft because the glans penis is already inserted in the vagina. It's an interesting trick that I don't think many human females can try.
The authors speculate on four possible explanations for the function of fellatio in C. sphinx:
Number 1: Lubrication. That makes sense - stimulation for the male and easier thrusting. It's a win-win.
Number 2: Mate-guarding. Apparently the male hangs around longer with longer copulation, and I suppose the females want that? I'm not so sure about this one.
Number 3: Prevention of STDs. Bacteriocidal effects of saliva can help a dude out ... and a girl for that matter. I think this is a great idea.
Number 4: Detection of chemical cues for mate choice. Eh. I mean, it'd be cool if this were true, but they didn't have an example of any female licking and then stopping coitus. Is this just an act of affirmation that she's picked the right mate? I think this is the weakest hypothesis.
Favorite part is at the end, "The behaviour presumably favours the donor, although it may also benefit both partners especially if fertilization success is increased. It is conceivable that the female manipulates the male by increasing sexual stimulation, so that she ultimately benefits."
Bottom line people: Have more oral sex. It's evolutionarily good for you.