I'm always fascinated by various unlikely and unpredictable behavioral strategies that have evolved in a wide variety of animals. Just this week I Iearned of one about which many people, including the researchers themselves, were surprised.
Spiders are fascinating and enigmatic animals and their mating strategies are very interesting to study. For example, a team of researchers led by Dr. Simona Simona Kralj-Fiser of the Slovenian Academy of Sciences and Arts have recently discovered that sex-may-be-a-life-saver-for-spider.html#.VQBmQDqKlpA">male Darwin's bark spiders have evolved a unique strategy for avoiding cannibalism during mating. The males perform oral sex that seems to relax females so they are less likely to eat the males. Dr. Kralj-Fiser suggests "the oral lubrication relaxes adult females so they are less likely to engage in sexual cannibalism - which would explain why the males don't make such an effort with the younger females that are unable to eat them."
It turns out that spiders aren't unique for having evolved what some might call "kinky" mating tactics (please see "Can Evolutionary Biology Tell Us What's Kinky?"). A web search for "oral sex in nonhuman animals" resulted in more than 425,000 hits! Although there are problems with definitions because people define oral sex differently - ranging from oral-genital contact to stimulation that feels good to stimulation resulting in orgasm - it's pretty clear that oral sex involves a mouth and lots of animals partake.