www.msnbc.msn.com/id/335877...e-science/

When they do their thing, female Chinese fruit bats add oral sex to get the males to prolong the act, scientists now find, suggesting the behavior confers evolutionary benefits.

Oral sex, or fellatio, is often used in human foreplay, the researchers noted, but rarely seen in other animals. As such, there have been few evolutionary reasons given for oral sex to date, and fellatio is largely thought confined to humans, although juvenile members of the chimpanzee-like bonobo do it for play.