Quick, someone tell Christine O’Donnell.  Ed Yong at Discover has already covered this quite well, but it’s certainly worth a mention here:  a paper published in PLoS ONE yesterday delves into masturbation’s function in African ground squirrels.  This is a really interesting behavior (why waste sperm and energy?), with several possible hypotheses having been suggested.

Waterman 2010

Dr. Waterman found that masturbation by males was most frequent on days of female oestrus, especially after sex with a female, and that more socially dominant males tended to masturbate more.  The sperm quality hypothesis maintains that increased sperm competition should result in increased masturbation.  In humans, masturbation has been seen to increase sperm quality by spurring on the creation of younger sperm.   In the case of these squirrels, this does not hold since most masturbation is occurring post-coitus.  So the likely answer?  The squirrels do it to reduce the possibility of infections resulting from sex.  The masturbation techniques the squirrels use employ both their forepaws and mouths, as salvia has bacteria and virus-fighting properties.

Waterman JM (2010) The adaptive function of masturbation in a promiscuous Africa ground squirrel. PLoS ONE 5(9): e13060. doi:10.1371/journal.pone.0013060