A team of researchers led by brain scientist John Yeomans at the University of Toronto in Canada used special microphones to eavesdrop on mice during sex. When they let a male mouse into a female’s cage, he approached her with a series of whistle calls and then, during intercourse, sang more complex chirp songs until ejaculation. (Females sing during social reunion with other females but only squeak uncomfortably during sex). Even without sex, the male mice sang complex chirps after the researchers injected them with a low dose of amphetamine, which activates the brain’s reward circuitry by boosting levels of the neurotransmitter dopamine.

Main article has a WAV file of the mice.

http://sciencenow.sciencemag.org/cgi/content/full/2008/403/4