Apparently it really does cost you brains

"MALE animals can produce a lot of sperm or grow big brains but cannot do both, according to a study that may confirm the suspicions of many women. Writing in Proceedings of the Royal Society, Biological Sciences, Scott Pitnick, of Syracuse University, New York, reported that species [of bats - as in Guambats] with promiscuous females had evolved extra large testicles but smaller brains."
Read all about it.