From Yahoo News: A research team lead by Syracuse University biologist Scot Pitnick, pictured in Syracuse, NY, Monday, Jan. 9, 2006, found that in bat species where the females are promiscuous, the males boasting the largest testicles also had the smallest brains. Conversely, where the females were faithful, the males had smaller testes and larger brains. (AP Photo/Kevin Rivoli).
According to William Saletan on www. Slate.com: "Reason for big testicles: If a female is taking sperm from you and another guy, the best way to pass on your genes instead of his is to deliver more sperm. (This is why chimps have testicles "many times larger than those of gorillas.") Reason for small brains: Male bats that spent their energy making sperm beat out the ones that spent their energy thinking. Researchers' conclusion: "Size does matter."
My personal take on this news: How empowering is it for those female bats knowing that their level of promiscuity can affect the size of the male brain and balls? They really have got them by the 'nads, don't they?