Scott Pitnick, a biologist at Syracuse University in New York, discovered that bat species with larger brains have smaller testicles.
The male who ejaculates the greatest number of sperm may win at this game, and hence many bats have evolved outrageously big testes," Pitnick said. "Because they live on an energetic knife-edge, bats may not be able to evolutionarily afford both big testes and big brains."
That's a handy explanation for a lot of men, I'd imagine! http://www.livescience.com/animalworld/051208_bat_testes.html