A recent scientific paper reveals that as sexual selection pressures increase in promiscuous bat species, males evolve larger testes and smaller brains. But in bat species where females remain faithful, males had comparatively smaller testes and larger brains. Conversely, male sexual behavior had absolutely no effect on either brain or testes size.
Because brains and testis are the most metabolically expensive tissues to grow and maintain, the balance between their relative sizes represents an evolutionary trade-off
The study prompted one (presumably) male to ask:
Um… does this mean that the next time my girlfriend makes a crack about men being stupid I can blame it on her being a slut?
thus proving he has very large balls.