In one of his most famous jokes, comedian Robin Williams explained occasionally dimwitted male behavior by saying, "God gives men a brain and a penis...and only enough blood to run one at a time."
Scott Pitnick, Ph.D., a biology professor at Syracuse University in New York, called Williams’ joke a “brilliant metaphor” for his comparative study of 300 bat species that showed a distinct tradeoff between brain and testicle size. Pitnick and his fellow researchers found that in bat species where females were most promiscuous, male bats evolved the largest testicles. In the most extreme case, bat testicles equaled 8.4% of their total body mass. And those species of bats with the largest testicles had the smallest brains in terms of body mass percentage. Apparently larger testes produce more sperm, in support of Pitnick’s hypothesis that male bats were forced by evolutionary pressures to grow larger testes in order to compete on the sexual playing field set by promiscuous females. Since testicular tissue takes a lot of energy to grow and maintain, he surmises that smaller brains were the evolutionary tradeoff.
How does this research apply to humans? “Our relative testes size suggests that our breeding system—our evolutionary past, but probably still true today—falls somewhere between monogamy and polygynandry (both sexes promiscuous),” says Pitnick. “The testes really are the window into the breeding system.”