Big Brains Means Small Testes

According to a new study reported by New Scientist there is a correlation between brain size ans testes size in...bats. Researchers examined 334 species of bats for the studies. They found an inverse correlation between brain size and testes size. They relate the results to breeding systems. In bat species with highly promiscuous females, male bats had smaller brains but larger testes - related to sperm competition (something also seen in chimps). They attribute the effect to:

Both brain tissue and sperm cells require a lot of metabolic energy to produce and maintain. The different species appear to have evolved a preference for developing one organ more than the other, presumably determined by which will help them produce more offspring.

Incidentally:

“An extraordinary range of testes mass was documented across bat species - from 0.12% to 8.4% of body mass. That exceeds the range of any other mammalian order,” says Scott Pitnick, from Syracuse University in New York, US, one of the research team. Primate testes vary between species from 0.02% and 0.75% of body mass.

Apparently, then, you can have a large brain (such as in humans) or large testes (such as in some bats and chimps) but not both.

If I remember corectly, one of the areas where researchers recently found differences between chimps and humans was in the genes for sperm production. So a change in mating patterns and social structure between chimps and humans may be one, of several, forces driving the evolution of large brains in humans.