Size does matter

For some male bats, sexual prowess comes with a price - smaller brains. A Syracuse University research team found that in bat species where the females are promiscuous, the males with the largest testicles also had the smallest brains. Conversely, where the females were faithful, the males had smaller testes and larger brains.

"It turns out size does matter," said Scott Pitnick the research team leader. The study offers evidence that males - at least in some species - make an evolutionary trade-off between intelligence and sexual prowess. "Bats invest an enormous amount in testis, and the investment has to come from somewhere. There are no free lunches," said David Hoskens, a biologist at the University of Exeter.

Pitnick's team looked at 334 species of bats and found a convincing contrast in testes size. In species with monogamous females, males had testes starting at 0.11 percent of their body weight and ranging up to 1.4 percent. But in species where the females had a large number of mates, Pitnick found testes ranged from 0.6 percent to 8.5 percent of the males' mass (in the Rafinesque's big-eared bat).

Promiscuity is known to make a difference in testicle size in some other mammals. For example, chimpanzees are promiscuous and have testicles that are many times larger than those of gorillas, in which a single dominant male has exclusive access to a harem of females.

Large brains, meanwhile, are metabolically costly to develop and maintain. Pitnick's research suggested that in those bat species with promiscuous females, the male's body used more of its energy to enhance the testes - giving it the greater adaptive advantage - and lacked the energy it needed to further develop the brain.

The study found that in more monogamous species, the average male brain size was about 2.6 percent of body weight, while in promiscuous species, the average size dipped to 1.9 percent.

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I knew the Republican Party was blind as a bat on just about every issue. I just didn't know they actually were bats. Big balls, little brains.