Last Updated Jan 24, 2006 12:05 PM EST
A research team led by Syracuse University biologist Scott Pitnick found that in bat species where the females are promiscuous, the males boasting 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 Pitnick, whose findings were published in December in "Proceedings of the Royal Society: Biological Science," an online journal.
The study offers evidence that males, at least in some species make an evolutionary trade-off between intelligence and sexual prowess, said David Hoskens, a biologist at the Centre for Ecology and Conservation at the University of Exeter in England and a leading authority on bats' mating behavior.
"Bats invest an enormous amount in testis, and the investment has to come from somewhere. There are no free lunches," said Hoskens, who did not participate in the study.
The relationship between the breeding system and relative brain size has received little investigation, said Pitnick, who teaches evolution and population biology and researches topics such as sexual selection and sexual conflict.
Bats are the second largest group of mammals, behind rodents, with about 1,000 known species. Because of their exceptional navigational and flying abilities, bats have been the subject of countless studies, providing Pitnick and his colleagues, Kate Jones of Columbia University and Gerald Wilkinson of the University of Maryland, with a bounty of data without having to slink off into caves.
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).
"If female bats mate with more than one male, a sperm competition begins," Pitnick said. "The male who ejaculates the greatest number of sperm wins the game, and hence many bats have evolved outrageously big testes."
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.