In Bat Evolution, Size Does Matter

SYRACUSE, N.Y. — For some male bats, sexual prowess comes with a price — smaller brains.

A research team led by Syracuse University biologist Scott Pitnick found that, in bat species where the females are promiscuous, 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.”

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.

With about 1,000 known species, bats are the second largest group of mammals (behind rodents).

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. 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).

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.

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