Many mammalian species: sexual endowment or intelligence...

Size Does Matter in Bats' Evolution By WILLIAM KATES, Associated Press
Writer

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, 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.