Okay guys, if you had a choice between having a big brain or big .. er, testes .. which would you choose?

A recent scientific paper reveals that as sexual selection pressures increase in promiscuous bat species, males evolve larger testes and smaller brains. But in bat species where females remain faithful, males had comparatively smaller testes and larger brains. Conversely, male sexual behavior had absolutely no effect on either brain or testes size.

Because brains and testis are the most metabolically expensive tissues to grow and maintain, the balance between their relative sizes represents an evolutionary trade-off, especially in species that have very tight energy budgets, such as bats. This energetic conflict between brains and balls caused Scott Pitnick from Syracuse University in New York and two colleagues, Kate Jones from Columbia University, also in New York, and Gerald Wilkinson from the University of Maryland, to look at how bats solve these evolutionary challenges.

In their paper, the researchers tested two hypotheses; first, that testes size increases as female promiscuity increases because larger testes produce more sperm and second, that as testis mass increases, brains become smaller due to the subsequent decrease of energy available for growing and maintaining a larger brain.

To test these hypotheses, Pitnick, Jones and Wilkinson compared brain dimensions and testis mass to the social and mating systems for 334 species of bats. Due to their small body size and their correspondingly high metabolic requirements necessary to meet the demands of flight, bats have a severely restricted energy budget and so they provide an excellent model system for studying this evolutionary balancing act. Additionally, even though bats are the second most species-rich mammalian group after rodents, they are well-known, so the researchers were able to access and analyze a tremendous amount of taxonomic data collected throughout the decades.

"In species with promiscuous females, the males are competing to fertilize her eggs and so need to produce a lot of sperm," Pitnick said. "And this may be especially true in some species of bats where the females store sperm for several months."

"If female bats mate with more than one male, a sperm competition begins," Pitnick elaborated. "The male who ejaculates the greatest number of sperm wins the game, and hence many bats have evolved outrageously big testes."

This relationship between testis size and mating system has already been demonstrated in primates. However, even the largest primate testes (in the promiscuous crab-eating macaque) were only 0.75% of total body mass, whereas the largest testes found in bats were an astonishing 8.4% of total body mass (in the Rafinesque's big-eared bat). Wow! If that was the case for humans, assuming that the average man weighs 160 pounds, the average guy would have balls that weigh almost thirteen-and-a-half pounds (and none of his pants would fit, either)!

As predicted by this study, brain size in these bats also varied with testis mass and mating system: as testis mass increased, brain size decreased. This study also showed that in the more monogamous bat species, average male brain size was about 2.6% of body weight, whereas in promiscuous species, average male brain size shrank to only 1.9%.

Surprisingly, the relationship between mating system and brain size has received little attention from researchers, says Pitnick, who teaches evolution and population biology and researches topics such as sexual selection and sexual conflict.

"Because they live on an energetic knife-edge, bats may not be able to evolutionarily afford both big testes and big brains," concluded Pitnick.

This paper [PDF] was published online in the top-tier peer-reviewed journal, Proceedings of the Royal Society of London B.

Thanks for the tip, Ian.

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