Bats and balls
Bigger testes mean smaller brains
Dec 8th 2005 | From the print edition

MEN are often accused by women of, to put it bluntly, having their brains in their balls. A joke, of course. But perhaps not as much of one as people might like to think. For a study of bats carried out by Scott Pitnick, of Syracuse University in New York State, and his colleagues, suggests that there really is a trade-off between the two organs.

With about 1,000 species, bats are the second-largest group of mammals (rodents are top), so there is plenty of material for interspecies studies. Dr Pitnick's project, published in this week's Proceedings of the Royal Society, looked at brain size and testis size in 334 of those species. Sadly, the team's research budget did not allow it to jet around the world and gather data directly. Instead of visiting bat caves, the scientists visited their universities' libraries. But bats are a well-studied group, and so the team was able to gather pertinent data on the anatomy and behaviour of a third of them.

The hypothesis they were testing came in two parts. The first was that in any given species, the average male's testis size as a fraction of body weight will depend on the behaviour of that species' females—in particular, how promiscuous those females are. The second was that, given that brain tissue and testis tissue are among the most expensive to maintain physiologically, and that bats have a very tight energy budget, bigger balls would result in smaller brains.

The team knew, from work done some time ago, that the first part of their hypothesis is true in primates. Greater promiscuity in females does, indeed, lead to bigger testes, presumably because a male needs to make more sperm to have a fighting chance of fathering offspring, if those sperm are competing with sperm from a lot of other males. Gorillas, which discourage dalliances between other males and the females of their harem, have small testes. Chimpanzees, among whom females mate widely, have large ones. Human testes lie between these two extremes.

And so it proved in bats. Bat testes range from 0.11% of body weight in the African yellow-winged bat, to a whacking 8.4% in the generously endowed Rafinesque's big-eared [sic] bat. (The largest primate testes by contrast, those of the crab-eating macaque, are a mere 0.75% of body mass.) And the small balls were indeed found in species where females were monogamous (though they might be members of harems), while the large ones were found in species where females mated widely.

Brain size, by contrast, and just as predicted, varied in the opposite direction. Nor was it dependent on the level of male promiscuity. In the bat world, it seems that you do not have to be cleverer to be a libertine than to be a faithful husband. But if the girls are putting it about, it is better to be virile and dim, than impotent and smart.