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ORGAN SIZES

Trade-off apparent in bats’ ‘costly tissues’

by Rowan Hooper

Here’s a rhetorical question that isn’t just an excuse to talk about something rude. Would you men out there rather have large gonads or large brains? For female readers, how about this: What do you think is most important in a male, testes size or brain size?

You might think that it’s not a case of one or the other, that it’s possible to have large testes and a large brain. Likewise, there might be some unfortunate males with both small testes and a small brain.

But new research in bats suggests otherwise.

Scott Pitnick and colleagues at Syracuse University in New York state, looked at brain size and testis size in 334 different species of bat. What they found was that there really is a trade-off between the two organs.

Put simply, bats with large testes had small brains.

However, whether a similar correlation exists in humans at this point still remains to be seen.

Bats are the second-largest group of mammals, after rodents. What Pitnick did was to examine the data collected by scientists all over the world on bat anatomy and behavior, and then look for patterns across species. He was following up well-established work in primates, which shows that testis size is related to the mating system.

Hence promiscuous species, that is those that have multiple partners, have larger testes.

That makes sense. If you need to copulate with lots of different females, it’s best to be able to manufacture lots of sperm.

Chimps have large testes because they copulate freely with many different females.

Gorillas also copulate with many females, but they have a harem system in which a mature, immensely strong male guards a group of females and prevents other males from getting a chance to copulate. So gorillas have relatively small testes.

In between chimps and gorillas are human males. This reflects our mating system: we are socially monogamous, but occasionally an opportunity will arise to copulate with a different female.

What about within humans? Research on testes size is not to the highest standard, though what has been done suggests there are ethnic differences in humans. But the new work Pitnick has done on bats is much more reliable. And certainly less controversial.

Pitnick and his colleagues had two ideas. The first was that testis size would be linked, as it is in primates, to mating system. In other words, species that were promiscuous would have large testes, reflecting the higher demand for sperm production.

But the second idea was where it got interesting.

Pitnick knew that the tissue in the testes that makes sperm, and also the tissue that makes the billions of neurons in the brain, are the most energetically costly types of tissue to maintain.

He also knew that bats are animals that live life on the edge. Bats are the only mammals able to truly fly, and flying is a damn tiring thing to do. It requires a huge amount of energy, which could otherwise be spent growing larger testes or larger brains. Perhaps because bats are animals that have such a tight energy budget, it turned out to be easy to see that there was a trade-off between testes size and brain size.

In fact, it turned out that testes size in bat species ranged from 0.12 percent to 8.4 percent of their body mass. The latter percentage is the size of the testes in Rafinesque’s big-eared bat, the bat with the biggest gonads. The former, the bat with the smallest testes, is the African yellow-winged bat.

In primates, the range is much more narrow: Among species, primate testes range from 0.02 percent of body mass to 0.75 percent.

In the bat species where the females were faithful, the male’s gonads were small. However, their brains were bigger. And the opposite was true, too. In bat species with large testes, the brains were smaller.

Pitnick said he was surprised. He expected that in species where females were promiscuous, the brain would be larger, so that males could keep track of what the females were up to. Not so. In bat society, at least, if females are going to have sex with lots of males, then the best solution is not to have a big brain — but to have huge testes, and for the males to also have sex a lot.

It may be a case of “if you can’t beat ‘em, join ‘em.”