(M)igration causes brain size to reduce, rather than the other way around.
The quote might be a bit misleading, though, because that was in reference to bird migration. All manner of animals migrate, and it is possible that birds face pressure other creatures don’t.
A good first place to look for a comparison would be bats. Because despite being separate by several hundred millions years of evolution, bats have one very obvious similarity to birds: they fly. And, like birds, they migrate, though their migration tends to be shorter than birds. So you would expect some of the same patterns of brain size to happen in bats as in birds.
McGuire and Ratcliffe decided to take this a step further, not only looking at overall brain size, but the size of the hippocampus. The hippocampus is deeply involved in learning about your position in space, and it has been shown repeatedly in many species that those with heavier demands on their spatial memory (say, from having a larger home range tend to have a larger hippocampus). You might expect migrating bats to have a larger hippocampus than those that don’t.
One of the recurring problems with trying to figure out these sorts of questions, though, is what data you use. Generally, you can’t go out and get new brains, and do detailed measurements of how much each bat species migrates. You have to go back into published reports on migration.
There are biases both ways: people might not realize a species migrates (bats being nocturnal). And for those bats that don’t migrate, it might not get explicitly mentioned, because it’s “normal” for bats, which means you get no information. And within one species, some individuals will migrate and others won’t. Still, they ended up with a list of over 300 bats species for which they had some brain and behaviour data.
Migrating bats indeed had smaller overall brain sizes for their size – just like the birds. This supports the idea that brains are expensive, and so is flying, and the combination of the two is incredibly difficult to fit into the energy budget.
I’d be interested to see if there were differences in the brains of migrating and non-migrating invertebrates, like butterflies. They tend to have smaller nervous systems relative to their body size than vertebrates, so would they also have the same energetic costs to shave off neurons here and there? Or vertebrates that migrate without flying?
More surprising was that the hippocampus showed no difference between the migrating and non-migrating bats. McGuire and Ratcliffe suggest there are just too many confounding factors for any signal to rise above the noise. What we need are some bat neuroethologists (and I know you’re out there) to do some studies on how bats use the hippocampus in a controlled lab setting.
McGuire L., & Ratcliffe J. 2010. Light enough to travel: migratory bats have smaller brains, but not larger hippocampi, than sedentary species. Biology Letters: In press. DOI: 10.1098/rsbl.2010.0744
Photo (Myotis myotis) by Jan Svetlík on Flickr; used under a Creative Commons license.