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January 16, 2007
Big brains and bird brains
How did I become so fascinated by bird brains?
Last week, NewScientist posted an interesting article describing several recent studies on birds and brains. The hypothesis that both studies consider is whether larger brains offer a particular evolutionary advantage for birds.
In the first, Susanne Shultz and her colleagues in the UK considered whether large-brained birds are better able to adapt to the changing environmental conditions induced by human farming activities than their small-brained cousins. It turns out that it's not just having a big brain that helps birds here. Instead, it's having a big cerebrum - that part of the brain that is overly developed in we humans.
Although their study is purely empirical, there are some interesting theoretical questions here. For instance, past studies on the decline of farmland birds succeeded only in pointing out that so-called "generalists" fared better than "specialists." This distinction is one I've encountered before in the ecology literature, but it's never very well defined. Is it possible to come up with, perhaps from first principles, a reasonable quantity that captures the notion that species vary in how specific their needs for survival are? Humans, for instance, would seem to be generalists, but are we more or less general than ravens? Until we have such a quantity, the question of, for instance, whether humans or cockroaches are more general, doesn't make any sense. But, such details haven't stopped some scientists in the past, and it doesn't stop Shultz et al. from suggesting that bigger cerebrums correlate with more generalist behavior, and that explains why these bird species are able to adapt to changing conditions.
In summary, our results suggest that the farmland birds whose populations have suffered most under agricultural intensification are those with more specialized resource and habitat use and lesser cognitive abilities.
The work of Sol et al. (the second article) seems to support this idea, however. They consider whether larger bird-brains correlated with a higher degree of success in the colonization of new areas. The impressive thing about this work is the extent to which the authors try to control for other factors that might misleadingly give the appearance of a correlation between brain size and success. (This kind of careful statistical analysis makes their conclusion - that big brains help - all the more persuasive.)
Our findings support the hypothesis that large or elaborated brains function, and hence may have evolved, to deal with changes in the environment... [The many hypotheses to the origin of large brains] are essentially based on the same principle, that enlarged brains enhance the cognitive skills necessary to respond to changes in the environment...
The fact that their results suggest such a connection seems to contradict the idea that in order to be successful, an invading species must fit into a previously unexploited niche or out compete previously established species. That is, here, successful species adapt to their new environment by figuring out ways to get food, avoid becoming food, and reproducing. I'm not sure this kind of argument applies beyond higher vertabrates, though. What's missing, it seems, is some kind of theoretical explanation that connects brain size with adaptability. Of course, such a theory would itself depend on knowing what exactly brains do and why making certain parts of them (i.e., the cerebrum) bigger seem to improve one's ability to do certain things.
Shultz et al. "Brain size and resource specialization predict long-term population trends in British birds." Proc. Royal Society B 272, p2305-2311 (2005).
Sol et al. "Big brains, enhanced cognition, and response of birds to novel environments." Proc. National Academy of Science USA 102 (15), p5460 (2005).
posted January 16, 2007 01:29 PM in Obsession with birds | permalink