How does one species get a bigger brain than another?A while ago, I wrote about a paper that argued that genes that define boundaries in the nervous system seemed to be responsible for differing brain structures in cichlid fish. This seemed to explain the data better than a competing hypothesis, which was that the differences in brain size were caused by the length of time the brain spent forming neurons (neurogenesis). A forthcoming paper by Charvet and Striedter suggest a third possibility.
Bobwhite quails (pictured) have kind of small brains. But that’s okay, because they’re kind of small birds; they’re smaller than a chicken. But despite the chicken having a bigger brain, the chicken goes from fertilization to hatching faster than the bobwhite quail.
Proportionately, both the quail and chicken start neurogenesis at about the same time. The chicken has a slight edge, but the extra 5% of time the chicken spends doing neurogenesis isn’t enough to explain that the chicken’s brain ends up being 100% bigger than the quail’s. When neurogenesis starts, the chicken’s brain is already 100-200% bigger than the quail’.
Before neurons form, the chicken’s cells are diving great guns. The whole cell cycle, everything mitosis related, is ramped up and going at a much faster clip in chicken than quail. They tested this using bromodeoxyuridine (BrdU), a chemical that cells incorporate into themselves if they’re actively dividing. The more a cell divides, the more BrdU it takes in. They couldn’t test all the way through development, but at around 11% development, chicken are showing cell division going about 100% faster than quail. After neurogenesis begins, though, that difference is almost nil.
To sum up: The chicken makes a mess of cells real fast, real early.
The other two mechanisms are more about changing the relative proportions of regions within a brain: enlarging over here, shrinking over there. This mechanism allows you to make brains that are different sizes, but that are otherwise structurally similar.
The unanswered question, though, is why do chickens need such large brains?
Ever wonder what chickens do when you’re not looking?
Reference
Charvet, C., & Striedter, G. (2010). Bigger brains cycle faster before neurogenesis begins: A comparison of brain development between chickens and bobwhite quail Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.0811
Photo by leshoward on Flickr, used under a Creative Commons license.
Interesting. If I remember correctly, development in chicken lasts 21 days. In most quails (including, I guess, Bobwhite), it last 24 days which is a little longer than chicken. On the other hand, Japanese quail develops in less time - 16 or 17 days (depending on the strain). Did this paper mention and explain that?
ReplyDeleteYes! Charvet and Streider start off with just that point -- how to compare the two animals when one develops faster. This is one of the things that makes chickens and quails good for the purposes of this study: you can't argue that the chicken brain is bigger because it undergoes [whatever] longer.
ReplyDeleteThey end up expressing most of their results as a proportion of development, to normalize events in development. When they do that, the chicken is leading the quail at some stages, but not as much as if you look at absolute time.
Oh sweet - the article is Open Access, will read now. Thanks.
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