[Cross-posted from NeuroDojo]

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.

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Comment by David D. Olmsted on July 31, 2010 at 7:14am
Dang, that is really interesting! Why would the brains of two different bird species having similar ecologies be so different, one new world and the other old world?

If I remember correctly the wild ancestors of chickens are the red jungle fowl out of southeast Asia. The only other variable I can think of is that the red jungle fowl must have an elaborate courtship.

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