Shell shapes evolved to avoid 'turning turtle'

reposted from: http://www.newscientist.com/article.ns?id=mg19626275.800&feedId=fundamentals_rss20
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Shell shapes evolved to avoid 'turning turtle'

• 27 October 2007

FOR a tortoise, being upside down is at best ignominious and at worst fatal. So to avoid it, some species have evolved a quite brilliant trick - perfectly shaped self-righting shells.

Gabor Domokos and Peter Varkonyi at the Budapest University of Technology and Economics in Hungary

modelled the geometry of turtle and tortoise shells of differing dome heights. Those with very tall domes, such as the star tortoise, effectively have just one stable orientation, right side up. Put another way, a star tortoise placed on its back will automatically roll back onto its front, righting itself with little effort.

In geometric terms, this tortoise's shell is known as a monostatic object, one that has only one stable way of resting on a horizontal surface. Such shapes are incredibly rare in nature, the researchers say.

Other, flatter species, such as the Argentine snake-necked turtle, have two stable points - on their belly and their back - and must use their neck as a lever to flip themselves back over. Animals with medium height domes have the worst shape of all, say the researchers. They can come to rest at three distinct points, right side up and on either side of the shell, meaning they can get stuck halfway through a roll

(Proceedings of the Royal Society B, DOI: 10.1098/rspb.2007.1188).

From issue 2627 of New Scientist magazine, 27 October 2007, page 22