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Why some animals have smaller eyes: Lifestyle matters

ITHACA, N.Y. — If brain size is proportional to body size in virtually all vertebrate animals, Cornell University biologists reasoned, shouldn't eye size and body size scale the same way? While they failed to find a one-size-fits-all rule for eyes, what they learned about the 300 vertebrates they studied helps to explain how animals evolved precisely the orbs they need for everyday life.

The biologists reported their findings in the journal Vision Research (August 2004, "The allometry and scaling of the size of vertebrate eyes"). Howard C. Howland, Stacey Merola and Jennifer R. Basarab say they did find a logarithmic relationship between animals' body weight and eye size for all vertebrates, in general: Bigger animals do tend to have bigger eyes, on average.

But breaking vertebrates into smaller groups -- such as birds, fishes, reptiles and mammals -- and trying to predict their eye size gets more complicated. And dividing all mammals into groups -- such as rodents and primates -- could make a scientist cross-eyed: Compared with all vertebrates, rodents' eyes are only 61 percent as large as they "should be" if all animals obeyed the general rule, while primates' eyes are 35 percent larger than those of vertebrates as a whole. Except for gorillas, that is.

Says Howland, a Cornell professor of neurobiology and behavior who led undergraduate students Merola and Basarab through the survey: "A carnival barker who correctly guesses your weight could also derive the weight of your brain with this allometric equation: log (organ size) = slope constant * log (body weight) + intercept constant . Allometry refers to the scaling of size of animals and their parts, and it works amazingly well for brain size and body weight.

"But the carny who wants to guess your eye size would have to know something more about how you use your vision and your other senses. Larger eyes are better for gathering light. Birds in general and owls in particular h
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Contact: Roger Segelken
hrs2@cornell.edu
607-255-9736
Cornell University News Service
6-Aug-2004


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