CARTOONISTS love to sketch endearing animals with larger-than-life eyes. But according to a new survey, the reality is rather different: for many vertebrates the relationship between the size of a creature鈥檚 eye and its body follows a classic mathematical formula.
A well-known mathematical relationship is known to link body weight and the size of certain organs. For instance, in many vertebrates brain weight is proportional to body weight raised to the power of 0.66.
One study has suggested that there is a different, semi-logarithmic relationship between vertebrates鈥� eye and body size. But in a much larger study, Howard Howland and colleagues at Cornell University, New York, have now found that is not so. By scouring the natural-history literature, they found front-to-back eye measurements and body weights for more than 300 vertebrate species and breeds. This showed eye size increasing with body weight along a logarithmic scale just as brains do 鈥� which is hardly surprising, given that the retina is intimately connected to the brain via the optic nerve (Vision Research, vol 44, p 2043).
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This basic relationship between eye and body size could reflect how each species occupies a particular ecological niche, especially where eye size deviates from the norm. 鈥淥nce we have this line you can ask 鈥榙oes it have a big eye or a small eye鈥� and then you can ask why,鈥� says Michael Land, a neurobiologist at the University of Sussex in Brighton, UK.
For instance, the eyes of birds and primates are around 35 per cent larger than those of the average vertebrate, probably because both groups rely heavily on vision to find mates and food. Owls, for instance, have the largest eyes of any vertebrate relative to their body size, an adaptation useful for spotting small prey in dim light.
Reptiles and rodents generally have small eyes, the researchers found. Many rodents are nocturnal, like owls, so their small eyes probably reflect the fact that they have forsaken their visual sense in favour of touch and smell.
Fish, however, do not seem to follow any particular trend. That seems to be because the buoyancy of water has allowed many species to evolve long bodies without growing a bigger head, Howland says. 鈥淔ish live in this wonderful gravity-free environment so they can assume these wonderful shapes that we can鈥檛. The results were surprising at first, but make complete sense.鈥�