Why some animals have smaller eyes: Lifestyle matters

August 06, 2004

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 have relatively large eyes because their vision is critically important to them. So do most nocturnal mammals," Howland says. "But a shrew's eyes are tiny because it

spends most of its time underground, where things are dark, but where the shrew's senses of smell and touch are more important [than vision] for making a living."Or the carnival barker could simply measure eye size, which is what Merola did for some dissected specimens, while she and Basarab scanned the scientific literature for weight-and-size information already gathered by other biologists on other species. Then the student biologists turned to their graphing calculators to enter the values in the allometric equation and compared the results for various animal groups to the average for all vertebrates.

Some results were reasonably consistent. Most reptiles, for example, tend to have eyes only 70 percent as large as vertebrates in general. But relative eye size in fishes was all over the map, in part because their body weight in the water is less constrained by gravity than the weight of an animal out of water. One aquatic creature that fails to obey the bigger body-bigger eyes rule is the reedfish, a lanky, well-camouflaged fish with no apparent head and little room for eyes.

The Cornell biologists introduced their Vision Research report with a brief comment on perception and reality, noting that "cartoonists often exploit the fact that the eyes of babies are much larger in proportion to body size than the eyes of adults." They point out that many of the smaller non-human primates -- the ones we humans think are cute -- tend to have much larger eyes than their body size might suggest, whereas the biggest primate, the gorilla, has especially small eyes for its body size. "I imagine gorillas don't have to worry whether we think they're cute," Howland says.

He hopes the eye-size research, which was sponsored by the National Institutes of Health, will encourage other investigators to ask the questions: Is that a big eye or a small eye, and why? What is it about environments in which animals live that determines eye size?

For his part, the Cornell neurobiologist has moved on to a new study of eye size throughout various stages of development. He observes that human eyes grow rapidly in the womb and for the first three months after birth. That explains why babies are so adorably cute: Their disproportionately big eyes gaze out from those little round faces.

By three months, our eyes are as big as they'll ever be -- at least from outward appearances: The corneas have reached their full width, although inside the eyes, the neurobiologist notes, the front-to-back length will increase somewhat. Then our eyes move farther apart in the face as the head grows. And we'll never be that cute again.

Cornell University

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