Animals’ eyesight has evolved to suit the differing needs of different species

As a columnist who tries to explain scientific and other puzzles, I get asked a lot of strange questions. Here’s one that has been bugging me for some time: Are there visually impaired animals? Are there nearsighted deer that could use glasses or farsighted elephants that could benefit from an enormous set of contacts? How about astigmatic alligators?

It seems like an animal question, but, at its core, it’s motivated by an astute comparison with humans.

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We’re undeniably visual creatures, yet many of us have trouble seeing well. According to some estimates, up to 42 percent of Americans are myopic, or nearsighted. Isn’t this a failure of natural selection? Shouldn’t our blurry-sighted ancestors have starved to death or been consumed by predators because of their visual handicaps? Does nature allow other animals to have such poor vision?

These questions turn out to be surprisingly complicated. Let’s start out with the non-human animals and work back to our own visual shortcomings.

Ophthalmologists can’t ask lions to read an eye chart or put glasses on a whale. Instead, they shine a light into the animal’s eye to see how it refracts and focuses on the retina. And with a trainable animal, such as a hawk or a horse, researchers can teach it to respond to a visual cue, then determine how well the animal picks up the cue when it is far away, very close or somehow obscured.

So what do these tests reveal?

“Wild animals don’t generally exhibit the same variation as humans in their visual capabilities,” notes David Williams, a veterinary ophthalmologist and fellow at St. John’s College Cambridge. “Wild animals either don’t live long enough in the wild to suffer age-related changes in their eyes, or they are better able to cope with old age than are humans.”

In other words, while there may be a nearsighted deer lurking around Rock Creek Park, myopia is pretty rare among animals that rely on their eyesight. Those genetically inclined toward visual impairment are less likely to pass their genes on, because animals with blurry vision have trouble finding food or spotting predators. Eventually, the genetic weakness should be bred out of the population over time.

That doesn’t mean that all animals see exactly the same. In fact, they exhibit an incredible array of visual eccentricities. These variations, however, occur on a species-wide basis. They’re adaptations, rather than handicaps.

Take whales, for example. Marine conditions pose a challenge to mammalian eyes, because water bends light differently than air. Humans deal with our underwater farsightedness by wearing goggles or masks, which place air between the water and our eyes. Whales have a different solution: The shape of their corneas accommodates the light-bending power of water. While whales are all astigmatic by human standards, their eyesight is a purposeful solution to a specific problem.

Animals that live between water and land cope with the demands of vision in various ways. “Penguins have relatively flat corneas,” says Chris Murphy, a professor of comparative ophthalmology at the schools of medicine and veterinary medicine at the University of California at Davis. The lack of corneal curve common in humans and other animals decreases the effective difference between being above and below water, allowing them to see in focus in both environments. “Crocodiles deal with the problem a different way: They simply live with slightly blurry vision.”