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

Animals have bad sight too. (ISTOCKPHOTO)
March 18, 2013

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.

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.”

Visual peculiarities aren’t limited to the sea. Rabbits have many of their color-detecting cells concentrated along a single line where the horizon normally occurs in their field of vision, which sets off predators against the blue background of the sky. Horses are farsighted, which is advantageous because their predators, such as mountain lions, tend to begin stalking them from far away. They can tolerate poor vision close to them, since grass is pretty easy to spot.

Domesticated animals exhibit somewhat more variation in their visual capabilities than wild animals do.

Some dogs, for example, see better than others, and some might do well with a pair of glasses. There are two reasons for this. First, human whim has displaced natural selection in dog breeding. Dogs have offspring not because of their hunting or predator-spotting skills but because we find them adorable. Second, we regularly intervene in a dog’s health. Even if a dog’s eyesight is so bad that it couldn’t forage for its own dinner, humans gladly bring the food to the pet, making it possible for the dog to survive and pass on its faulty sight genes to the next generation.

Which brings us back to humans. For the most part, poor vision wasn’t something that we inherited from our hominid ancestors or the apes that preceded them. Rather, myopia is a result of modern life. Researchers have found that the Inuit, for example, had low rates of myopia when they followed their traditional lifestyle. Other tribal societies exhibit the same low prevalence.

“Myopia, like many diseases, is the result of interaction between genes and environment,” notes Dan Lieberman, a professor of evolutionary biology at Harvard. “It’s still a controversial topic, but it appears that myopia results from either too much close-work [like book-reading] or too much time spent indoors. The contraction of muscles around the eye changes the shape of the eyeball.”

This explains why natural selection didn’t rid us of myopia. When humans lived in a state of nature, people with a genetic inclination toward nearsightedness weren’t actually nearsighted, because the environmental conditions didn’t trigger the disorder. They had no trouble breeding and spreading their genes.

Once myopia became a problem, human culture and technology protected those genetically inclined to nearsightedness, the same way we breed dogs regardless of their ability to see. Eyeglasses, and now laser surgery, mean that there’s no evolutionary cost to myopia. In fact, Lieberman notes, there may even be an advantage.

“If you look in a [fashion] catalogue today,” he says, “you’d be led to believe that glasses are suddenly sexy.”

Charles Darwin observed that sexual selection, or the perpetuation of traits that become attractive to members of the opposite sex, is among the most powerful forms of natural selection. Some anthropologists even believe it’s responsible for bipedalism. Perhaps it’s now working its effects on myopia, turning us into a permanently bespectacled species.

Show Comments
Most Read National