Study Says Eyes Evolved For X-Ray Vision

The advantage of using two eyes to see the world around us has long been associated solely with our capacity to see in 3-D. Now, a new study from a scientist at Rensselaer Polytechnic Institute has uncovered a truly eye-opening advantage to binocular vision: our ability to see through things.

Most animals — fish, insects, reptiles, birds, rabbits, and horses, for example — exist in non-cluttered environments like fields or plains, and they have eyes located on either side of their head. These sideways-facing eyes allow an animal to see in front of and behind itself, an ability also known as panoramic vision.

Humans and other large mammals — primates and large carnivores like tigers, for example — exist in cluttered environments like forests or jungles, and their eyes have evolved to point in the same direction. While animals with forward-facing eyes lose the ability to see what's behind them, they gain X-ray vision, according to Mark Changizi, assistant professor of cognitive science at Rensselaer, who says eyes facing the same direction have been selected for maximizing our ability to see in leafy environments like forests.

All animals have a binocular region — parts of the world that both eyes can see simultaneously — which allows for X-ray vision and grows as eyes become more forward facing.

Demonstrating this X-ray ability is fairly simple, the researcher says: hold a pen vertically and look at something far beyond it. If you first close one eye, and then the other, you'll see that in each case the pen blocks your view. If you open both eyes, however, you can see through the pen to the world behind it.

To demonstrate how eyes allow us to see through clutter, hold up all of your fingers in random directions, and note how much of the world you can see beyond them when only one eye is open compared to both. You miss out on a lot with only one eye open, but can see nearly everything behind the clutter with both.

"Our binocular region is a kind of 'spotlight' shining through the clutter, allowing us to visually sweep out a cluttered region to recognize the objects beyond it," says Changizi, who is principal investigator on the project. "As long as the separation between our eyes is wider than the width of the objects causing clutter — as is the case with our fingers, or would be the case with the leaves in the forest — then we can tend to see through it."

To identify which animals have this impressive power, Changizi studied 319 species across 17 mammalian orders and discovered that eye position depends on two variables: the clutter, or lack thereof in an animal's environment, and the animal's body size relative to the objects creating the clutter.

Changizi discovered that animals in non-cluttered environments — which he described as either "non-leafy surroundings, or surroundings where the cluttering objects are bigger in size than the separation between the animal's eyes" (think a tiny mouse trying to see through 6-inch wide leaves in the forest) — tended to have sideways-facing eyes.

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Uganda's Mountain Gorillas Increase in Number

The most recent census of mountain gorillas in Uganda’s Bwindi Impenetrable National Park—one of only two places in the world where the rare gorillas exist—has found that the population has increased by 6 percent since the last census in 2002, according to the Uganda Wildlife Authority, the Wildlife Conservation Society, the Max Planck Institute of Anthropology and other groups that participated in the effort.

“This is great news for all of the organizations that have worked to protect Bwindi and its gorilla population,” says Wildlife Conservation Society researcher Alastair McNeilage, who is also the director of the Institute of Tropical Forest Conservation in Bwindi. “There are very few cases in this world where a small population of a endangered primates is actually increasing.”

According to the census, which also successfully used for the first time genetic samples from fecal specimens, Bwindi’s gorilla population now numbers 340 individual gorillas, up from 320 in 2002, and 300 in 1997.

The census was conducted between April and June 2006 to determine the size and makeup of the Bwindi population, in addition to their distribution and to gauge human impacts on the gorillas. During that time, survey teams set out with the intention of counting every family group in the population, a method possible only with small animal populations in a relatively small area; Bwindi Impenetrable National Park is approximately 127 square miles in size.

In addition to census methodologies such as counting trails and nests (gorillas construct a nest each night), researchers used genetic analyses as well, specifically because many of the groups were clustered in the same area of the park, which presented survey teams with the risk of double-counting the same individuals or groups. DNA analysis, however, allowed individuals to be identified and distinguished, even among groups which have never been seen by people.

Fecal specimens were analyzed at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Researchers were encouraged by the overall growth of the population, showing an annual increase of one percent over the past decade. The age composition of the population shows a healthy distribution of adult and immature classes, including infants and juveniles. Other findings are that gorillas are not using the eastern side of the park, a possible result of human disturbances there. Particularly encouraging, however, is that one group of gorillas are starting to use the northern sector of the park for the first time in living memory.

Bwindi is also home to a thriving gorilla ecotourism program, which generates much needed foreign revenue for Uganda annually by bringing paying travelers into the company of mountain gorillas. Four of the 30 family groups are habituated for tourism, or conditioned to tolerate the presence of humans for short durations.


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