Our Window on Nature

. . . exploring the world around us

The Eyes (and the Teeth) Have It

Filed under: Critters — Lowell Christie -- September 1, 2015 @ 8:06 pm

BarnOwl-LHere in our desert campsite, daybreak means that coveys of quail will soon strut in for a morning meal. They dash this way and that while vocalizing cheerfully, competing with the cottontails for the seed that we toss out at first light. It means furry little ground squirrels popping their heads out of their burrows and, if the coast is clear, chasing each other around with just awakened vigor. It means a female coyote who, with her half-grown pup, comes in daily for water. When the coyotes near, other animals scurry for cover.

The morning hours are a time to seek food. There’s no morality in nature; each animal eats that for which its body is designed. Predatory creatures capture and devour other animals, while prey species try to escape becoming a predator’s next meal.

Examining eye placement is one way to determine whether an animal species is more likely to be predator or prey. There are exceptions to this rule, but when an animal’s eyes are situated to the front of the skull facing directly forward, that generally indicates that the animal is a predator – a meat eater. When the eyes are located farther back on the skull, and face more to the side, they indicate that the animal is typically prey.

Predators – coyotes, foxes, bobcats, and mountain lions to name a few – need to have binocular vision. Both eyes must focus together to produce the depth perception necessary to catch prey. In order for a Mountain lion that is perched on a ledge to spring down upon a deer, the lion must be able to precisely measure the length of its lunge. That deer isn’t going to stand around and provide the lion with a second chance. Off it goes, and a mountain lion isn’t fast enough to chase down a deer on the run. A predator with faulty vision may starve to death.

Not so for animals designed as prey. As a vegetarian, a deer doesn’t need binocular vision to find food, because plants aren’t going to escape while the deer browses from bush to shrub, sampling a twig here and a leaf there. No – a deer’s eyes are primarily needed to help it avoid predation.

With eyes located on the side and well back in the skull, a deer’s depth perception is poor, but with only a small movement of the head it can keep track of anyone sneaking up from behind. And that’s what’s most important to the deer. In contrast, animals with binocular vision must turn their heads all the way around if they wish to see what’s behind them.

Rabbits’ and rodents’ eyes are positioned similarly to those of a deer. That they are heavily preyed upon is supported by the prolific rate at which they breed; everyone likes to eat bunnies and squirrels and mice, so these creatures must constantly remain alert.

Nocturnal animals have huge eyes that are ever watchful for the movement of a fox or an owl, and the eyes of ground-burrowing animals, such as ground squirrels, kangaroo rats, and prairie dogs, are placed high on their skulls to allow them to peer from the mouth of a burrow without emerging far enough to become a target. With only its head exposed, the rodent can duck below ground if a hawk swoops down to snatch a tasty morsel.

Obviously, the dogs and cats that share our homes are designed to be predators, in that their eyes face forward. But so do ours. We don’t have fossil records to prove that our early ancestors ever caught prey with their teeth, but it takes good vision to accurately aim a spear as well. Without it, we would all be vegetarians.

Although they are less easily observed in the wild, you can also tell much about an animal’s eating habits by examining the structure of its teeth. Should you happen upon a mountain lion that is gentle enough to allow you to open its mouth, you would see that this meat-eater, or carnivore, has teeth that are designed to capture and kill prey. Sharp, piercing canines, or fangs, are used to bring down the deer. By sinking deeply into the flesh, these teeth give the lion a stout hold on the animal – long enough for those powerful jaws to snap the victim’s spinal cord.

Behind the canines are the carnassials – sharp cheek teeth that, instead of meeting top and bottom jaw, overlap to provide a scissoring action. With them the lion can shear off chunks of meat that are small enough to swallow. No chewing though; cats are incapable of chewing. The fact that the upper teeth overlap the lower means that there is no lateral movement to the jaw. Cats literally gulp their food and leave it to their digestive systems to take care of the rest.

Herbivores – plant-eaters – need totally different dentition. Canine teeth would be useless to a deer or to a rabbit, so these creatures don’t have them. Herbivores need incisors in front to crop off grasses, leaves, and twigs, and they need flatcrowned molars behind to grind up the food and break up the cellulose. These vegetarians have those traits in common, but the rest of their teeth vary greatly. Most hooved animals, such as deer, elk, bighorn, and bison, have incisors only in the lower jaw, and a bony plate in the upper jaw where the incisors would be.

Rodents have two incisors – top and bottom – that grow continuousIy. Enamel covers only the anterior surface of those teeth, which means that the inside surfaces wear down, keeping the teeth chisel sharp. Cottontails and jacks also have two incisors in a row – top and bottom – but if you look very closely at a rabbit’s skull, you see two additional tiny incisors behind the ones on top. Biologists studying bunnies have discovered no purpose for those teeth, but they speculate that they are evolving in, rather than out.

Although we humans have the eye placement of the hunter, our teeth more closely parallel those of vegetarians. In fact, we are omnivores, along with the highly successful raccoons and coyotes – animals that eat both plant and animal matter. Because our ancestors hunted with tools, however, we lack sharp canines for catching prey. (We do have canines, but, except for those of Count Dracula, they are in a line with the rest of our teeth.) Our molars are relatively flat, and our teeth meet, which allows for considerable chewing motion. But is our versatile dentition a cause of, or a response to, our success as a species?


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