Have you ever stumbled across an animal skull in the woods and found your mind filled with questions about it? What kind of animal was it? What did it eat with those teeth? Were the eyes really that big? Was the brain really that small? Finding a skull tends to bring out the private investigator in all of us. As we examine these bony shells, built to protect the brain, hold the teeth, and house many of the sensory organs, we can find clues about the animal’s life.
The first thing to notice when you find a skull is its size. Is it the length of your thumb (squirrel, rabbit, weasel); does it fit in your hand (fox, bobcat, raccoon, beaver, opossum); is it as long as your foot (deer, bear), or even bigger (cow, horse, moose)? Size can be deceptive since skulls lack the covering of muscles, skin, and fur of a live animal and thus often seem much smaller than you would expect. Still, by considering its size first, you can often narrow down the possibilities.
Among the most obvious and most telling features of a mammal skull are the teeth. These can give an idea of what the animal ate, for the characteristics of the teeth have been molded by evolution to fit an animal’s habits and food. The teeth of plant-eaters or herbivores are different from those of carnivores, the meat-eaters. Omnivores, animals that eat both plants and animals, need a combination of herbivore and carnivore teeth. Insectivores have teeth that are specialized for grasping and biting through the hard exoskeletons of insects.
Mammals have four basic kinds of teeth. In the front of the mouth are the incisors, sharp, chisel-shaped teeth used for biting off or cutting into hard foods. The canines are long, pointy, conical teeth, especially prominent in meat-eating animals. Canines are used for killing prey and for ripping and tearing the flesh. Behind the canines are the premolars, small molars with two humps (bicuspids), and molars, the large teeth farthest back in the jaw, used to shear off or grind food. It may be easier for children to remember these different teeth by the way they function in our mouths. We use our incisors as “nippers,” our canines and premolars as “rippers,” and our molars as “grinders.”
Herbivores can be divided into gnawing animals – like rodents and rabbits – and grazing animals, like deer, sheep and goats. Rodents (including squirrels, mice, beavers, and porcupines, among others) have large, sharp upper and lower incisors for cutting through tough plant material, and broad, ridged molars for grinding it up. Because they lack canines and premolars, they have a gap between the incisors and molars. Beavers use their incisors like chisels to chip off bark, like clippers to nip off twigs, and like scrapers to peel off the thin inner bark they like to eat. The incisors of all rodents grow constantly throughout their lives. With hard, orange-colored enamel on the front and softer dentin underneath, the incisors wear down faster on the inside than the outside, keeping a sharp edge. Rodents must keep gnawing to both shorten and sharpen their teeth. Grazing herbivores, like deer and moose, sheep and goats, have no upper incisors. Rather than cutting their food off neatly, they rip and tear off vegetation by working their bottom incisors against their hard upper palate. They have wide, flat molars with hard enamel ridges for grinding up twigs, buds, nuts, and other tough plant material.
Carnivores, such as bobcats, fishers, and weasels, have all four kinds of teeth. Their incisors are small, but their canines are especially long and sharp to grab, hold and kill their prey. Carnivores also have special blade-sharp premolars and molars that work like scissors to shear through meat. With its long canines and sharp molars, a bobcat can kill a full-grown deer. Omnivores, such as foxes, coyotes, bears, and raccoons, also possess all four types of teeth, but theirs tend to be more generalized than those of either herbivores or carnivores, since they eat both plants and animals. Like carnivores, foxes and bears have large, sharp canines for catching prey, but they also have broad molars for mashing and grinding fruits and nuts.
Insectivores like moles and shrews eat beetles and other invertebrates in the soil, while bats eat flying insects such as moths and mosquitoes. In order to break through the tough exoskeletons of many invertebrates, insectivores have extremely pointy teeth. Shrews have long incisors that point forward so they can grasp their prey, and the short-tailed shrew is unique in having venom that it uses to paralyze and subdue prey larger than itself. The reddish-brown coloring on the tips of shrew teeth is a helpful clue to identifying their tiny skulls.
While the upper jaw, or maxilla, is attached to the skull, the lower jaw, made of two bones called mandibles, is not, so you may stumble across one without the other and have to piece together the evidence from what remains. Sometimes the teeth are missing as well, and then the challenge is even greater. However, other features of the skull can give us clues about the animal too.
The position of the eyes, which we can gather from the eye sockets or “orbits,” may tell us whether an animal was a predator or prey. Most predator eyes face forward, giving them three-dimensional binocular vision, useful for hunting. Prey animals, on the other hand, usually have eyes positioned more on the sides of their heads, thus increasing their peripheral vision so they can spot predators in all directions. The size of the eyes is another clue about the animal’s habits, being larger in nocturnal than in daytime animals, since larger eyes can gather more light.
The shape of the nose may tell us how much an animal relies on its sense of smell. Deer have a very keen sense of smell. Their long noses have thin, flaky bones inside that increase the surface area for detecting odors. Coyotes and foxes also have long muzzles and depend on their sense of smell for finding prey. Bobcats, on the other hand, have short noses and hunt by sight more than by smell.
Various other features of skulls can be helpful in identifying an animal. Sutures are places where the bones of the skull are knitted together like pieces of a jigsaw puzzle. These are especially noticeable on deer skulls. Rabbits have lacey nose bones, a distinctive characteristic of their skulls. The size and placement of various holes where nerves and blood vessels pass through the skull are useful in identification. In some animals you may find a raised crest or ridge on the top or back edge of the skull, surfaces to which jaw muscles are attached. All of these are useful for skull identification, provided you have a guide in hand.
The next time you happen across a skull while on a walk in the woods or field or swamp, take a close look at the teeth, consider the position of the eyes, the shape of the nose, the overall size. Let your inner sleuth get to work looking for clues, and see what you can discover about the animal that traveled here before you.
Burt, William H. A Field Guide to the Mammals of North America. Peterson Field Guides. New York, NY: Houghton-Mifflin, 1980.
Elbroch, Mark. Animal Skulls: A Guide to North American Species. Mechanicsburg, PA: Stackpole, 2006.
Searfoss, Glenn. Skulls and Bones: A Guide to the Skeletal Structures and Behavior of North American Mammals. Mechanicsburg, PA: Stackpole, 1995.