Squirrel Skull: How to Identify It (and Tell It From a Rat or Rabbit)

Hold the skull so you are looking at it from the side. A squirrel skull is broad and rounded, with a relatively short, deep snout. The Maryland Department of Natural Resources skull key and the University of Texas at El Paso account of the squirrel family list the same set of features.

At the front sit the incisors, one large pair above and one below. A squirrel has no canines. Behind the incisors is a long, empty stretch of jaw called the diastema, and only after that do the cheek teeth begin. On the side of the snout you will find a small opening on each side called the infraorbital foramen. In a squirrel it is small; keep that in mind, because it matters when you compare with a rat.

Now look down on the skull from above. Just behind each eye socket, a pointed flange of bone juts outward. These are the post-orbital processes, and in squirrels they are large and sharp. Maryland's key uses them to separate squirrels from other rodents, and once you have seen them, they are hard to miss.

The tooth count is written as a dental formula. For the eastern gray squirrel it is I 1/1, C 0/0, Pm 2/1, M 3/3: one incisor, no canines, two premolars and three molars on each upper side, with one incisor, no canines, one premolar and three molars on each lower side. Double it for both sides and you get 22 teeth. The squirrel family as a whole, Sciuridae, ranges from 20 to 22 teeth, the University of Texas at El Paso notes, so a count gets you to the family rather than naming the species.

Squirrel incisors are orange-brown on the front because of a thin, specialized surface layer on the enamel, and they never stop growing. Both facts explain why the teeth look the way they do in a skull you pick up from the grass.

Start with the growth. Rodent incisors are what scientists call elodont: they keep growing and erupting for the animal's whole life, without the ordinary roots our teeth have. The front face is covered with hard enamel, and the back is softer dentine. Every time the squirrel gnaws, the soft back wears away faster than the hard front, which leaves a sharp chisel edge. Smithsonian Q?rius describes these incisors as self-sharpening. A squirrel, in other words, never needs a dentist, only a steady supply of things to chew.

The color is where the science moved in 2024. Iron has long been linked with the orange of rodent teeth, and the iron is real: a study published in *ACS Nano* and summarized by the American Chemical Society found that iron-rich structures in the enamel strengthen and protect it. The strong orange-brown color, though, comes mainly from a thin surface layer of organic and inorganic material on the outside of the tooth, rather than from the iron-rich pockets alone. The Max Planck Institute for Solid State Research has its own summary of the work for anyone who wants the details.

Tree squirrels, ground squirrels and chipmunks all belong to the squirrel family, so they share the same basic plan: paired, ever-growing incisors, no canines, a diastema and squirrel-type post-orbital processes. Telling them apart takes a closer look.

Among the tree squirrels, the Maryland key compares the eastern gray squirrel with the eastern fox squirrel, *Sciurus niger*. In that key, a fox squirrel skull is under about 76 mm long and has developed temporal ridges on top that come together into a small sagittal crest. A gray squirrel skull lacks those ridges. If you want to compare a skull on screen, the University of Texas Digimorph collection has a 3D scan of a fox squirrel skull.

Ground squirrels vary a great deal by genus, and there is no single ground-squirrel skull shape to compare against. The one big relative that is easy to separate is the groundhog, *Marmota monax*, which the Maryland key sets apart from smaller squirrels by its skull length. If your skull is noticeably bigger and heavier than a squirrel's, our story on the groundhog skull is the next stop.

Chipmunks are the small end of the family. The Oregon State University Mammal Collection describes the eastern chipmunk, *Tamias striatus*, as having proportionally smaller post-orbital processes, a larger braincase for its size, and round infraorbital foramina that pierce a flat plate of bone on the cheek called the zygomatic plate. Under magnification its incisors can show fine grooves running lengthwise. These skull features are more reliable than where the skull turned up.

A young rabbit, an adult squirrel and a rat can look alike at first glance, since all three are small skulls with big front teeth. A few checks settle most cases.

Rabbit first, because it is the easiest. Turn the skull over and look behind the big upper incisors. A rabbit has a second, smaller pair of peg-like incisors tucked right behind the first. The Maryland key identifies the eastern cottontail, *Sylvilagus floridanus*, by its four or six upper incisors and by a rostral fenestra, an open, window-like area of bone on the side of the snout. Rabbit cheek teeth also keep growing, unlike the rooted cheek teeth of squirrels and rats. Count two pairs on top and you are holding a rabbit; our rabbit skull story picks it up from there.

A rat is the closer call, since it has the same single pair of ever-growing incisors and the same diastema. The differences are in the cheek teeth and the skull's shape. Rat-like rodents lack premolars, which squirrels have. Their infraorbital opening is typically larger and more slit-like, and the skull tends to be narrower and more smoothly sloping, without the squirrel's sharp spurs over the eyes. Our story on the rat skull goes through rats, mice and voles in detail.

One more look-alike deserves a mention. If the skull has orange incisors but is much bigger, with long ear canals pointing upward underneath, it is worth comparing with a beaver skull.

Once you know it is a squirrel, two practical questions follow: whether you may keep it, and how to clean it without wrecking those delicate little bones.

> Legal note: This is general information for readers, not legal advice. Whether a skull may be moved or kept depends on the species, its protected status, who owns the land, and the state or country you are in. Nothing in this story says that a particular skull is legal to keep, so check with your state wildlife agency or the U.S. Fish and Wildlife Service, and with the landowner or land manager, first.

The rules are real and they differ. Alberta's found-dead-wildlife guidance tells people to contact Fish and Wildlife before keeping a skull and explains its permits, while the Wisconsin DNR sets out its own possession and sale rules for wild animal parts, including squirrel skulls. Our story on what to do when you have found an animal skull walks through the general questions to ask.

If you have the go-ahead, a small skull calls for a gentle hand. Wear disposable gloves and eye protection, and remove any remaining tissue carefully. Cold-water maceration, a long soak in cold water, is one of the gentler home methods. After that, a soak in soapy water lifts out grease, and a bath in 3% hydrogen peroxide can whiten the bone if you want it brighter. Two things to avoid: household chlorine bleach, and long boiling, which can crack bone, split the teeth, break the fine nasal bones and loosen the joints between skull bones. The Alaska Department of Fish and Game guide to skulls also stresses gentle handling of those thin nasal bones, and it notes that protected species can be illegal to possess. Let the skull dry completely before it goes on a shelf.

Our full guide to cleaning an animal skull covers each step, and how museums use dermestid beetles explains the method collections rely on for small, fragile skulls. The *Journal of Mammalogy* was already writing about them in 1933.

A few more questions come up once the skull is on the table.

If your skull is a squirrel, the rules in found an animal skull come first, then how to clean an animal skull once you know it is yours to keep. If it looks weathered and you are curious about its age, how old are these bones explains what the condition of bone can tell you. And if the skull is a little too narrow or a little too big to be a squirrel, the rat skull and groundhog skull stories are the next places to compare.

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