Rat Skull or Mouse Skull? How to Identify the Small Ones

A rat skull is a narrow rodent skull. At the front sit four prominent incisors, two above and two below, which grow throughout the animal's life and commonly look yellow-orange. Behind them comes the diastema, a long stretch of jaw with no teeth at all, because rats have no canines and no premolars to fill it. Then come the molars, three on each side, top and bottom. Four incisors plus twelve molars gives the 16-tooth total.

Size depends on the species and the animal's age. The Michigan State account gives the laboratory Norway rat, *Rattus norvegicus*, an average total length of nearly 400 mm and a body mass of 140 to 500 g; those are whole-animal figures, tail included, not skull measurements. For the skull itself, the University of Florida key to rodent skulls describes large rat skulls as generally longer than 25 mm. A young rat will measure smaller, which is why the ruler is a first clue rather than the last word.

If you are wondering why the incisors are colored at all, that question has its own answer in our story on the squirrel skull, which covers the 2024 science on orange rodent teeth.

These are the three small skulls most likely to turn up in a shed, a garden or an owl pellet, and they can look alike at a glance. Each one gives itself away in a different place.

Mouse against rat comes down to size first. In the UTEP key, under 20 mm suggests a house mouse and over 20 mm suggests a rat, and the rat skull is generally longer and more heavily built, with more pronounced ridges on the cranium.

If you have a rat, the ridges on top tell you which one. Look down on the skull at the two temporal ridges that run back along the top of the braincase. In the Norway rat they run relatively straight and parallel. In the black rat or roof rat, *Rattus rattus*, they bow outward and diverge. Chile's Ministerio del Medio Ambiente also describes the black rat skull as shorter and more oval than the Norway rat's, with curved temporal and occipital ridges.

Voles are the trickiest, because size alone will not separate them safely. Their cheek teeth are the giveaway: the enamel on a vole's molars forms distinctive folded, prismatic patterns. The University of Florida owl-pellet key separates voles using molar shape, the shape of the tooth bed, the edges of the eye socket and the ridges on the skull. For any of these, a complete skull or a full upper row of molars tells you far more than a single small fragment.

A rat and a squirrel both have ever-growing incisors, no canines and a diastema, so the front of the skull will not settle it. The cheek teeth will. Rat-like rodents lack premolars and have three molars on each side. Squirrels have premolar-like cheek teeth in front of their molars, and the number and arrangement of upper cheek teeth differ among squirrel species.

The University of Georgia Mammal Skull Key separates squirrels by total skull length, by where the eye socket sits relative to the cheek teeth, and by the number of upper molar-like teeth. If you want to see the differences side by side, the University of Illinois *Fieldbook of Illinois Mammals* illustrates the molar patterns of the Norway rat, meadow vole, house mouse and fox squirrel.

Orange incisors on their own never prove squirrel, since rats have colored incisors too. Our squirrel skull story covers the squirrel features in detail. And if the skull you are holding has a second, smaller pair of upper incisors tucked behind the first, it belongs with the rabbit skull instead.

Owls swallow many small prey animals whole, then bring up the parts they cannot digest, the bones, fur and feathers, as a compact pellet. The Texas Parks and Wildlife Department explains that the skulls, jaws and other bones inside a pellet show what the owl ate.

The National Park Service lesson "Owl Pellets from the Past", based at Tule Springs Fossil Beds National Monument in the Las Vegas Valley, Nevada, turns that into a classroom activity for grades 3 through 5. Students dissect pellets, sort the bones on black paper, examine the lower jaws and skull pieces, record what they find and compare the rodent skulls against a chart. The lesson ties the work to fossils: identifying a vole from a pellet uses the same habits of looking as identifying an animal from bones in the ground. Our story on how old are these bones picks up that thread for older finds.

The Maine Department of Inland Fisheries and Wildlife adds the practical steps: sort the bones, look closely at the teeth, and wash your hands afterward. A good pellet kit is simple. Gloves, a magnifier, a ruler and a skull chart for your region will do more than guessing, and they keep you from assuming every tiny skull is a mouse when it might be a vole or a young rat.

Owls also turn up beside skulls as a symbol, and our story on the owl and skull covers that side of the pairing.

A small skull from a shed, a garden bed or a pellet is easy to handle safely with a few habits. The CDC's guidance on cleaning up after rodents and its hantavirus prevention page set out the general steps. This is public-health guidance for everyday handling, not medical advice.

None of that needs special equipment, and it takes the guesswork out of picking up a find. A skull that came from a classroom pellet kit gets the same gloves and handwashing.

That depends on the animal and the place it came from.

> Legal note: Here you will find general information only, not legal advice. Rules on possessing wild animal parts vary by species, by the status of the land and by location, and they can change. Whether your particular skull may be kept is for your state wildlife agency or the U.S. Fish and Wildlife Service to answer, so ask them first.

As one example, the Wisconsin DNR treats clean skulls of unprotected animals differently from protected wildlife, and it requires permission to collect on private property. The Alaska Department of Fish and Game explains that possession rules vary by species, by agency and by location. A rat skull from your own shed and a vole skull from a state park may land in very different places in those rules. Our story on what to do when you have found an animal skull covers the general questions to ask.

If you have the go-ahead and want to clean it, our guide to cleaning an animal skull covers the steps, and how museums use dermestid beetles explains the method collections rely on for skulls this small.

A few more questions come up once the ruler comes out.

If your skull is a rat or a mouse, found an animal skull covers the rules, and how to clean an animal skull covers the next step once you have the go-ahead. If it is bigger and broader, with pointed spurs over the eye sockets, compare it with the squirrel skull; and if it is heavy and about three inches long, the groundhog skull is the one to check.

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