A Key-Guide to Mammal Skulls and Lower Jaws: A Nontechnical Introduction for Beginners

1. What they did

People walking in fields, mountains or along beaches often find the bones of a dead mammal and want to know what it was. Aryan Roest, of the Biological Sciences Department at California Polytechnic State University, San Luis Obispo, wrote this booklet to answer that question for outdoor people, teachers, students, wildlife biologists and game wardens. It is not meant for specialists who need exact identifications of small mammal skulls.

The booklet contains:

• How to use the keys. A key is a series of numbered steps, each offering two or more contrasting choices (a, b, c or d). Following the right choice at each step leads to a name. A worked example takes a skull about 75 mm (3 inches) long, with canines, three incisors on each side and six cheek teeth, through the key to a raccoon.
• A glossary of skull and jaw terms, from alveolus (tooth socket) to zygomatic arch (cheekbone), with a labelled raccoon skull.
• A key to skulls (49 steps), starting with skull length and then using features such as whether the eye socket is closed at the back by a bony bar, the number of incisors, canines and cheek teeth, the size and position of the infraorbital foramen (the hole in the side of the upper jaw), nasal bones, the lacrimal bone and pit, horn cores and antler bases, crests, and ear bones.
• A key to lower jaws (39 steps), using jaw length, the toothless gap in front of the cheek teeth, canine size, the shape and height of the coronoid process, the condyle and the angle of the jaw.
• Scientific names for every common name used in the keys.
• Cleaning skulls: brushing, simmering, sun bleaching, a weak hydrogen peroxide soak, rotting in water, and carrion beetles (dermestids) or meal worms.

Drawings show key features of the skulls and jaws of horse, cow, elk, deer, coyote, foxes, squirrels, pocket gopher, kangaroo rat, muskrat, house and harvest mice, shrew, bat, jackrabbit and beaver. They are not drawn to the same scale.

2. What they found

How to measure and count

• Measurements are taken with the cheek teeth in a nearly horizontal line. Skull length runs from the front of the incisors or premaxilla (or nasals) to the back of the skull; width is measured across the cheekbones. 25 mm equals 1 inch.
• Teeth are counted on one side only, so “4 cheek teeth” means 8 in all. Counts in the skull and lower jaw may differ.
• Skulls of young mammals may be too small to key out correctly.

Broken or incomplete remains

• Skulls are symmetrical, so a part broken on one side may be intact on the other.
• Missing teeth leave holes (alveoli) in the bone, but a cheek tooth can have up to five roots, so counting needs care. Very tiny teeth are easily missed.
• In hoofed mammals, the front bones of the upper jaw (premaxillas) often break off, which affects skull length; allow for the missing bone.

Getting a reliable name

• Keying both the skull and the lower jaw to the same species gives a check on the identification. Lower jaws have fewer distinguishing parts, so jaw-only identifications are less certain.
• Some small rodents are particularly hard to identify; treat them as tentative until checked with more formal keys.
• Many names in the keys cover several species. Range maps in field guides can narrow them down: of the woodrats, only four species occur in California, and north of San Francisco Bay a woodrat skull can only be a dusky-footed woodrat.
• Some breeds of domestic dog will not key out at all, though they can usually be recognized as carnivores.

Cleaning skulls

• Small skulls can be dropped into boiling water for about five minutes. Large skulls should start in cool water with a little detergent and be warmed to a simmer only, because a sudden temperature change can split large teeth. The author advises doing this outside, not in the kitchen.
• Flies should be kept off fresh skulls: maggots clean the bone but may stain it. Commercial bleaches can leave a white crust.

3. What they concluded

• The author states that the keys, revised many times as students ran into problems, now “work fairly well” for most wild and domestic mammals of the United States and southern Canada.
• They help beginners learn to recognize mammal skulls at a glance, but cannot separate many closely related small mammals to species; for those, the author points readers to more formal keys and field guides.

4. How tracking was or was not used

This is an identification guide, not a field study. It deals with one kind of sign: the skulls and lower jaws of dead mammals found in the field, including broken and incomplete remains.

6. Other technologies, skills or innovations

• Plain-language keys with few technical terms, every term explained in a glossary, and size thresholds given in both millimetres and inches.
• A small flexible plastic ruler is suggested for measuring; a scale is printed on the back cover.