Animal Skull ID: Identifying Animal Skulls By Their Teeth
Animal Skull Dentition
In my previous post about animal skulls I provided you with some basic animal skull identification resources, but in this post I want to help you begin to narrow down what type of animal skull you might have found. The easiest way to start is by looking at the teeth of of the skull. If the teeth are present, this is easiest, though you can sometimes muddle through by looking at the skull if only the tooth sockets remain. It's also helpful if you have both jaws available,(upper and lower mandibles) though it's not required. Often one or the other is enough to help you.
Different Types of Teeth
If you feel around your mouth with your tongue, it becomes apparent that you don't have all the same types of teeth. This is true for most animals. Teeth are specialized to do different jobs, depending on the diet of the creature (if you ever want to see a really weird skull, check out the anteater, they don't have any teeth, just a long bone snout!)
Incisors (dark blue) are at the front of the mouth and are usually for scraping or biting, so they are scoop shaped and smaller. Canines (green) are usually for ripping or tearing meat, so they are long and pointed. Premolars (pink), are behind the canines, and can be flat for grinding, like in the mouth of beavers, or they can be sharp and serrated like in dogs and canines, for tearing meat. Molars (turquoise) also vary, depending on their use. Often they are for grinding food, like in humans, but in meat eating creatures they too may be serrated and have sharp edges for ripping and tearing meat.
The number, shape, and size of teeth can help you determine what type of animal skull you've found. Knowing where molars and premolars begin and end can be tricky. There's no hard and fast rule for telling them apart, and often they can look very similar. The other types of teeth are much easier. You'll have to practice skull identification, and looking at different types of teeth to get comfortable. For now, if it helps, focus mostly on the incisors, canines and molars (farthest back two pairs of teeth) if you're not sure.
The last tidbit you should know, is about bilateral symmetry. All mammals, like you and me, have bodies with mirror halves. The right side of our skeleton, and our external body, matches the left. This means, that when you study the teeth of a mammal you only need to study one side of the mouth, or count teeth on one side of the skull. The other half of the jaw is exactly the same (for dental formulas you then multiply by two). For example, in the image above, there are three incisors, one canine, four premolars, and two molars on one side. You would double that for a full tooth count.
Remember that animal skulls will not always have the same number of teeth in their upper and lower jaws. This is because each jaw may have a different function. In deer (and sheep, horses, etc.), the upper jaw has no incisors, but the lower jaw has a full set of incisors. This is because they use their lower jaw to "scoop" grass and leaves, and then the vegetation is passed to the back molars for grinding (this scooping action is what animal trackers look at to know the difference between the shearing cuts of rabbit teeth and the ragged scoop of deer teeth on vegetation).
Now that you know the basics, let's try to narrow down the type of animal you have.
- Carnivores- These are true meat-eating-only animals that have sharp teeth for ripping and tearing. Often their front canine teeth are elongated and sharp and their incisors are often small and reduced in size. Their back teeth, or molars and premolars are what we call "carnassial" meaning that they too are serrated and sharp, like the blades of a saw. This allows them to hold, rip, and tear meat from prey. For this classification I'm going to include insectivores like bats and some voles and moles and pescavores, or fish eaters. These creatures often have very wicked looking teeth, and sharp carnassials as well, just like the other carnivores.
- Herbivores- Herbivores are the plant eaters. For our purposes I'm going to include grass, plant, and grain eaters (granivores). They creatures usually lack carnassials, though not all. Some deer like, sitka deer, have carnassial fangs!
Most herbivores have very flat molars and premolars in the back of their mouths (like we do) and they usually have scoop-like incisors (upper and/or lower). In some cases, like that of rabbits, beaver and other rodents, they may have orangish curved teeth for incisors. In these creatures, the teeth are "indeterminate" and grow continually. This is why animals that have them (except for rabbits) are called "rodents." Rodere in Latin means to gnaw or chew. They must chew, or their teeth can grow so long that they can't open their mouths. I've seen a very tragic case of a rodent skull, where the incisors actually grew unchecked in a loop and then into the animal's cheek bone. The rodent could only eat from one side of its mouth which led to starvation, apparently the cause of its death.
This is an example of a rodent skull, with bright orange incisors (Photo: Mike Simpson, Flicker Sharing).
- Omnivores- Omnivores eat both meat and plant material. Their teeth are usually a combination of meat eating and plant eating teeth. A great example can be seen in the dentition of raccoons. In the image below you can see that they have both flat molars for grinding plant matter, and sharp canines for ripping and tearing meat. If your skull has a combination of teeth then you're probably looking at an omnivore.
DENTITION: COUNT IT OUT
Each type of animal has its own unique dental formula. These formulas can be used by biologists to help accurately identify skulls and to assist in categorizing animals into families and subgroups. In the resources I mention in my other post about good references for skull identification, you'll often find dental formulas listed for each type of animal. My favorite book, "The Wild Mammals of Missouri" has a great combination of pictures of animals, their skull, lower jaw, tracks, descriptions, and a dental formula.
A dental formula is quite simple, they just use letters to represent each type of tooth:
After each number you will find two numbers that look like a fraction. These aren't really fractions. The top number represents the number of a particular teeth in the top mandible (or mouth) and the bottom number represents the number of a particular type of teeth in the lower mandible or jaw. For example, here is the dentition of an adult human:
Adult Human: I - 2/2 C - 1/1 P - 2/2 M - 3/3 = 16 x 2 = 32 total teeth
Notice that the total number of teeth is counted and then multiplied by two. This is because of what I mentioned earlier, bilateral symmetry. You only need to count the teeth on one side of the animal's skull and jaw, and then double it to get a full count of teeth. It simply saves you some work.
This is the dental formula of an American beaver (Castor canadensis):
American Beaver: I-1/1 C- 0/0 P- 1/1 M- 3/3= 10 x2 = 20
TRY DENTAL FOMULAS YOURSELF
Try your hand at the dentition of this red fox skull. I'm going to try to stump you here, because this was a relatively young animal when it perished. Look at the lower jaw, you can see that one side has erupted teeth, and one side does not, just behind the canine. Assume that if it had lived, it would have had a complete set of erupted teeth.
First, is it a carnivore, herbivore, or omnivore?
I __/___ C __/___ P __/___ M___/___= ______ x2= _____ total teeth
You can search the Wildwood Tracking website for the dentition of specific animals of North America if you'd like to see if you're on the right track with your count and ID. Here's the answer if you're not sure: I 3/3, C1/1, PM 4/4, M 2/3 = 21 x 2 = 42.
The image above came from a great website, called Will's Skull Page. It has great online images of different types of animal skulls, and close ups of teeth.
Often times you'll find skulls that are missing their lower jaws, teeth have fallen out, or various and sundry things have happened to the skull, so the teeth are hard to count or may be missing. Here are a few examples. See if you can guess the dentition.
Ok, this opossum skull is rough. It has both upper and lower jaw, but many teeth are missing and the jaw image is from only one direction. Try the dentition yourself first, then scroll to the bottom of the "jaw" image to see if you're correct.
American Opossum Dentition: I 5/4 C 1/1 P 3/3 M 4/4 = 25 x 2 = 50
Here's a really challenging skull. It's a rabbit, specifically a jackrabbit, not a rodent. If you're unsure of the difference, check out my earlier blog post. Notice, some of the back molars are just erupting, and we only have the top mandible.
Because this is a rabbit, it has an additional set of teeth behind its long front teeth (unique to hares and rabbits, not found in rodents), so the count is: I 2 C 0 P 3 M 3= 8 total or 16 in the top jaw alone.
Ok, so there you have it, skulls and dentition at a glance. This only touches the tip of the iceberg for identifying skulls, but it's a great place to start, and can help you hone your naturalist skills.