Rodents and Squirrels

The best way to learn something is to teach it.  This week, I had to teach a lesson on squirrels…

Forty percent of all the mammals worldwide are in the Order Rodentia or Rodents, over 2,000 species.

To be a rodent, you must have four incisors in front – two top and two bottom – which never stop growing throughout your life.  As you gnaw on things, they wear down, so the constant growth is necessary.  These incisors have a hard enamel on the front with softer material behind which makes them wear unevenly resulting in sharp, chisel-like edges.  The faces of your front teeth are yellow or orange.

Porcupine Skull Replica
Porcupines are rodents, but they aren’t squirrels.

Behind the incisors there is a gap with no teeth at all.  The gap is called the diastema.  In back you have “cheek teeth”.  The bones and muscles in your jaw are arranged in such a way that you can either gnaw with your incisors, or chew with your cheek teeth… but you can’t do both at the same time.

Of all the rodents worldwide, a little over 10% are in the Family Sciuridae – Squirrels – around 270 species.

If you were a squirrel, you would have 4 cheek teeth on each side of the top of your jaw and 4 or 5 on each side of the bottom jaw, depending on species.

Chipmunk SkullYou would also have a well-defined post-orbital process.  Hunh?  What’s that, you say?  Well, on the top of your skull, just behind each eye, there is a slender, sometimes pointy projection from the frontal bone and skull-studying nature nerds (and I use that phrase with the utmost affection) call it a post-orbital process.

Woodchuck Skull Replica
Woodchuck and chipmunk skulls both have post-orbital processes.

There.  Now you know how to tell if you are a rodent or not, and specifically if you are a squirrel or not.

Learn more:

The information for this post comes mostly from Allen Kurta’s excellent book Mammals of the Great Lakes Region, available from the University of Michigan Press.  There are 10 species of squirrels in his book, but only 7 of them live where I live.

Over the next few days, I’ll tell you about “my” squirrels…

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12 thoughts on “Rodents and Squirrels

  1. I loved this lesson! I agree that to learn, teach. I am presenting outreach nature programs to after school daycare programs in my area for the museum of natural history and I present a lot of different skulls. I truly appreciated the extra knowledge I learned here today. Thanks!

  2. Is it “whoopee” time for grey squirrels? The 2 in my yard spend most of their time chasing each other around the trees! I saw a Fox squirrel today for the first time this winter.
    Waiting for more “squirrely” posts.

  3. @Cis and Karen – You’re welcome!

    @Linda – Well… For the Gray Squirrel, Kurta doesn’t specify when “Whoopee” time is, however, by doing a little math we can figure it out. Gestation time is 44 days. The first of two litters is born in March or April… Yup! It’s Whoopee Time!

  4. I love this lesson too… in all your research, did it come up with a reason for the post-orbital process? A pointy thing near my eye seems dangerous to me 🙂

  5. Well, it took me a while to get around to digging my mammalogy texts out, but I finally did and had a look through for any clue as to the purpose of the post-orbital process. In fact, the texts didn’t have much to say about it at all, other than noting it was there and a distinguishing feature.

    My guess is that it has something to do with muscle attachment or structural support for the eyeballs or jaw hinge or something, and the fact that it’s pointed is just kind of incidental.

    Perhaps the key to its purpose is figuring out what squirrels do with their heads that would be different from other rodents. For instance, beavers would need big masseter muscles for power in the jaw when chewing down trees. Big eyeballs usually need an extra amount of bony support. Beyond that, though, I really don’t know. 🙂

  6. Well dang! Thanks so much for looking though!!!

    Maybe it’s something that they don’t do with their heads or eyes, compared to other rodents and mammals since it is pointier than others. Something tells me it’s too difficult of a doctoral thesis subject for anyone to have attempted 🙂

  7. woot, squirrels, i was taking anthro 200 this quater, and we just learned about dental formulae, and this squirrel wont leave my window alone, gonna have to stop giving him food.

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