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…