16 September 2008

Squirrel teeth marks on bone

One day last week I let my cat out into the backyard and a few seconds later followed her. As soon as I stepped on the deck, I heard the chirp-like sounds eastern gray squirrels (Sciurus carolinensis) seem to make when they spot a predator, in this case, probably the cat. I looked up at the nearby pine tree and saw a squirrel jumping from one branch to another and, at the same time, noticed a large white object falling to the ground.

It turned out to be the pelvic bone of a deer. No, the squirrel hadn't dragged a deer up the tree for dinner; it was one of the many deer bones that we had once collected in the nearby woods and now are scattered throughout our yard.


The close examination of the bone solved a mystery that I had first written about in this post: who's been gnawing on our deer bones? The pelvic bone had numerous distinct scratch marks along its edges. These are probably the squirrel's teeth marks.


So the squirrels are the bone eaters. But, why do they do that? A quick look at The Encyclopedia of Mammals (David Macdonald, ed., 1984) suggested one possible answer. As in other rodents (for example, beavers), squirrels' incisor teeth grow continuously. So they have to keep chewing on hard objects, nuts and the like, to wear their teeth down lest they get too long to fit into their mouths.

Another possibility that I can think of is that squirrels satisfy their calcium requirement by gnawing on bones.

Here is another detail from the same bone.



Irradiatus said...

My dad told me when I was a kid that squirrels eat bones to get their calcium (and more importantly to him, they eat deer antlers. He would get so pissed when his stash of antlers in his toolshop would get raided. This is why when you come across shed antlers in the woods - chances are that it's been chewed. What can I say, I grew up with rednecks.)

My zoology teacher in high school told me the same and mentioned the tooth problem as well.

Now that I work with mice, I can definitely say that rodents must constantly chew. Otherwise, their teeth overgrow their mouth (occlusion) and they starve.

I've never actually verified the calcium thing though. But it makes sense. I'm not sure about the calcium content of nuts and such, but it intuitively seems like it would be small.

Anonymous said...

Interesting. Your pictures appear to favor the tooth maintenance theory.

If they were eating the bone for nutrition, you would expect that they would eat the spongy bone, which is much easier to chew.

On the other hand, if they were chewing for the sake of chewing, they might prefer the much harder dense bone.

That is exactly what we see in you pictures. The squirrels chewed the dense bone, ignoring or even avoiding the spongy parts.

That's not to say that they are not simultaneously getting calcium, or some other nutrient as a side benefit.

I wouldn't think squirrels would be calcium deficient anyway. Nuts contain enough calcium that they can be a significant source for humans, and relative to their body weight squirrels eat a lot of nuts.


Squirrels eat mostly acorns. I couldn't find reliable-as opposed to anectodal-information on the calcium content of acorns on the Internet.

Eric Heupel said...

That is quite cool.
Hmmm.. I wonder if you could train a squirrel to notch the shells for you. ;)


It would be great if I could train a squirrel to FIND shells for me!

Bailey said...

This evening I took a short walk in the woods behind our house. I noticed a Gray Squirrel gnawing on something that made a peculiar sound, kind of a high pitched squeaking. When I looked closely, it appeared to be a bone. I'd never heard of squirrels gnawing on bones, so I was quite surprised. I sat and watched, and it kept gnawing. When I tried to get closer to verify if it were actually a bone, the squirrel ran, holding the bone in it's mouth. It made a wide circle, and as I watched it hop from one fallen tree to another, it stopped behind a limb, and when it emerged, the bone was missing. I went to place where the squirrel had been, and after a minutes of searching, I found the bone buried under some leaves. It looked like a thigh bone of a small mammal, and there were several chewing marks on it similar to the photos posted here. It also seemed to have avoided any spongy areas.

forestwalk/laura k said...

i was just googling 'bone collecting'...as this is one of my passions...and this OLD post came up...hopefully this blog is still good...and you still post! i have some cow skulls i've collected...and have them in my gardens...and YES!! the squirrels gnaw on them! i have even been lucky enough to get a picture of a squirrel sitting atop one of the skulls!! anyway...interesting post! and i just had to comment! now i'll check out the rest of your blog! thanks!