A tale of three skulls

I am never quite sure what label to use in describing myself these days, but something I would never claim to be is a zoologist. Still, I did take some of those courses, and one of my aims when I first went to draw in the museum was to re-acquaint myself with skulls. Drawing is a great way of observing, and looking around the collection, I quickly fell in love with the bear specimens. Even among one species, however, (in this case Ursus americanus, the black bear) the variety is astounding, so the question became, which one?

In the end, I decided to make that variety part of the project, so I drew three: a cub and two larger animals, all of them males.

Ursus americanus skulls. Not to scale

Judging by on differences in tooth wear, bone texture, and the fusion of cranial sutures, one of the larger animals is older than the other, but that is as far as I am willing to take my interpretation. Telling the exact age an animal based on the skull alone requires more expertise than I have. Moreover, according to the specimen labels, these skulls represent at least two different subspecies: U. a. cinnamomum (top) and U.a. kermodei (middle). I am not sure about the one on the bottom.

There is a lot to take in here, and some of it is too small to see in this image. But to me, the most obvious differences are in overall skull proportions. Some of the details may reflect subspecies differences, others biomechanical constraints that come with changes in overall body size; I am not sure. But what strikes me is how the width of the zygomatic arches (the cheek bones) and the height of the sagittal crest (the bony ridge at the back of the skull) differ between specimens. Both of these features are important to the muscles that move the lower jaw, so does this mean the bigger the animal, the stronger its bite? Ecologically, this seems to make sense: cubs are suckled by their mother, and larger animals presumably will try to take down larger prey.

The point here is not to lecture about bears. As you may have worked out, I do not know much about them. It is that having looked at these skulls for so long, I now have all kinds of questions! I think they would make an excellent starting point for a class or lab exercise. There is something here for all ages, from the immediately relatable (bear cubs have baby teeth!) to questions about bone growth, skull mechanics, and feeding ecology and behaviour.

What is not to love?

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Sincere thanks to the Royal BC Museum for access to specimens, and to Danielle Rivet for setting me straight on some misconceptions about teeth. All errors are mine and mine alone.

Goswami A, Foley L, and Weisbecker V (2013). Patterns and implications of extensive heterochrony in carnivores cranial suture closure. Journal of Evolutionary Biology 26: 1294-1306.

Guskov V Yu (2014). Skull-Based Method of Age Determination for the Brown Bear Ursus arctos Linnaeus, 1758. Achievements in the Life Sciences 8: 137-141.

Lariviere s (2001). Ursus americanus. Mammalian Species 647: 1-11.