The Skull Biomechanics of the Polar Bear

Skull of a Polar bear. Image taken from http://www.marinebio.net/marinescience/04benthon/arcplrbr.htm

Building on from the last article I looked at in association with the polar bear I came across this one by Slater et al. (2010). Again the animals jaw is examined in this paper but focuses on the craniodental adaptations that have evolved in relation to an exclusive carnivorous diet.

The polar bear evolved fairly recently, about 150-700 kya, from coastal populations of the brown bear. Over this time the polar bear’s molars have reduced their surface area and the skull is much lower and slender compared to a brown bear. By examining the cranium of the polar bear and taking a biomechanical approach  some evolutionary adaptations can be seen when compared to a brown bear.

The study found that there is a similar amount of bone in polar and brown bear skulls and both bears produce a similar bite force. However, although the bite forces may be similar the stress distributions were different and this was most marked in the molars; with polar bears producing a larger strain value. This indicates that a polar bear’s skull goes under deformation in order to produce a similar bite force and suggests that a polar bear’s skull is weaker and less efficient making it less suited to dealing with high masticatory loads.

Although a polar bears skull may appear weaker it is likely that due to its diet less reinforcement of the skull is needed. This is because the prey of a polar bear is much smaller and requires less force to bring it down. This is in contrast to other carnivores, such as lions, whose prey is generally larger than them therefore needing a stronger, robust skull. The diet of the polar bear also provides some answers for why their premolars and molars have reduced in size and why they don’t have blade like canines like other hunters. A polar bear usually consumes the blubber and flesh of seals which require little processing before it is swallowed, unlike bone. This means that there is no selective advantage for maintaining large molars and therefore produces a decrease in size. In contrast the brown bear is omnivorous and consumes a lots of plant material which needs to be broken down before swallowing and therefore requires larger molars.

Comparison of a polar and brown skull. Images taken from http://nature.ca/education/cls/lp/lppbskl_e.cfm

In addition to their dentition a difference can be seen in the polar bear’s skull shape when compared to a brown bear. A polar bear has a much flatter and lower skull and the eye orbits are more elevated. This shape, along with denser hind and fore limbs, is consistent with semi-aquatic and faunivorous behaviours. All of these adaptations make the polar bear a successful hunter in a harsh and difficult environment. They are also a good example of how animals evolve in response to their environment and the surrounding ecology.

As well as the suitability to their environment the polar bear can be used as a prime example of evolution in action and that becoming bigger and more robust isn’t always what is needed. As large molars and an extremely strong skull, compared to other carnivores, are not any more beneficial to the bear these attributes have decreased. These reductions do not have an impact on the hunting success of the bear but will potentially save energy in terms of maintaining and growing the bone in the first place.

For the full article click here. The citation in full is:
Slater, G. J., B. Figueirido, et al. (2010). “Biomechanical Consequences of Rapid Evolution in the Polar Bear Lineage.” PLoS ONE 5(11): e13870.