Differences among bears

This page focuses on differences in skulls and dentition among the various bear species. For a broader view that places bears in context of other carnivores see the front page on Skull and dentition. As described there, bears, as a group, tend to have skulls and teeth that match a generalized often omnivorous diet, which means no features are particularly extreme. The one exception is the giant panda, for reasons that are elaborated here. Like virtually all carnivores and primates, bears have forward-looking eyes that give them the kind of stereoscopic vision that facilitates grasping and manipulating things--which matches forelimbs that engender remarkable dexterity (see Forelimbs). They also have molars that are, for a carniviore, well-adapted to grinding vegetation--the kind of molars that are described as "bunodont." Plus, they have the robust canines (in their case, moderately so) common to all species of carnivorous descent, which helps them grasp and subdue prey when that is the order of business. They also have a moderately elongate snout, which hosts modestly elaborated turbinals, as well as a gap in the teeth (called a diastema) that facilitates the manipulation of food within the mouth. As a final feature, they have a comparatively broad skull featuring flared zygomatic arches that encompass moderately robust temporal muscles. Meaning that, for their size, they can deliver a modestly powerful bite (although, as elaborated under Skull and dentition, their shear size means that their bite is extremely powerful in absolute terms). So, the through thread (by and large) is "moderately so," barring the panda. Now for the details.

The figures at left are the work of Borja Figueirido, who has devoted much of his academic career to plumbing the depths of carnivore morphology. The top graphic plots individuals of the various bear species relative to dominant trends in cranial morphology (topmost graph) and jaw morphology (bottom left graph). Different symbols represent individuals of different species. The "RW" along each of the axes denotes "Relative Warp," which is too arcane to explain in exact terms, but reflective of basic differences in morphological configuration. The skull and jaw shapes shown around the periphery of each figure give a profile of what variation along each axis denotes. Perhaps most germane to the results shown at left: the black dots represent giant pandas, the brown dots brown (or grizzly) bears (U. arctos). At the opposite extreme, the white squares represent polar bears and the open circles, sloth bears (Ursus ursinus, the ant-eating bears of southeast Asia).

 

The take-away points from the two graphics at left are, first, that giant pandas are an extreme case; second, that polar bears and sloth bears (the specialists on vertebrates and invertebrates, respectively) stake out the other extreme; and, third, that brown bears occupy the morphologic middle ground. Put another way, bear species with skulls and jaws that are built thick, robust, and tall and broad relative to length are adapted to an herbivous diet--exemplified by the bamboo-eating panda. Bear species with skulls that are low and relatively elongate, coupled with a shallow angle to the bottom of the jaw, tend to eat animals; vertebrates in the case of the larger polar bear, and invertebrates in the case of the smaller sloth bear. The shape of the brown bear skull and jaw places this species on the borderline between omnivory and carnivory, which matches the observed variation in brown bear diets worldwide ranging from largely meat in some places to largely vegetation (including berries) in others (see Nutrition). 

 

 

The graphic at right provides a synoptic photographic view of (from left to right) a top (or planar) view of one-half of the jaw, featuring the molars; a side view of the jaw; a side view of a molar; and a side (left) and front (right) view of a top canine--this for each one of the extant bear species. The red and orange arrows superimposed on the canines emphasize characteristic differences in curvature (side view) or the extent of displacement outward from the main axis of the snout (front view). These graphics are adapted from papers by Per Christiansen (canines) and Tyson Sacco and Blaire Van Valkenburgh (jaw and molars), who have contributed substantially to our understanding of variation in dentiary adaptations among the bear species.

 

There are several take-away points from this graphic. Again, the robustness of the jaw, molars, and canines of giant pandas is striking. Pandas clearly have proportionally the largest molars of any bear, combined with less extreme cusps on the molars--all obviously adapted to crushing bamboo. At the other extreme, polar bears and sloth bears (U. martimus and U. ursinus, repectively) have molars that are proportionately quite small. Polar bears are further distinguished by having molars with tall cusps (much like a carnassial) and canines that are comparatively straight and aligned with the strong axis of the snout--all, no doubt, faciliating the acquisition (stabbing) and consumption (shearing) of its vertebrate diet. By contrast, the ant-eationg sloth bear has by far the smallest molars, along with curved somewhat splayed canines--all the better to process a diet of invertebrates. All of the other bears, including brown bears (U. arctos), fall somewhere in between, although brown bears share the stabbing configuration of their canines with the meat-eating polar bear. (Parenthetically, the closely-related Asian bear species [U. thibetanus, U. malayanus, U. ursinus; see Evolution] all feature somwhat splayed canines.)

Another take on dental differences is provided by the graphic to the left (data thanks to Tyco Sacco and Blaire Van Valkenburgh). The top surface area of the two largest (rearmost) molars (y-axis) is plotted against the total length of the jaw, from hinge to canine. This indicates the robustness of molars relative to size of the jaw. Each dot represents an individual bear: golden dots, brown and polar bears (U. arctos & U. maritimus); black dots, pandas (A. melanoleuca); white dots, sloth bears (U. ursinus); and gray dots, all other bear species.

 

This plot clarifies just how robust the molars of the bamboo-eating panda are, especially in contrast to those of the insect-eating sloth bear. All other bear species fall along the mid-line of the relationship, suggesting that, in a relative sense, none have any pronounced differences in molar size that might be associated with a specialized diet.  

 

 

As a supplementary tid-bit, the illustration above left shows the whole teeth, including roots, of various bear species (courtesy of Kornelius Kupczik). The take-away from this is the massive roots of the giant panda's (Ailuropoda melanoleuca) molars and the proportionately massive canines of the polar bear (U. maritimus). These distinctions yet again emphasize adaptations of the panda to a diet of bamboo and adaptations of the polar bear to a predatory life-style (i.e., large canines "designed" for stabbing and holding prey).

 

Adaptations to diet can also be found even at the finest morphologic resolutions. For example, the graphic above right shows differences in patterns of tooth enamel among bear species, referenced to a phylogenetic tree (this thanks to the work of Clara Stefen). These enamel patterns are called Hunter-Schreger-Bands (HSBs), which vary in terms of their waviness and angulation (each is illustrated by the inset to the right of a species). As noted in the graphic, specific patterns are identified with specific diets--"durophagy" referencing a diet typified by foods that need to be crushed. Such is the case for pandas as well as the extinct giant short-faced bear (Arctodus simus), consistent with speculations that this species did a lot of scavenging and, hence, bone crushing.