DENTITION: GENERAL DESCRIPTION
 
    Among the chief functions of the skull and teeth are grasping food (e.g., prey or vegetation) and ingesting it.  Consequently these structures are highly specialized for feeding and best understood from that perspective.  
 
    Miacids, the Miocene ancestors of all bears, were carnivores, with sharp carnassials and similar rear molars for shearing meat. Increasing herbivory among their descendents, the ursids, have modified dentition and skull structure.  (Ewer 1973; Kurte'n 1976; Kurte'n & Anderson 1980).  
 
    Carnivores have 4 major types of teeth: incisors, canines, premolars, and molars (Fig. 4:__).  To varying degrees, each type of tooth is specialized for serving different functions, including mastication of different kinds of food (Van Valkenburgh 1989:410):
Bones, meat, fruit, and insects differ in texture and hardness and thus are more efficiently fractured by teeth with different designs.  For example, meat can be considered a soft food that is much more readily comminuted by a bladed than a pointed tooth.  By contrast, bone is hard and brittle and broken more easily by conical teeth.  And plant material, although highly variable in texture, can be factured into small particles with a mortar-and-pestle tooth design (see Lucas 1979...).
Omnivores such as bears show high diversity between these 4 types of tooth.  
 
 
Incisors
    The incisors of a bear are typically small, suggesting that they are of far less importance than in ungulates and rodents, for example.  Nevertheless, some bears whose molars show little wear have worn their incisors down to stubs (Storer & Tevis 1955).  Perhaps they had to depend more than most conspecifics on grazing or on stripping bark from conifers to eat the cambium.
    In sloth bears, the medial pair of incisors has been lost, leaving a diastema through which the animal can suck or blow air (Schaller 1967??, Ewer 1973, Laurie & Seidensticker 1977, Sunquist 19__).
 
 
Canines
    The first Ursinae were skeletally similar to modern Asian and American black bears, and about the size of a modern sun bear, but with more slender canines (Fig. 4:__, Kurtn 1976).  Modern black bears and grizzly brown bears have stout canines.  These teeth are especially long and sharp in the predaceous polar bear  (Stirling & Guravich 1988).  The canines of Arctodus were also relatively large (Kurten 1967).
 
    Canines serve 4 major functions: display, grasping an "opponent" (enemy or prey), injurying or killing it, and dismembering the carcass (Van Valkenburgh 1989).  The canines of most felidae, for example, are specialized for slipping between vertebrate and cutting the spinal cord of prey.  Different prey select for somewhat different canine designs.  (Ewer 1973).
 
    Bears apparently lack a specific killing bite.  But they do kill prey either by biting or clubbing with a paw.  A grizzly bear can supposedly break the neck of a moose or bison with a single blow.  Indeed, they break live trees up to 12 or 15 cm diameter in this way.
    When bears fight one another, they commonly jaw-wrestle, which can break canines.  Grizzly and American black bears are occasionally found with a dangling canine, perhaps the result of damage from jaw wrestling.  And many show healing of jaw fractures which could have resulted either from jaw-wrestling or paw swats.  A primary strategy of fighting in animals, as in humans, is to disable the opponent's weapons.  (Geist, pp.___ in Herrero 1972, 19__).  Fighting has probably exerted stronger selection pressure on the design of canines in bears than in other carnivores that don't jaw wrestle and swat.
 
 
Premolars
    Ancestral carnivores had 4 sets of premolars.  The anterior premolars served as piercers and secondarily as slicers.  Just as the lower (mandibular) canines are anterior to upper (maxillary), so too lower premolars and molars lie partially ahead of upper ones, such that a molar of the jaw partially overlaps two molars of the skull and vice versa; the posterior upper premolar (P4) overlaps the lower (P4) as well as the first lower molar (M1).  P4 and the anterior portion of M1 (the trigonid) were especially modified for slicing meat, and are known as the carnassial teeth.  In some taxa, the posterior portion (the talonid) of M1 is adapted for grinding, as are the post-carnassial molars (M1-2 and M2-3).  (Van Valkenburgh 1989).
 
    Ursavus retained all 4 sets of premolars.  The anterior 3 sets were still adapted for piercing and shearing flesh.  But its carnassials had begun to flatten and broaden for chewing, a trend that continued over the next several million years among ursidae, increasing the proportion of surface area devoted to crushing and grinding vegetative food, relative to that for slicing meat.
 
    Such dental changes were apparent in Tremarctinae even with the Florida cave bear, whose anterior premolars were reduced and the molars enlarged.  (Kurten 1976).  Similar changes occurred among Ursinae.  In Ursus minimus/abstrusus, ancestor of modern black bears (Ch. I.V.A), all 4 sets of premolars were still present as functional shearing teeth.  The anterior 3 sets of premolars were smaller than those of Ursavus.  But the 4th premolar and molars were even larger, better adapted to grinding food.  
 
    The Etruscan bear also retained all its premolars, although they were very small.  In Savin's bear, all but the 4th premolars were vestigal or absent.  (Kurten 1976).  Among modern Ursinae, all anterior 3 sets of premolars are vestigal--a trait more advanced in the grizzly/brown bear than in the Asian and American black bears.  (Kurten & Anderson 1980).  One or more of the anterior 3 sets of premolars occur in some cubs, but are commonly lost as the bear matures (Storer & Tevis 1955).  The premolars most typically retained in adults are the 4th (carnassial) set.  Unlike other bears, the adult panda retains functional second and third premolars and sometimes the first; these and the molars are broadened and flattened for chewing bamboo which is tough and fibrous.
 
    Kurten (1967) reported that P4 and M1 of Arctodus are shaped to provide a carnassial shear.  That trait, along with the allegedly "trenchant" shape of M1, were major points supporting his suggestion that Arctodus was especially predaceous.  That suggestion is countered by the arguments of Emslie & Czaplewski (1985).  They maintain that M1 was not especially "trenchant", and that the carnassials were poorly adapted to shearing meat, tending to quickly wear flat with age, rather than remaining sharp through abrassion of the carnassials against one another.  They view M1 as having a superficial resemblance to the premolars of hyaenas, and thus better designed for crushing bone than for shearing meat.  In Crocuta and Hyaena, the premolars have lost some of their blade-like form, becoming more conical and robust for crushing larger bones than any other carnivore (Ewer 1973) (Fig. 4:__).  
 
    It would be revealing to learn whether the jaw action of Arctodus is more scissor-like than that of the typical bear, more like that of a predaceous canid or even a felid.  Available drawings and photographs (Kurten 1976, Emslie & Czaplewski 1985:Fig. 4, etc.) to not suffice for determining this.
 
 
Molars
 
    The high sectorial molars of early carnivores have been replaced by seco-bunodont teeth in bears and bunodont teeth in the giant panda (Sicher 1944).  Even ancient Ursavus had begun the long trend towards blunter and more massive molars with elaborate wrinkling of the enamel, for better mastication of fiberous vegetation.
 
    Such herbivorous dentition among ursids reached its zenith in the spectacled bear, the Florida and European cave bears, Indarctos, and the giant panda--although even the panda's teeth are not as well adapted for herbivory as the dentition of ungulates (Davis 1964, Kurten 1976).
    More omnivorous bears, such as the grizzly/brown, Asian black, and American black, have more generalized teeth, which are well suited for their diet of varied plants (most with low fiber content), animal prey, animal carrion, and insects.  Indeed, their molars resemble those of swine (Colbert 1955, 1980)--with whom those bears compete for soft fruit and nuts (mast) which have fallen to the ground, as well as for roots, bulbs, corms, tubers, carrion, and other rich foods.  
 
    Hecht (1963) describes the evolution of cheek teeth in the polar bear as a shift from the heterodont form of other ursidae to a homodont form similar to those of other aquatic carnivores.  Molar cross-sectional area has been reduced; cusps have been elevated and sharpened into a more carnassial-like form (Kurtn 1964, 1966; Kurtn & Anderson 1980).  The direction of dental change in the polar bear is definite, but not far advanced.  Lack of more thorough divergence from the brown bear norm--more specialization for feeding on pinnipeds--may simply be a consequence of the recent (Pleistocene) colonization of the sea ice habitat.  But it might also reflect pressures to retain generalized dentition, as has happened to a lesser degree in canids (Ewer 1973).  When polar bears are forced ashore during summer and cannot prey on seals, they have an omnivorous diet somewhat like that of a barren-ground grizzly (Perry 1966??); but in general, they fast (Bruemmer 19__; Taylor, pers. comm.).  So it is not obvious how omnivory could exert enough selection pressure to retain omnivore teeth.  The dentition of Arctodus, a fully terrestrial species, is more likely to have been under competing selection pressures by predation vs. herbivory.
 
 
DENTITION:  QUANTITATIVE COMPARISON AMONG TAXA
    The above comparison of dental traits across species of bears and other taxa are more subjective than one would prefer.  To introduce more objectivity and quantification, and to derive further insight about carnivore guilds, Van Valkenburgh (1989) compared several dental traits among 47 species of extant carnivores, including 5 bears, with 1 adult of each sex.  These measurements consisted of (1) upper canine shape (CS: the ratio of width to length at the dentine-enamel junction); (2) premolar shape (PMS: maximum width to maximum length); (3) relative premolar size (RPS: maximum width of the largest premolar divided by the cube root of body weight); (4) relative blade length (RBL: length of the carnassial blade--trigonid--of M1 to total length of the tooth); and (5) relative grinding area (RGA: square root of total grinding area of lower molars [M1-3] divided by length of the trigonid).  The figures for bears are given in Table 4:__).
 
Upper Canine
     For carnivores in general, canine shape depends less on diet than on prey-killing behavior.  Species like hyenas that crush bones with their teeth also have robust canines with a round cross-section (CS=0.71).  The same is true for species such as felids that kill prey by biting between vertebrae (CS=0.74).  Species like canids that kill by biting soft tissue have canines which tend to be more flattened side-to-side (CS=0.70).  (Van Valkenburgh & Ruff 1987).  
    Canine shape in polar and brown bears is as round as in bone-crushing specialists and typical canids; polar bears are eat mainly meat, whereas grizzly bears do so only occasionally (Ch. 3:__).  Canine shape is narrower in the more herbivorous ursids.
 
Largest Lower Premolar
    Across all 47 species of carnivore, the premolar is most conical in predators that crush bone, such as hyenas; premolar width is greatest relative to premolar length (PMS=0.65) and to body weight (RPS=3.79).  The premolar is bladelike (PMS=0.48-0.53) and relatively large (RPS=2.14-2.18) in more specialized predators such as felids and canids.  A blade shape is most efficient for slicing meat, but more fragile than a conical shape for crushing bone.  Crushing bone with premolars, rather than postcarnassial molars, which can exert more pressure, may relate to the wider gape of the japs at the premolars.  More omnivorous species such as most ursids and some procyonids have premolars of intermediate shape (PMS=0.60) and width relative to body weight (RPS=1.57).  Indeed, premolars of bears, especially polar bears, are the narrowest relative to body size of any of the carnivores examined (Table 4:_).  The function of especially narrow premolars is unclear.  (Valkenburgh 1989).
    For the predatory polar bear, premolar width relative to body size is unusually narrow; but premolar width relative to premolar length is comparable to that of American and Asian black bears, and progressively less blade-like in brown and spectacled bears.
 
Relative Length of the Trigonid Blade
    Carnassial blade length is greatest in felids (near 1.0), and progressively shorter in more omnivorous species--0.85 for those that also crush bone (e.g., hyenas); 0.61 for most canids and other predators that are eat more meat than vegetation; and 0.55 for omnivores that only occasionally eat meat.  Blade length is particularly short for bears, including the polar bear, but especially more omnivorous species.  (Van Valkenburgh 1989:Table 15.3).
 
Relative Grinding Area
    Predominantly carnivorous felids have no grinding area on the carnassial molar and have lost other the rear molars.  The index for bone-eating hyenas averages 0.24--all on the talonid, since the postcarnassial molars have been lost. Viverrids and canids have a grinding area index near 1.00.  The index increases for omnivorous mustellids and procyonids (roughly 1.0-1.5), and is highest for ursids (1.83-2.23).  Note that the index is lower in the polar bear than in any other ursid; it is highest in the brown bear.  
*          *          *
    Van Valkenburgh's dental analysis expands previous insights on predator guilds, and to a lesser extent about ways in which dentition is related to the primary foods each species eats.  It is most useful for comparison across broad groups of species organized by diet, but less useful for comparing within a taxon such as ursidae.  For example, it reveals little difference in grinding area of molars between polar bears vs. Asian black bears, than between Asian black bears vs. American black and brown bears, with spectacled bears intermediate.  This contrasts sharply with other information indicating that the spectacled bear is the most herbivorous species considered, but should be closer to other omnivorous ursids than they are to polar bear.  Clearly, to fully distinguish among these species on the basis of diet will require consideration of more traits, perhaps including additional dental characteristics (e.g., heterodont vs. homodont molars) and skull traits.  But this is an important first step towards deriving correlates that will enable us to better understand the diets of extinct ursidae.  Multivariate analyses of these traits, for instance factor analysis, discriminant analysis, and canonical correlation, would be especially welcome.
 
Table 4: .  Dental traits and body size for 5 bear species.1
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Trophic Niche             Log body         _      Dental indicies     ___          
& Species                    weight       CS     RPS     PMS    RBL     RGA
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Carnivore
 Polar bear                    2.57       0.713    0.94     0.55     0.52      1.83
 
Omnivore
 Brown bear                  2.42      0.724    1.25      0.59     0.48      2.23  
 American black           2.18      0.662    1.06     0.56     0.48      2.17
 Asian black                     2.00      0.632    1.37     0.56     0.52      1.96
 Spectacled bear                2.13      0.633    1.01    0.64    0.52      2.06
  Mean                            2.18      0.663    1.17     0.59     0.50      2.10
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  1 Data from Van Valkenburgh (1989:Table 15.1).
 
 
 
 
Chapter 13. SKULL & DENTITION
The most revealing bones of all are those making up an animal’s skull. Each cheekbone (zygomatic arch) begins near an eye socket (orbital) and arches out away from the brain case (cranium) before anchoring near an ear. Passing through that arch from the temple to the jaw (mandible) is the large temporalis muscle which powers cutting, crushing and grinding actions by the teeth. The bigger that arch, the larger the temporalis muscle it formerly contained. So the harder it could bite or the tougher the food it could chew. Other clues to chewing power and diet are sizes of flanges on the skull. Bigger muscles need more surface area for anchoring to the skull. Size of the brain case is very limited; so additional skull surface area is added by growing bigger flanges. The narrow flange running front-to-back across the top of the skull is called the sagital crest. Although humans lack such a crest, most large mammals, including gorillas and bears have one. There is also an occipital crest where the neck joins the skull. Figure 0.1. Diagram of skull with features labeled, including jaw muscles and hinge A third clue to biting/chewing power and to diet is hinging of the jaw. If the hinge is more-or-less level with the surface of the lower teeth, the jaw closes first at the back, then towards the front, much the way scissors do, and for the same reason: slicing power. Any point along there length where two blades of scissors come together is where most of the slicing power is concentrated; same for slicing by cheek teeth and biting by the canines. In contrast, if the hinge is well above the lower teeth, the top and bottom rows of teeth close together all at once, like pliers. And, like pliers, they are better at crushing than at slicing. Tooth structure also offers clues. Strict meat-eaters like felids (cats) have scissors-like cheek teeth, with almost no flat surface for crushing food. Strict vegetarians like moose have crushing-grinding cheek-teeth with no slicing surface. The cheek teeth of a canid, such as a wolf, have a small crushing surface for eating occasional vegetation and berries. The earliest ancestors of bears also had cheek teeth designed mainly for slicing meat. Over the millennia, however, their molars (like our own) became increasingly bigger and flatter for crushing most of their food. Even their premolars (which lie between the molars and the canines) lost their meat-slicing shape or shrank to small pegs that may be useless – something that varies among species, even in the polar bear, which has reverted to a predatory diet. Whereas a canid chews meat off a carcass mainly with its premolars, even a polar bear bear does so almost entirely with its big sharp incisors. Limb bones also provide important clues to extinct species. The speed with which a 4-footed mammal can run depends on flexibility of its spine, the length of its legs, and the ratio of its upper to lower limb bones. High speed is promoted by a very flexible spine and long legs, where the upper arm and thigh (humerus and femur) are much shorter than the forearm and lower leg. Even if such anatomical details still seem bone dry to you, have patience. As I describe and compare the eight surviving species of bear and their extinct ancestors, you’ll quickly see how these clues help you recognize what’s rare about a species and how this adds to your enjoyment of coming to know each of them.