Body size in bears is highly variable over paleontological history and geography within a species, as well as among species.  Paleontological and interspecific differences are reviewed here.  Geographical variations within extant species, and factors governing body size, are introduced here but discussed at length in Chapter 9.  [Regarding Shane Mahoney's "giant" black bear on New Foundland, consider remarks by Pielou on p. 9--scarcity of small mammals as prey which are abundant on the adjacent mainland, but abundance of large mammals (hares to caribou), in addition to the island situation and maritime climate, including salmon].

 

 

    Parallel variations in size of cave bears are documented by 2 lines of evidence (Kurte'n 1976).  First, aging of strata in which bears are found indicate large size (Deninger's cave bear) during the Elster glaciation (about 500,000 to 600,000 years ago), smaller size during the Holsteinian interglacial, a slight increase during the Saale glaciation, small size in the Eemian interglacial, followed by peak size for cave bears during the Weichselian glaciation.  Second, for late Pleistocene cave bears, body size tends to be inversely related to the altitude at which caves occur.  Bears tend to be smaller in caves above 1000m than in lower caves.  The higher caves were probably covered with ice and inaccessible to bears during glaciations.  Although the lower caves also contain small bears, these are in older strata.  

    Kurte'n (1976) also noted size differences among what seem to be contemporary populations.  Cave bears in Britain and Belgium were much larger than those in most of continental Europe.  

 

    The foregoing arguments assume that variation within a species over millenia and space can be reliably distinguished from short-term variation due to nutritional status, pathogens, and other factors.  They also assume reliable discrimination of male vs. female specimens, and adults vs. immatures.  Small sample sizes, along with lack of definitive criteria for sex and age, warrant caution, for reasons especially obvious when one views data on extant bears (Section II.B.2, Ch. 9:__).  

    Because body size varies with ontogney, comparisons are best made between animals of comparable maturity.  Age itself may be a poor indicator, since some individuals or populations grow and mature faster than others.  Reasonable standardization can be achieved by basing comparisons on only fully mature adults; their skulls are distinguished by complete closure and perhaps obliteration of main sutures (Manning 19__; Rausch 1961, 1963).  For example, when Rausch compared condylobasal length--the distance from the prosthion to the level of the posterior surfaces of the occipital condyles--for male bears, he considered only those mature enough that the SL had been maximized, as determined by complete closure, calcification, and obliteration of the basiooccipital-basiosphenoid suture.  Adults may also be distinguished by heavy wear on permanent teeth (Kurte'n 1976).  Extant bears are commonly aged by counting annual rings within teeth, comparable to the rings of a tree.  Perhaps this could be done with fossil teeth too.

    Bone size also varies with sex, requiring that this variable be standardized too before comparisons are made between species, populations, or time periods.  There is too much overlap in skull sizes of adult males vs. females for skull size alone to identify sex.  But sizes of the canine teeth are maximized long before adulthood; and canine size has strong sexual dimorphism, with few intermediates.  This has been demonstrated in modern brown bears; similar dimorphism is found for cave bear fossils (Koby 1949; Kurte'n 1976).  Thus, comparisons between areas and times for cave bears, based on skulls with closed sutures and caines, are likely to be reliable.  Indeed, Kurte'n (1976:71-72) claims that:

Size differences in canines correspond to size differences in rest of the skeleton, except for the cheek teeth.  (p. 71)  "In effect, this means that, as soon as we have established the typical male and female dimensions for a given bear population, practically every [adult?] bone may be determined as to sex. (p.72)

    Some European caves contain fossils from tens of thousands of cave bears; brown bear remains are abundant in some British caves.  These are the largest samples available for any extinct species or subspecies (Kurte'n 1976).  Thus for cave bears, and secondarily fossil brown bears, comparisons over time and between locations may be reasonably reliable.  But caution is warrented concerning comparisons where sample sizes are miniscule.  

    This problem is illustrated by Kurte'n's statements concerning variation in body size for American black bears.

In the Holocene, there was a marked decrease in size in this species, a phenomenon also seen in many other species of large mammals.  The present-day black bears of Florida, which still retain something of the giant stature of their Pleistocene ancestors, are an exception.

    This problem of distinguishing size variation due to evolution from that due to diet, sex, and age, is critical to understanding speciation, habitat segregation, competition, etc. during the Pleistocene, both with regards to bears and other animals--most of whom are even less well known as fossils.

 

 

    Species differences are illustrated here with data on adult males, since they have been sampled more thoroughly than adult females or younger bears of either sex (Table 2:2).  On average, polar bears tend to be substantially larger than grizzly/brown bears.  Mean weights in polar bear populations range from about 275 to 500 kg; a few individuals exceed 700 kg.  Grizzly bears on the Pacific coast of North America average almost 290 kg; this is comparable to brown bears on the Pacific coast of Manchuria and Siberia, for instance the Kamchatka Peninsula (Novikov 1962; Kistchinski 1972); exceptional bears approach 600 kg.  Grizzlies and Eurasian brown bears in inland habitats, by contrast, tend to be smaller, averaging 200 kg (maximum about 450 kg) in western North America and 165 kg in Finland.  The larger size of polar bears is apparent even when one considers polar and grizzly bears from the same general region.  For example, near the Bering Strait, polar bear skull length averages 407 mm (Manning 1971:Table 2), compared to 352 mm for grizzly bears on the nearby Alaska coast (Rausch 1963:Table 2).  In turn, American black bears tend to be considerably smaller than grizzlies, particularly in areas where these species have been sympatric.  In the same general inland biomes where adult male grizzlies average about 200 kg, sympatric black bears average nearly 115 kg--contrasted to about 140 kg in eastern North America (Table 2:2).  Eastern black bears are comparable in size to Asian black bears and spectacled bears.  

    Wild sloth bears supposedly average about 100 kg (Walker 1964).  Prater (1965) reports weights of 126-145 kg for large males and at least 64 kg for females.  A female recently captured in India weighed 57 kg (Seidensticker, pers. comm.).  Two obese male sloth bears in the National Zoo (Washington, D.C.) reached weights of 204 kg and 227 kg (Seidensticker, pers. comm.)  

    According to Walker (1964), wild male sun bears average about 45 kg.  However zoo specimens commonly reach twice that weight.  In the Knoxville Zoo, a male reached at least 97 kg; 2 weights for an adult female were 75 kg and 91 kg; another adult female weighed 80 kg (C. Jones, pers. comm.).  One of those adult females was sold to the Seabold Animal Farm in Keokuk, Iowa, where it became "enormously fat," like a female American black bear preparing for hibernation, and seemed about the same size to P. Seabold, (pers. comm.), owner of Seabold Animal Park, who has reared and bred numerous sun bears and other ursids.  T. DeLerenzo (pers. comm.) has collected weights on other captive sun bears, among whom adult males average roughly 90 kg and females about 70 to 80 kg.  Individuals this large are not necessarily obese.  Indeed, their short pelage makes them appear much lighter than they really are, compared to species with thick coats.  Apparently, sun bears have the capacity to mature at sizes of anywhere between about 45 to 90 kg for males and 10-20% less for females.  We are not yet sure whether sun bears in each habitat have this much phenotypic plasticity, or whether mean weight varies 2-fold between different ecotypes (those on Borneo, Java, and other islands perhaps being smaller than mainland subspecies).  

 

    Numerous factors have been proposed to account for evolutionary, short term, and geographical variations in body sizes of bears.  These are discussed in Chapter 9:II.