Differences in sizes of brown bears
Given that bears, as a group, are large, and that brown/grizzly bears are amongst the largest of the lot, there is substantial within-species variation in body size. As with so many mammal species, males are consistently larger than females. Superimposed on this sex-related difference, body size varies substantially among populations as an apparent function of the average amount of meat consumed. More dietary meat translates into larger bears, which is probably also a driver of differences between the sexes; on average, males consistently eat more meat than do females in any given population (see Foods). Insofar as some details are concerned:
The graphs at left show the average size of adult females (A) and males (B; on the vertical axis) relative to the amount of meat in the diet (horizontal axis). Each dot represents the average for a population, all of which are North American. (Estimates of meat consumption come from an extensive data set compiled and generated by Garth Mowat, a researcher from British Columbia). The colored dots with a white center denote averages for the Yellowstone grizzly bear population.
I fitted a trend line to each of these data sets, differentiating coastal populations with access to substantial numbers of spawning salmon from interior populations without. A quick scan of the data clearly indicated that these two subpopulations exhibited different responses in body size depending on whether the meat consumed was salmon as opposed to terrestrial mammals. Although all of the relationships were positive, fish-eating populations were larger at any given level of meat consumption and, on average, increased in average size at a greater rate in response to each additional increment of dietary meat.
Why this difference in population-level response to consumption of salmon versus terrestrial meat? This question has not been directly addressed by researchers, but it is easy to imagine that the per kg energetics of eating fish are more favorable than those of eating meat from, say, caribou, moose, or elk. Terrestrial meat tends to come in larger packages that are often difficult to procure, and spread thinly and unpredictably on the landscape. By contrast, spawning salmon are highly concentrated, predictable, and digestible, and, under the right circumstances, easy to obtain.
The nutritional and energetic connection between dietary protein in the form of meat and growth and body size is straight-forward. Which means that these population-level responses are not surprising nor hard to explain (for details on nutritional and energetic aspects see Nutrition).
The link between dietary protein--especially from salmon--and body size of brown bears is reinforced by hemispheric patterns. The map below shows the historical distribution of brown bears (along with denotations of genetic clade; see Evolution), overlain with isopleths of skull size (specifically, length of the skull [condylobasal length]); with skull size indicating overall body size. The darker the green, the larger the size. Large size is clearly associated with coastal areas hosting spawning runs of salmon. . By contrast, the smallest bears are in Europe, the Tibetan Plateau, and much of interior North America. (Data on skull size were compiled and georeferenced by Masaaki Yoneda using measures made by Bjorn Kurten and Bob Rausch).
The shrinkage of brown bears
One interesting phenomenon that potentially links to dietary meat relates to the fact that brown bears seem to have diminished in size over the course of the last ice age (the Pleistocene). This was observed early in the research on evolution of bear body size (during 1950s) by researchers such as N.K. Vereshshagin and Bjorn Kurten, the one famous in Russia, the other in Europe as pioneering paleontologists.
More recently, Adrian Marciszak and associates published results, based on measures of bear skulls found in Europe, that seem to have pretty much nailed this trend. These results are summarized at left, arrayed according to time, from left to right, differentiating males from females and measures of skull length from skull width. (The acronym MP refers to "middle Pleistocene," progressing in time to the recent, "R"). One feature of these results is the suggestion that males dwarfed more substantially than did females.
This begs the question: Why? There was certainly a lot of meat around during the Pleistocene, perhaps in the form of carrion able to be obtained from the carcasses of the numerous large herbivores dying from any number of causes (see History). According to Herve Bocherens, at expert on reconstructing diets from the analysis of ancient tissues, brown bears were especially carnivorous during the Ice Age, especially in the steppe tundra of Eurasia. This is all consistent with the larger size of Ice Age brown bears being a consequence of ingesting more meat. Even the differences between trends in male and female sizes (see below) is consistent with a meat effect given that males tend to eat comparatively more meat whenever it is available, which means they would have been comparatively more affected by the rapid decline of the meat resource caused by widespread extinctions of the large herbivores towards the end of the last Ice Age.
As I noted at the beginning of this page, male brown/grizzly bears are, on average, consistently larger in size than females, especially as they grow beyond cub-hood. Beyond this baseline there are several interesting trends. One (illustrated in the graph above left) is that the enlarging of males relative to females (i.e., deviation above the 1:1 line) increases with average size of bears in the population. The rate of this increase is denoted by the power value attached to the X-coefficient above (1.44, with indicates a rapid rate of increase, in contrast to a value of 1, which would denote parity of increase in the sizes of males and females).
As to the driver of this difference? The graph at upper right again suggests that dietary meat plays a role. As you can see, the ratio of male to female body size increases with an increase in percentage of meat in the averaged population diet. The rate of this increase seems to be greater for populations consuming terrestrial meat than for populations in coastal areas that consume primarily salmon. Remember that, by contrast, the increase in overall body size of bears is greater in fish-eating versus ungulate-eating populations. The difference could simply be a mathematical artifact in that the denominator of this ratio (female body size) tends to be greater at any given level of meat consumption among fish-eating versus ungulate-eating populations. That said, the overall trend in this ratio (increase with increasing consumption of meat) defies this bias, and strongly suggests that males do, indeed, get larger at a more rapid rate compared to females with each increment of meat consumption.
Again (as noted above), this divergence could have something to do with the greater ability of the larger less-security-conscious males to dominate concentrated sources of meat. Plus it could have something to do with the extent to which, compared to females, males more efficiently convert dietary protein into lean body mass. Although such a phenomenon has not been definitively shown for bears, it has been well-documented in swine and humans, two other large-bodied omnivore.
As a parting note, the graph at the left places the sexual dimorphism of bears in context of other carnivores, thanks, again, to the work of Per Christiansen. The height of each bar denotes canine (and skull) size as the ratio of males to females for each of the carnivore families comprised of larger-bodied species (excluding, for example, weasels and mongooses). Ursids are denoted by the dark brown bar.
So, the main points here are that: cats (Felids) tend to be the most dimorphic of all families; that the greater differences between males and females of all families are found in canine strength; and that bears are not exceptional in the degree of expressed dimorphism, at as expressed by key skeletal indicators such as skull length and canine size and strength. Parenthetically, it is interesting to note that hyaenas are truly exceptional in being one of the few families of terrestrial mammals in which females are larger than males.