Grizzly bears and meat from Large Herbivores
Meat is a uniquely valuable grizzly bear food because it is proteinaceous, easily digested, and potentially rich in fat, contingent on season and source. Protein and fat are central to bear diets primarily because ursids retain the simple digestive tract of a carnivore despite being dietary omnivores--which means that, unlike ruminants or hind-gut fermenters, they have little capacity for digesting the complex carbohydrates that comprise much of vegetal foods. Fat fractions are particularly important because of the unparalleled density of digestible calories in this tissue, efficiencies of conversion to adipose reserves, and importance to ursid life-strategies organized around hibernation and obesity. Despite at one time being a major source of meat for grizzlies in the northern US Rocky Mountains, fish have essentially disappeared from the menu. Salmon and steelhead spawning runs were largely extirpated during the early and mid-1900s. More recently, during the 1980s and 1990s, spawning runs of native cutthroat trout and non-native kokanee salmon--dietarily significant to grizzlies in Yellowstone and Glacier National Parks--were functionally extirpated by human-caused environmental changes. As a result, bears in the northern Rockies currently obtain most meat from large herbivores such as elk, cattle, deer, bison, and moose; hence the focus here.
Meat as Bear Food
The figure at left graphically illustrates the unique qualities of tissue (e.g., meat) from fish and large herbivores relative to other bear foods. The dots colored red, pink, or burgundy denote average protein content of each food; the brackets bound the standard errors of each. Red denotes an annually averaged estimate; burgundy a spring estimate; and pink a midsummer or fall estimate. The black, gray or white dots represent estimates of digestible energy--black where annually averaged; dark gray for spring; light gray for midsummer; and white for fall. The horizontal pink band represents the approximate optimal dietary protein for bears centered on 20%.
Tissue from fish and large herbivores stands out as being the richest in digestible protein and energy of any bear food, vegetal or otherwise, although some roots, insects, and fruits and seeds offer similar magnitudes of digestible energy. Perhaps paradoxically, the exceptionally high concentrations of protein in meat pose challenges for bears because they are so far in excess of optimal.
The figure at right puts bears in context of other large mammals insofar as digestive efficiencies are concerned. The boxes and vertically associated bars and dots--called box and whisker diagrams--summarize dry matter digestibilites of various foods reported for various mammals, including grizzly bears, ursids in general, giant pandas, other large carnivores, and herbivores differentiated by whether they are ruminants (foregut fermenters of fiber such as elk and deer) or non-ruminants (hindgut fermenters such as elephants and horses).
Bears are less efficient than specialized herbivores at digesting foliage, but every bit as efficient as hyper-carnivores at digesting meat. Roots (and fruits, not shown) are the only vegetal foods digested by bears as well as foliage is by large herbivores. Interestingly, despite subsisting almost wholly on bamboo, giant pandas are remarkably inefficient at digesting it. This begs the question why bears eat any foliage at all, or why pandas subsist on bamboo--which, to answer, requires digging deep into the life strategies of ursids.
Despite the high quality of meat as bear food, different bears eat different amounts even with equal amounts available to them in their larger environments. Most of this variation organizes around sex, age, and species, although idiosyncratic differences among individuals of the same sex, age, and species have also been documented.
The bar graphs at left are emblematic of these patterns. The figure farthest left provides a breakdown of average differences among sex-age groups of grizzly bears (adult and subadult males and females) documented for the Yellowstone ecosystem. Burgundy denotes results for males, green for females. The different hues of each bar correspond to different published sources: (1) darkest, percentage of total diet by Jacoby using isotopic analyses; (2) lightest and cross-hatched, also percentage of total diet, but by Chuck Schwartz, also using isotopes; (3) dark cross-hatched, percentage of total energy requirements by myself; and, finally, in kg, estimated total meat (dry matter) consumed annually, also by myself.
The bar graphs in orange and brown summarize results for black and grizzly bears occupying the same ecosystems, but differentiating those where fruit (i.e.. berries) versus meat dominates the diet--essentially differentiating the fruit-centric system spanning the US-Canada border near northern Idaho and northwestern Montana, versus ecosystems such as the East Front and Yellowstone farther east and south.
The results are not subtle. In Yellowstone, adult males are, by every measure, much more carnivorous than other sex-age classes--perhaps by as much as 2-3-fold. Results from other ecosystems show a similar tendency for greater carnivory among adult males. In systems where meat is a dietary mainstay, grizzlies tend to eat twice as much as do black bears. By contrast, where fruit dominates and meat matters comparatively little, differences in consumption between black and grizzly bears are essentially non-existent.
The greater role of meat in diets of adult males probably arises from both greater demand and greater access. Protein predictably plays a more important role in the reproductive success of males compared to females, first, because larger males tend to be more successful in head-to-head violent competition with other males for reproductive opportunities; second, because larger males tend to have larger testes and produce more sperm, which also lends competitive advantage in a promiscuous mating system; and, third, because omnivorous males tend to accumulate more lean body mass when eating a high-protein-content diet compared to females of the same species. All of this predictably adds up to a greater preference by male ursids compared to female ursids for protein-rich diets, especially given that dietary fat seems to be more important to the reproductive success of females via the critically important role of adipose reserves. And given that males tend to be larger than females--often much larger--they are also predictably better able to dominate sources of protein-rich food often concentrated in the form of larger-bodied carrion or dense runs of spawning salmonids. This latter factor also probably explains the greater fraction of meat in diets of larger-bodied grizzlies compared to smaller-bodied black bears occupying the same ecosystems (see this entry for more on differences between the two species).
Differences Among Bears
Differences Among Ungulates as Sources of Meat
Compounding differences among bears in their motivations to eat meat and their ability to access this food, the large herbivores exploited by grizzlies also offer substantially different rewards for the involved individuals. The graphs immediately above (A) summarize these differences for herbivores that are the source of most meat consumed by grizzly bears in the Yellowstone ecosystem--elk, bison, and moose, differentiated by sex-age class of the exploited carcass and by whether the meat was obtained by scavenging or as a result of outright predation. The height of the bars correspond with the amount of mass (mostly meat, but also including fat, hide, and hair) consumed by bears from an average carcass of the corresponding species (bison v moose v elk), differentiated by whether the carcass was of an adult or an animal less that 2 years old and, for bison and elk, whether a bull or a cow. The bar graph farthest left shows data for scavenged carcasses, the bar graph to the right, data for animals killed outright by bears. In addition, the narrower cross-hatched bars represent the average amount consumed by an average individual bear, recognizing that more than one bear was sometimes involved in consuming the tissue from a carcass, which is represented by the total.
One key proviso for these results is that they are derived from data collected during 1976-1993, which predates the current greater reliance of grizzly bears in this region on domestic livestock concentrated in the ecosystem's periphery (see Trends).
The main results are: first, that, all else equal, bears obtained far more tissue from larger-bodied bison and moose compared to smaller-bodied elk; second, that bears obtained substantially more meat from a predated animal versus a scavenged one, especially in the case of elk; third, that bears typically shared larger-bodied carcasses with other bears, regardless of whether these large-bodied carcasses were scavenged or obtained by predation; and, fourth, that this competition among individual bears was greatest for scavenged bull elk and bull bison and for any adult elk that had been killed outright. The absence of data for predated adult bison is because grizzlies very rarely prey on such large-bodied, herd-dwelling, aggressive animals in open environments lacking ambush cover (see Predatory behavior). Parenthetically, the comparatively small amount of meat obtained from scavenged v predated elk arises from the rapidity with which these animals are exploited by other competing scavengers such as coyotes and ravens (see Scavenging), which magnifies the incentives for grizzlies to kill rather than scavenge kill elk, especially in the case of calves. In any case, elk are a less risky prey item for grizzlies compared to bison and moose.
The x-y graph immediately above (B) recasts data from the bar diagrams at left to clarify the effects of predation versus scavenging on mass-specific differences in amounts of meat consumed, total and per individual bear, from a given type of carcass. Consumed mass, whether total or per individual bear, is on the vertical axis and available mass (as a function of carcass type) on the horizontal axis. The size of the various dots is proportional to number of sampled carcasses of each type, which is also reflected in the vertical error bars bounding each dot. Relations between total mass consumed by all bears and total amount available are shown by the thick solid lines--burgundy for by predation and pink for by scavenging. The relations for average mass consumed by individual bears are shown as dotted lines.
As I summarize at left, the basic trends are for total amounts consumed to increase with increasing available amounts, but for amounts consumed by predation to be consistently greater, and increasing at a proportionally greater rate with carcass size compared to when exploitation is by scavenging. The fact that total consumption tracks availability is not surprising, or that predation consistently yields a greater reward for involved bears. At some point though, the pay-off from predation is offset by the hazards of attacking ever-large prey, which, barring rare instances, makes adult bison available to bears only as carrion.
The final pattern of note is the extent to which amounts scavenged by individual bears increasingly deviates from the totals consumed by all bears as carcass size increases, even to the point of declining for the largest-bodied carrion of all (e.g., bull bison). The reason is actually pretty straight-forward. As carcass size increases, the time required for total consumption also increases, which means that ever more bears and other scavengers will have an opportunity to discover the carcass and compete for the remaining edibles, reducing the amount available on average to any given individual (see Scavenging).