Predation on ungulates
Even though all three North American bear species can be predators, the grizzly bear is more likely to kill larger terrestrial prey; either because of opportunity (compared to polar bears) or ability (compared to black bears). An adult grizzly is large enough to kill most adult ungulates, and equipped with claws, teeth, and power sufficient to deliver a killing blow. Meat's ready digestibility also means that the typical bear will reap considerable rewards from predation (see Nutrition).
Bears locate larger land-dwelling prey, such as deer, elk, and moose, by a variety of methods and with varied intent. A certain number of encounters with potential prey are strictly by chance, apparently without deliberate stalk or search. This is typified by one incident where two grizzlies approached some elk behind a blind rise, without either species being aware of the other's presence. However, as soon as the elk came into sight, one of the bears lunged after them, but without success. This unfruitful result probably characterizes the vast majority of these chance encounters, however there are enough recorded kills in rolling terrain or otherwise visually opaque conditions to suggest occasional success under similar circumstances. On the other hand, the majority of observations suggest that grizzly bears most often encounter their prey either after a deliberate search, characterized by movement in the direction of the hoped-for kill, or, with even greater forethought, by putting themselves in areas with concentrations of vulnerable prey. Elk and caribou calving grounds or traditional caribou river crossings are good examples of such areas where, either because of extreme young age, the preoccupations of birth, or the mass confusion of swimming animals, otherwise elusive ungulates become easy prey. This is most dramatically evident in graphic descriptions by observers such as Adolf Murie, Harry Reynolds, Kerry Gunther, and Steve and Marilyn French, of pursuits, kills, and confrontations by grizzlies concentrated on calving grounds in Alaska and Wyoming.
Bears clearly use all of their senses to track down prey. However, most observations suggest a prominent role for olfactory and audio cues (see Senses). Bears have often been observed following scent trails associated with potential prey, sometimes for miles, or more convincingly, been seen killing calves at the culmination of a complex search mostly with nose to the ground. There are other phenomena that argue anecdotally for the prominence of hearing in tracking down meat. A number of hunters bugling for elk have surprised themselves by soliciting the apparently predatory interest of a grizzly bear. Similarly, there is an observed tendency world-wide for increased bear predation on both bull elk and bull moose during their fall ruts. Bears also seem to be attracted to the sound of gun-fire during the hunting season, one can only suppose with the intent of scavenging the gut pile and other remains of a hunter's kill. Although this likely orientation towards sound has not been conclusively demonstrated, it remains an intriguing possibility worthy of further study.
Bears can chase down prey in open country, and are more successful at this tactic after separating a young calf from a cow-calf herd. Capture usually occurs after the pursuing bear cuts the corner on a fleeing calf that has been foolish enough to turn. However, most successful bear predations are not the result of long pursuits, but rather the result of a short-range rush either from ambush or after a stalk. Given the relative slowness of bears (see Front limbs), at least compared to most wild ungulates, there is a distance at which this rush is more likely to be successful or, if not, then terminated. A number of records by different observers in different areas suggest that this "optimal" distance is around 20-50 m. Without the aid of mitigating conditions or tactics, it would be difficult for a bear to get this close to a reasonably alert and healthy ungulate, and so it should be no surprise that most kills are associated either with cover, conditions that hinder prey mobility, or the afflictions of exhaustion, disease, or raging sexual hormones in their prey. Many documented predations have been in or near the edge of willow thickets and dense timber, often near running water; presumably under conditions that facilitate the unobserved approach of a predatory bear. Other predation occurs on forest game trails, often as the prey negotiates deadfall or a small draw; under conditions where temporary imbalance or distraction would favor a successful ambush. Otherwise, bear researchers throughout the northern hemisphere have noted the remarkable success of bears preying upon winter-weakened ungulates in spring snow conditions. Surface crusts form that can support the weight of a broad-footed bear but not a larger-bodied ungulate such as an elk, moose, or muskox (follow this link to a paper on foot loadings of grizzlies). Under these conditions the delivery of a killing bite is more often a constraint than over-taking the prey. Similarly, exhausted and/or hormonally-deluded bulls seem to make relatively easy bear prey during and after the fall ruts. One extraordinary story from Siberia recounts a brown bear that lured a rutting bull moose into a willow thicket by imitating the bull's mating roar.
Grizzly bears kill land-dwelling prey primarily one of two ways. Most often they maneuver to approach over the back, after which they grab the animal around the rib-cage and, if successful, deliver a killing bite to the back of the neck or skull. A bite to the small of the back may be a prelude to this finishing move, or an animal that escapes the first attempted neck bite may be severely wounded on the flanks by the bear's claws as they are disengaged from their hold. An approach from the rear is also the logical consequence of a chase, and on occasion the bear may use its weight to collapse the hind-quarters of an animal that has just been caught. The next most common kill technique involves a bite to the nose and face that at least paralyzes and possibly kills the animal. Either way, the animal does not offer subsequent resistance. Interestingly, there are only a few reports of bears delivering killing or debilitating blows with their paws, and in most cases their paws come into play for such things as pinning down a newly-captured calf or grasping larger prey to facilitate delivery of a killing bite.
Bears are able to consume between 12 and 40 kg of fresh meat in a day. However, consumption is not indiscriminate and typically reflects marked preference for some carcass parts. Almost universally, the most favored or at least first eaten portion of a female is the udder. After that, most bears eat the brisket and adjacent rib meat or the meaty proximal part of the front legs. Most carcasses are cleanly eviscerated soon after they are killed or found by a bear, with the heart, liver, and lungs selected for priority consumption. The skin, hair, rumen, and stomach are the most consistently unused soft parts of a carcass. In fact, a more-or-less neatly pealed off hide is one of the more diagnostic signatures of bear involvement. Bears are also one of the few consumers that can readily crack the major leg bones of a larger ungulate in pursuit of the marrow. Some bears even get into the habit of cracking the skull so that they can consume the brains.
There are other features that distinguish carcass use by grizzly bears. If there are enough edibles to warrant the investment, a bear will typically bury the entire carcass or selected parts of it by scraping adjacent soil and litter over the tidbit with its front paws. Presumably this inhibits decomposition and the dissemination of scent that might attract competitors. As further sign of ownership, you might even find a bear sprawled directly on top of a carcass, attempting to rest while at the same time prevent pilfering by other scavengers. A bear may also move an intact or even partly consumed carcass to a spot more to its liking. If the carcass is not fresh, a drag trail of shed ungulate hair will connect the new carcass location with the rumen contents left at the original site of death. In addition, there are often bear beds nearby and a bear trail leading to the nearest available water (an apparent necessity when consuming such a protein-rich diet).
As a final note on this type of predation - grizzlies exhibit marked preference for different sizes and species of ungulates (see this paper for results from Yellowstone). They are least likely to attack very large herd-dwelling animals that rely upon aggression for predator defense, epitomized by bison. Among the larger ungulates, they seem most likely to attack moose, presumably because of a solitary forest-dwelling existence that lends itself to ambush. Among the medium- to large-bodied prey species, bears preferentially prey upon young animals, especially the highly vulnerable neonates. On the other hand, although they are not much larger than a calf elk or moose, adult deer are relatively infrequent bear prey, presumably because of their greater agility and endurance.