The wolf in North America, is a predator of primarily large ungulates, that is, animals like moose, elk, and deer. All biological and social aspects of the wolf make it adapted for this role. No other carnivore in the western United States replaces the ecological importance of the wolf. Other wild animals that regularly prey on large mammals in North America include mountain lions and black and grizzly bears. Although the mountain lion regularly preys on large ungulates, its methods of hunting (primarily "ambush") and social organization (solitary) contrast sharply with the socially cooperative methods of the wolf. Black and grizzly bears, usually solitary by nature, stalk and kill moose, elk, and deer and take mostly calves but occasionally take vulnerable adult ungulates as well. While coyote diets occasionally include young, old, and vulnerable ungulates, they mainly take only small animals.
In general, wolves depend upon ungulates for food year round. They have, however, been known to eat almost every available type of small prey, including small mammals, birds, snakes and lizards, fish, and even insects and earthworms. Grass and berries too are sometimes eaten but none of these items can be regarded as making a significant contribution to the diet. In northern Montana, elk, moose, and deer (mule and white-tailed deer) are the principal prey species. Smaller mammals can be an important alternative to ungulates in the snow-free months. These small mammals include beaver, marmots, ground squirrels, snowshoe hare, pocket gophers, and voles. In various areas of North America, and during the years of abundant beaver populations, beaver comprise 25-75% of the spring-fall diet of wolves. In those areas or situations, wolves may prey less on ungulates. However, when these figures for beaver are converted to a biomass basis, ungulates still constitute the bulk of the summer diet and certainly of the annual diet. In areas where beaver are not so abundant, ungulates usually account for more than 90% of the biomass consumed by wolves.
On an average, wolves eat 9 pounds of meat per wolf per day during winter. Although the wolf is capable of eating large
quantities of food in a short time, such quantities are not always available. Thus, wild wolves may have to go for several days at a time without eating. Wolves probably could fast for periods of two weeks or more while searching for vulnerable prey. When food is available, wolves can replenish themselves to prepare for another period of fasting. With its large stomach capacity, the wolf seems well adapted for this cycle of feasting and extended fasting.
The frequency of kills by a wolf pack varies tremendously, depending on many factors including pack size, diversity, density, and vulnerability of prey, snow conditions, and degree of utilization of the carcasses. Because the wolf's prey varies in size from small mammals to beaver to bison, the kill rate of each species varies according to the amount of food each provides. In Minnesota, where wolves eat white-tailed deer almost exclusively, estimated kill rates range from 15-19 deer per wolf per year. In areas where elk are the dominant prey, these kill rates are generally lower, as elk are larger than white-tailed deer and provide more food per kill.
There is an ongoing debate about whether wolves deplete ungulate populations in a significant manner, enough to keep humans from getting their hunting share. This belief has resulted in much "wolf control" throughout the decades. What kills most ungulates is winter itself (winter-kill). Humans also kill more ungulates, and are more likely to deplete populations, than wolves do or would. Why are humans more likely to deplete populations than wolves? To answer this question we'll examine a basic ecological principle which distinguishes modern humans from other animals. This principle states that any predator has a prey image, or images, in mind during a hunt, and as a particular prey declines due to continued hunting, the predator switches prey images to hunt a more abundant prey. This period gives prey time to "bounce back", increasing their numbers, at which time the predator may switch back to the original prey image. Likewise,
wolves are more likely to turn to a more plentiful food source and might even starve before they would deplete ungulate populations to dangerously low levels. Early humans probably also hunted in this manner. Unfortunately, modern humans continue to target a depleted animal (this includes any animal species), either for sport, for tradition, for economic gain (often the rarer the animal becomes the greater the financial reward), or to feed ever increasing human populations until it becomes threatened, endangered, or extinct.
While wolf predation is one component of total annual mortality in many ungulate populations, and they may keep some prey species at a low level if they are already low and other limiting factors exist, wolf predation of larger ungulate populations usually results in smaller fluctuations in ungulate numbers over the years. Wolves and ungulates have survived over millions of years in a balance, and through all their prey-predator interactions, they have made each other stronger and better adapted to their environments.