Wolves are carnivores, or meat eaters. Gray wolves prey primarily on ungulates – large, hoofed mammals such as white-tailed deer, mule deer, moose, elk, caribou, bison, Dall sheep, musk oxen, and mountain goats. Medium-sized mammals, such as beaver and snowshoe hares, can be an important secondary food source. Occasionally wolves will prey on birds or small mammals such as mice and voles, but these are supplementary to their requirements for large amounts of meat. Wolves have been observed catching fish in places like Alaska and western Canada. They will also kill and eat domestic livestock such as cattle and sheep, and they will consume carrion if no fresh meat is available. Some wolves eat small amounts of fruit, although this is not a significant part of their diet. If prey is abundant, wolves may not consume an entire carcass, or they may leave entire carcasses without eating. This is called “surplus killing” and seems inconsistent with the wolves’ habit of killing because they are hungry. Surplus killing seems to occur when prey are vulnerable and easy to catch – in winter, for instance, when there is deep snow. Since wolves are programmed to kill when possible, they may simply be taking advantage of unusual situations when wild prey are relatively easy to catch They may return later to feed on an unconsumed carcass, or they may leave it to a host of scavengers. Additionally, they may cache food and dig it up at a later time. Red wolves primarily prey on white-tailed deer, raccoons, rabbits, nutria and other rodents.