Prey-model feeders look to the wolf's diet for a model of what to feed. The gray wolf is the living ancestor of the modern dog, and can offer us valuable insight into what and how domestic dogs are designed to eat.

The following passage is from Of Wolves and Men by Barry Holstun Lopez, 1978, published by Charles Scribner's Sons, New York. It describes a wolf's diet as Mr. Lopez has gathered from field experience and a variety of sources including other biologists and naturalists:

"The wolf's diet consists mostly of muscle meat and fatty tissue from various animals. Heart, lung, liver, and other internal organs are eaten. Bones are crushed to get at the marrow, and bone fragments are eaten as well; even hair and skin are sometimes consumed. The only part consistently ignored is the stomach and its contents. Some vegetable matter is taken separately, particularly berries, but Canis lupus does not seem to digest them very well. The red wolf commonly consumes a higher proportion of vegetable matter and subsists on smaller game, like swamp rabbit, and such things as fiddler crabs. All wolves eat grass, possibly to scour the digestive tract and remove worms. Consisting mostly of cellulose, the grass itself is never digested.
Wolves consume an average of five to ten pounds of meat a day and wash it down with large quantities of water to prevent uremic poisoning from the high production of urea associated with a meat diet. The wolf has a large liver and pancreas to aid digestion, and the feces provide an interesting example of efficiency in its large intestine. Droppings in the wild typically consist of chips and slivers of bone neatly packaged along with such items as the rubbery remains of deer hooves in a capsule of hair that moves very smoothly down the colon.

The major sources of meat in the wolf's diet are deer, moose, elk, musk ox, Dall sheep, Rocky Mountain sheep, caribou, reindeer, or beaver, depending on the area, the season, and the year -- a good one or a bad one for, say, moose. Wolves also prey on buffalo (in Wood Buffalo National Park, Canada), snowshoe hares (on Ellesmere Island), flightless ducks (in the James Bay region of Canada), marmots, mice, squirrels, grouse, geese, and rabbits. Wolves fish, too, wade-herding salmon, arctic grayling, or whitefish into shallow pools where they're trapped. They also mouth-spear them in swift water from the bank with well-timed lunges. They eat carrion, and occasionally insects, especially whey they encounter them in epidemic populations. And they feed on domestic stock. They hunt by intent but are opportunists, too."