Wolves must work very hard to catch and kill their prey. Prey is usually much larger and can fight back. This famous study proves that every meal for wolves could be their last. http://www.isleroyalewolf.org/overview/overview/wolves.html has provided the following information:
“For most North American and European humans eating a meal is a pretty simple affair: get some food from the cupboard, heat it up, and eat. What if every meal required exerting yourself to the point of exhaustion, holding nothing back? What if every meal meant risking serious injury or death? Under these circumstances, you might be happy to eat only once a week or so – like Isle Royale wolves.
Isle Royale wolves capture and kill, with their teeth, moose that are ten times their size. Think about it for a moment – it is difficult to comprehend. A successful alpha wolf will have done this more than one hundred times in its life. Wolves minimize the risk of severe injury and death by attacking the most vulnerable moose. Somehow wolves are incredible judges of what they can handle. Wolves encounter and chase down many moose. Chases typically continue for less than ½ a mile.
During chase and confrontation wolves test their prey. Wolves attack only about 1 out of every ten moose that they chase down. They kill 8 or 9 of every ten moose that they decide to attack. The decision to attack or not is a vicious tension between intense hunger and wanting not to be killed by your food.
Wolves typically attack moose at the rump and nose. The strategy is to inflict injury by making large gashes in the muscle, and to slow the moose by staying attached, thereby allowing other wolves to do the same. Eventually the moose is stopped and brought to the ground by the weight and strength of the wolves. The cause of death may be shock or loss of blood. Feeding often begins before the moose is dead.
A moose, with a wolf clamped to its rump is still formidable. They can easily swing around, lifting the wolf into the air, and hurl the wolf into a tree. Most experienced wolves have broken (and healed) their ribs on several occasions. Moose deliver powerful kicks with their hooves. Wolves occasionally die from attacking moose.”
The Wolf Preservation Blog 