weigh at birth and begin to see after nine to 12 days. The milk canines erupt after one month. Pups first leave the den after three weeks. At one-and-a-half months of age, they are agile enough to flee from danger. Mother wolves do not leave the den for the first few weeks, relying on the fathers to provide food for them and their young. Pups begin to eat solid food at the age of three to four weeks. They have a fast growth rate during their first four months of life: during this period, a pup's weight can increase nearly 30 times. Wolf pups begin play-fighting at the age of three weeks, though unlike young coyotes and foxes, their bites are gentle and controlled. Actual fights to establish hierarchy usually occur at five to eight weeks of age. This is in contrast to young coyotes and foxes, which may begin fighting even before the onset of play behaviour. By autumn, the pups are mature enough to accompany the adults on hunts for large prey. Hunting and feeding Single wolves or mated pairs typically have higher success rates in hunting than do large packs; single wolves have occasionally been observed to kill large prey such as moose, bison and muskoxen unaided. This contrasts with the commonly held belief that larger packs benefit from cooperative hunting to bring down large game. The size of a wolf hunting pack is related to the number of pups that survived the previous winter, adult survival, and the rate of dispersing wolves leaving the pack. The optimal pack size for hunting elk is four wolves, and for bison a large pack size is more successful. Wolves move around their territory when hunting, using the same trails for extended periods. After snowfalls, wolves find their old trails and continue using them. These follow the banks of rivers, the shorelines of lakes, ravines overgrown with shrubs, plantations, or roads and human paths. Wolves are nocturnal predators. During the winter, a pack will commence hunting in the twilight of early evening and will hunt all night, traveling tens of kilometres. Sometimes hunting large prey occurs during the day. During the summer, wolves generally tend to hunt individually, ambushing their prey and rarely giving pursuit. When hunting large gregarious prey, wolves will try to isolate an individual from its group. If successful, a wolf pack can bring down game that