discarded food containers like tin cans. Behaviour Social structure The wolf is a social animal. Its populations consist of packs and lone wolves, most lone wolves being temporarily alone while they disperse from packs to form their own or join another one. The wolf's basic social unit is the nuclear family consisting of a mated pair accompanied by their offspring. The average pack size in North America is eight wolves and in Europe 5.5 wolves. The average pack across Eurasia consists of a family of eight wolves (two adults, juveniles, and yearlings), or sometimes two or three such families, with examples of exceptionally large packs consisting of up to 42 wolves being known. Cortisol levels in wolves rise significantly when a pack member dies, indicating the presence of stress. During times of prey abundance caused by calving or migration, different wolf packs may join together temporarily. Offspring typically stay in the pack for 10–54 months before dispersing. Triggers for dispersal include the onset of sexual maturity and competition within the pack for food. The distance travelled by dispersing wolves varies widely; some stay in the vicinity of the parental group, while other individuals may travel great distances of upwards of , , and from their natal (birth) packs. A new pack is usually founded by an unrelated dispersing male and female, travelling together in search of an area devoid of other hostile packs. Wolf packs rarely adopt other wolves into their fold and typically kill them. In the rare cases where other wolves are adopted, the adoptee is almost invariably an immature animal of one to three years old, and unlikely to compete for breeding rights with the mated pair. This usually occurs between the months of February and May. Adoptee males may mate with an available pack female and then form their own pack. In some cases, a lone wolf is adopted into a pack to replace a deceased breeder. Wolves are territorial and generally establish territories far larger than they require to survive assuring a steady supply of prey. Territory size depends largely on the amount of prey available and the age of the pack's pups. They tend to increase in size in areas with low prey populations, or when the pups reach the age of six months when they have the same nutritional needs as adults. Wolf packs travel constantly in search of prey, covering roughly 9%