When do black bears enter and leave their den?
Denning and hibernation among black bears varies with latitude, from 5 to 7 months in northern Canada and Alaska to a week or so in northern Mexico. In areas with mild climates, bears may den for 3 to 4 months. However, some bears-particularly males-may remain active all or most of the winter, especially in mild or snow-free years. In one Massachusetts study, bears denned for about 4 months in 1980-81 and about 5 months in 1981-82. Several studies have concluded that den entry is dependent on fall food supply. Bears remain active longer when food is abundant, but enter the den earlier when food is scarce and fruitless searching results in a negative energy balance. However, gravid females may den as soon as they have accrued sufficient fat reserves for reproduction. The cumulative effects of increasing precipitation and lower temperatures may also play a role in the onset of denning. In Ontario, den entry dates ranged between September 20 to November 29 over a 5-year period, while in T
