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The "Bear" Facts
Hibernation & Reproduction

Black bears do not hibernate in the classic sense, like rodents do, so 'partial hibernation' or 'winter lethargy' are often-used terms. This period of reduced activity, for various lengths of time, occurs in all black bear populations because winter lethargy is an adaptation to low/lack of available food, not just low temperatures. Bears in southern states, from North Carolina south to Louisiana, den for shorter periods and sleep less deeply than bears in colder climates. While denned bears in northern states are very lethargic and unresponsive to people, bears in the South readily run away when people come close to their den. 

In Florida, males and non-pregnant females may den up in dense vegetation for only a few weeks or a month. Pregnant females will den up for the entire winter, and because their cubs will be born in the den, they often select more protected sites. Dens may be in tree cavities, under blow-downs or fallen logs, or ground 'nests' in dense thickets.

The breeding season for black bears runs from June to July, but cubs are not born until late January to early February; a total gestation period of about 7 months (215 days). Bears have delayed implantation, which halts fetal development at a very early stage until late November or early December, when the sow will enter her den. If she is in poor condition and nutritionally stressed, the partially developed fetus will not develop further or cubs will be born and eaten by the female.  This adaptation to periodic food shortages prevents the sow from producing offspring for which she cannot care. 

Under normal circumstances, when the female enters her den, the fertilized egg will implant in the uterus and grow normally until birth.  The actual developmental gestation is only 6-8 weeks! Bear cubs are very small at birth, only 290 - 450 grams (10 - 15 ounces) and the size of a small squirrel. They have hair but their eyes are closed. Litters range from 1 to 5 cubs, but 2 or 3 are most common. 

The cubs nurse and play in the den until spring, when the family will leave the den for good. Cubs stay with their mother for a year and a half, and will den with her their second winter. During their second summer, the family group breaks up, the juveniles wander off on their own, and the sow comes into estrus again.  Taking care of the cubs for 2 summers means that sows will only breed every other year, if the litter is successful.  Should the litter fail, she will come into estrus earlier. This is thought to be one reason that large males kill unrelated cubs, to bring their mother into estrus again for breeding.