The size of bear cubs at birth depends on the species. They can range from 300 grams (sun bear) to 450 grams (brown bear or polar bear) which is more or less the size of a pound of butter. All of the species are born with their eyes and ears closed, a generous sprinkling of fine fur, and are completely helpless. They are born in den-like settings protected by their mother. Cubhood is a critical time not only for physical and social growth but for brain development. Healthy brain growth is the product of a complex series of dynamic and adaptive processes operating within a highly constrained, genetically organized but constantly changing context. Stiles 2010
Bear rehabilitators have observed common stages of behavioral development in cubs. Sally Maughan founder of Idaho Black Bear Rehab, Inc. has recorded extensive notes on cub development over her rehabilitation career. In general, the infant stage ranges from birth to 8 weeks old. In that time, the eyes and ears open, teeth erupt, and exploration and wobble walking begin. Between 8 to 12 weeks, cubs seem to pass through the bear equivalent of the terrible twos. There are swift emotional changes from calm to biting, attacking, scratching, and crying tantrums. At 4 to 7 months old cubs are at the age of destruction and social learning as they roughhouse with siblings and other orphans. From eight months to dispersion, peace breaks out as the cubs mature and ceaselessly learn about their environment. Behavior-based husbandry accommodates their mothering needs and provides a complex environment to allow for healthy physical and mental growth.