Size

Black bears in the Sierra Nevada are variable in size and color which reflects the influence of environmental conditions over its range as well as genetic plasticity. Male black bears rarely reach 400 pounds (180 kg) and females may grow to 250 (112.5 kg). Male black bears will gain weight until they are 10 years of age or more, while females gain weight slowly after 3 years and stop after 7 years of age. Differences in size are indistinct until puberty; thereafter females redirect nutrition from growth to reproduction. Males show a 27% difference between early summer and fall weights. They lose weight rapidly through the spring and then gain from July until denning. Seasonal weight change differences may be explained by male mating behavior – male bears expend substantial time and energy visiting the home ranges of eligible females at a time when food resources are at their lowest. Female bears show a 17% difference between spring and fall. They are at their lowest weight upon emergence from the den and gain steadily through the year. Females pay a substantial weight penalty for reproduction; females with young are 15% lighter than cubless females and may gain only a fourth as much weight from May to September.

Color

Despite their name, the coat color of black bears range from yellow through various shades of brown to black. About 95% of bears in the Sierra Nevada are some shade of brown and only about 5% are a true black color. Color changes through the year are common in bears because the guard hairs start bleaching immediately after the molt which may occur anytime between May and September. Molting is signaled by the appearance of new guard hairs, which are shorter and darker. They first emerge around the eyes and lower limbs, and then extend up the limbs and down the face and flanks. The molt is completed along the spine.

General Physiology

Black bears are similar to humans in many ways. One of these similarities is their normal body temperature which is approximately ninety-eight to ninety-nine degrees Fahrenheit. However, a bear’s temperature can vary based on levels of activity and whether the bear is hibernating. Like all animals, bears must regulate their body heat. Bears, like dogs, do not have sweat glands. They must cool themselves through several unique methods such as: balancing energy expenditure and food intake, resting in shady day beds, lying with their bellies fully touching the cool ground, dissipating heat through their tongues by panting, submerging in water or taking mud baths. Dissipating heat through the paws is a primary means of heat loss for a bear. The pads are well supplied with blood vessels and are flat on the cool ground. Areas with minimal hair such as the face, ears, nose and the insides of hind legs are also sources where heat is lost. The thick fur on a bear serves as excellent insulation during the cold winter months.

The longevity for bears in the Sierra varies, but the average lifespan is 18 years. Principal causes of mortality include accidents with vehicles, disease, enemies, old age, and starvation due to loss of functional teeth.