American black bears (Ursus americanus) are an integral part of the Sierra Nevada ecosystem and are only one of the many animal species protected in Yosemite National Park. Black bears forage on a wide variety of natural foods, including grasses, insects, berries, and acorns. No scientific census of bears has occurred in Yosemite , but it is estimated that there are roughly 300 to 500 black bears in the park.

Despite their name, most black bears in Yosemite are not black in color. Most are some shade of brown, ranging from almost blond, to reddish brown, to a dark chocolate color.

One common question is how big are Yosemite 's bears? This is a difficult question to answer because bears, like people, can vary greatly in size. Also, an individual bear's weight can change greatly throughout the year. Before entering winter hibernation, a bear's weight can be double what it was when it emerged from its den the previous spring, if food sources are rich enough.  In general, however, the weight of an average, adult male Yosemite black bear in summer is 300 to 350 pounds (136 to 159 kg). Females are smaller, with typical weights ranging from 200 to 250 pounds (91 to 113 kg). Much bigger bears, however, do occur. The largest black bear ever captured in Yosemite weighed 690 pounds (375 kg)!

Bears are classified as carnivores, but a majority of a black bear's diet is made up of vegetable matter. In the spring, after emerging from winter dens, the bears feed largely on meadow grasses, which are relatively low in nutrition, but sustain the bears until more nutritious foods become available. As berries of various plant species ripen in the summer, the bears shift to these higher-calorie foods. Animal matter that is eaten consists primarily of ants, termites, and insect larvae ripped out of logs or dug from the ground. Black bears sometimes kill young deer or scavenge the kills of other predators, such as mountain lions and coyotes. In the fall, black bears gorge on acorns, which are especially important to the bears as they fatten before going into winter dens for hibernation.

Winter dens are typically established in hollow trees or logs, under the root mass of a tree, or in caves formed by the jumble of large rocks on a talus slope. Here, the bears enter a state of reduced body temperature, pulse rate, and respiration that enables them to conserve energy. Their "sleep," however, is not a deep one, and bears may leave the den periodically. Cubs are born in the winter den, weighing less than 1/2 pound (0.23 kg) at birth, and typically number from one to three cubs in a litter. The fast-growing cubs will remain with its mother through another winter, before leaving her in the spring at about 16 to 17 months of age. By spring, a hibernating bear can lose as much as half of its body weight. This is especially true of females nursing cubs.