Mammals
Bear
Ursidae · also called bears
There are only eight kinds of bear, and they range from a 3 kg newborn in a snowed-in den to a polar bear that hunts seals across the frozen ocean and a panda that has committed its whole life to eating a grass that barely feeds it. What they share is a big flat-footed body, an extraordinary nose, and, in the cold-country species, the ability to spend a winter asleep on their own fat.
Overview
Bears are the family Ursidae, just eight species — brown, American black, Asian black, polar, sun, sloth, spectacled, and the giant panda — across North and South America, Europe, and Asia, with none in Africa or Australia. Most are opportunistic omnivores; the polar bear is a near-pure carnivore and the panda a near-pure bamboo eater. They walk on the soles of their feet and have one of the sharpest senses of smell of any land mammal.
Winter dormancy
Bears in cold regions spend months in a den without eating, drinking, or passing waste, living off fat and recycling their own urea. But they drop their temperature only a few degrees and wake fast, so it is lighter than true hibernation. A female gives birth in the den to tiny cubs, a few hundred grams each, and nurses them on very rich milk while barely awake.
Delayed pregnancy
A female mates in early summer, but the embryo does not implant until autumn, and only if she has stored enough fat — so a bad year quietly ends the pregnancy.
Status
Bears as a family are Not Evaluated, trend decreasing. The American black bear is abundant (~800,000), and the brown bear is secure across Russia and North America, but six of the eight species are threatened — the polar bear (Vulnerable, losing its sea ice), the sun, sloth, and Asian black bears (deforestation and the bile-and-paw trade), the Andean spectacled bear, and the panda, now Vulnerable after a long recovery.
Bear: common questions
Do all bears hibernate?
No. Only the bears that face a long season with little food den up for the winter, chiefly the brown, American black, and Asian black bears in the colder parts of their ranges, and pregnant polar bears. Bears in mild or tropical climates, including the sun bear, sloth bear, and spectacled bear, and brown and black bears in warmer regions, stay active year-round. And what denning bears do is not the deep hibernation of a rodent. Their body temperature falls only a few degrees, their metabolism drops by around half rather than to a trickle, and they can rouse and defend themselves within minutes, which is why a denning bear is best described as being in a state of winter dormancy or torpor.
What should you do if you meet a bear?
Stay calm, do not run, and make yourself known by talking in a low voice and slowly backing away while keeping the bear in view, giving it space and an escape route. Running can trigger a chase, and a bear can easily outrun a person. If a bear stands up it is usually just trying to get a better look or scent, not threatening. For a defensive bear that is protecting cubs or a carcass, back off. For the rare predatory bear that follows and stares, or any black bear attack, fight back aggressively, aiming for the face. Bear spray is the most effective deterrent where it is legal, and in bear country the priority is avoiding surprise encounters by making noise and storing food securely.
Why is the panda a bear if it eats only bamboo?
Genetically the giant panda is unambiguously a bear, the earliest branch to split from the rest of the family, and it keeps the digestive system of a carnivore, a short simple gut with no specialised chambers for fermenting plants. It has adapted to bamboo only through behaviour and a few tweaks, a false thumb from an enlarged wrist bone for gripping stems, strong jaws and flat molars, and a switched-off taste receptor for meat. But it digests bamboo poorly, extracting only a fraction of the nutrition, which is why a panda must eat 12 to 38 kilograms a day and spend most of its waking hours feeding. It is a bear making the best of a strange, low-quality diet.
Are polar bears really white, and are they marine mammals?
A polar bear's fur is not white but made of hollow, colourless, translucent hairs that scatter light and look white or cream, over black skin that absorbs heat. In some conditions the fur can look yellow from oils, or even green in zoos when algae grow inside the hollow hairs. As for classification, the polar bear is formally listed as a marine mammal because it depends completely on the sea, hunting seals from the sea ice, swimming long distances between floes, and spending much of the year on the frozen ocean rather than on land. It is the only bear given that status, and the loss of Arctic sea ice is the central threat to its survival.