Bears

Bears are large carnivores of the family Ursidae. They are members of the order Carnivora, which also includes dogs, cats, and seals, although these animals are in different families than bears. All of these carnivores have a pair of modified teeth in the upper and lower jaw, called carnassials, that are used to tear meat into smaller chunks during feeding. Bears are not strictly meat-eaters, however, and their molars are well adapted for grinding plant food. In fact, bears are opportunistic, omnivorous feeders.

Bears first appear in the fossil record of about 27 million years ago, as a fox-sized animal known as the dawn bear. About 6 million years ago, there were numerous species of bears, some of them huge, but all now extinct. Seven species of bear survive today. The most recent one is the polar bear (Thalarctos maritimus), which evolved from the brown bear (Ursus arctos) only about 70,000 years ago. The most recent extinction was the cave bear (Ursus spelaeus), a huge animal that co-existed with humans as recently as 20,000 years ago. The giant panda (Ailuropoda melanoleuca) is sometimes classified in the bear family, based on morphological and genetic similarity.

All species of bears but one live in the northern hemisphere. The exception is the spectacled bear (Tremarctos ornatus), which inhabits the northern Andes of South America.

Bear species are quite variable in size. The smallest is the Malayan sun bear (Helarctos malayanus), males of which are about 4 ft (1.2 m) long from head to tail, stand about 28 in (70 cm) tall at the shoulder, and weigh up to 440 lb (200 kg). The largest species is the polar bear, which may be up to almost 10 ft (3 m) long, and weigh a ton (about 900 kg). Most male bears, called boars, are considerably larger than the females, or sows. This is especially important when boars compete for females. Sun bears and sloth bears (Melursus ursinus), however, take only one mate and so the sexes are nearly the same size.

Bears from cold regions do not truly hibernate. Instead, during the coldest part of the winter when food is not readily available, they enter a long period of lethargy. However, their body temperature and heart rate do not drop much, as would occur during true hibernation. During this time bears sleep a great deal and do not eat; instead, they live off energy stored in their body fat. The exception to this pattern is the polar bear, which continues to hunt for seals during winter.

Bears have an amazing ability to adjust physiologically to seasonal ecological changes. Non-tropical bears mate during the spring or summer, but the fertilized eggs float free in the uterus, rather than immediately implanting and developing. Later, in early winter, the sow finds or creates a den in which to sleep away the winter. At that time, the eggs implant and their gestation starts. The cubs are poorly developed when born, being blind, nearly hairless, almost helpless, and extremely small. For example, a female brown bear weighing 450 lb (205 kg) produces cubs weighing less than a pound (about 450 g). The cubs are born during the winter, and are so small that they are incapable of regulating their body temperature. The warm den in which the mother winters provides a snug place for them to nurse and grow until they can maintain their body temperature. The sow's milk, which is extremely rich in fat, sustains the rapid growth of the cubs during the winter denning. By the time the sow is ready to leave her den, the cubs have grown enough to follow her.

All bears are thickly furred, often with a coat of a single color. With the exception of the sun bear, all species have fur on the bottom of their feet, around the pads of the soles. This is especially important for the polar bear, which spends most of its life walking on snow and ice. The sun bear, on the other hand, has furless feet as an aid in climbing trees. The feet of bears are well-armed with heavy claws. Bears walk in plantigrade fashion, meaning they walk on the heel and sole of the foot. Most other mammals walk on their toes. Although they may look rather lumpish and clumsy, bears can run for short distances at speeds up to 40 MPH (64 km/h).

Bears have a reputation for having poor eyesight, but their sight is actually quite good. Their sense of smell, however, is extremely acute. Bears can identify the odor of animals that passed by as much as several days before. Unlike other carnivores, bears do not have their lips attached to their gums. This means that they can make facial expressions and use their lips to suck in food, such as insects or honey.