Biology

Biology of Bears

General characters
Bears are generally bulky and robust animals with relatively short legs.  Unlike other land carnivores, bears stand and walk on the soles of their feet rather than on their toes.  They distribute their weight toward the hind feet which makes then look lumbering when they walk.  Bear can stand on their hind feet and sit up straight with remarkable balance.  Bears have non-retractable claws which are used for digging, climbing, tearing and catching prey. Their ears are rounded (Garshelis et al. 2009).
Dentition
  Unlike most other members of the Carnivora, bears have relatively undeveloped carnassials teeth, and their teeth are adapted for a diet that includes a significant amount of vegetable matter. The canine teeth are large, and the molar teeth flat and crushing. There is considerable variation in dental formula even within a given species. It has been suggested that this indicates bears are still in the process of evolving from a carnivorous to a predominantly herbivorous diet.  Polar Bears appear to have secondarily re-evolved fully functional carnassials, as their diet has switched back towards carnivore.  The dental formula for living bears is 3.1.2-4.2/3.1.2-4.3 (Bunnel and Macdonald 1984).

Reproduction
The bear's courtship period is very brief.  Bears in northern climates reproduce seasonally, usually after a period of inactivity similar to hibernation, although tropical species breed all year round. Cubs are born toothless, blind, and bald.  The cubs of Brown Bears, usually born in litters of 1–3, will typically stay with the mother for two full seasons.  They feed on their mother's milk through the duration of their relationship with their mother, although as the cubs continue to grow, nursing becomes less frequent and learn to begin hunting with the mother.  They will remain with the mother for approximately three years, until she enters the next cycle of estrus and drives the cubs off. Bears will reach sexual maturity in five to seven years.  Male bears, especially Polar and Brown Bears, will kill and sometimes devour cubs born to another father in order to induce a female to breed again.  Female bears are often successful in driving off males in protection of their cubs, despite being rather smaller (Garshelis et al. 2009).

Behaviour
While many people think that bears are nocturnal, they are in fact generally diurnal, active for the most part during the day.  The belief that they are nocturnal apparently comes from the habits of bears that live near humans which engage in some activities, such as raiding trash cans or crops, are nocturnal in order to avoid humans.  The Sloth Bear of Asia is the most nocturnal of the bears, but this varies by individual and females with cubs are often diurnal in order to avoid competition with males and nocturnal predators.  Bears are overwhelmingly solitary and are considered to be the most asocial of all the Carnivora. Liaisons between breeding bears are brief, and the only times bears are encountered in small groups are mothers with young or occasional seasonal bounties of rich food (Garshelis et al.  1999).

A Asiatic Black Bear claw mark

Winter dormancy
Many bears of northern regions are assumed to hibernate in the winter. While many bear species do go into a physiological state called hibernation or winter sleep, it is not true hibernation. In true hibernators, body temperatures drop to near ambient and heart rate slows drastically, but the animals periodically rouse themselves to urinate or defecate and to eat from stored food. The body temperature of bears, on the other hand, drops only a few degrees from normal and heart rate slows only slightly. They normally do not wake during this "hibernation", and therefore do not eat, drink, urinate or defecate the entire period. Higher body heat and being easily roused may be adaptations, because females give birth to their cubs during this winter sleep. It can therefore be considered a more efficient form of hibernationbecause they need not awake through the entire period, but they are more quickly and easily awakened at the end of their hibernation. They have to stay in a den for the whole hibernation (Garshelis et al. 2009).

Nature
The nature of bears Biologically, bears are large-bodied members of the mammalian order Carnivora, family Ursidae. They evolved from smaller, tree-climbing, predatory ancestors (Miacids) about 25 million years ago. The Polar Bear is the largest bodied of the modern bear species and the largest non-aquatic carnivore in the world. Adult males may weight from about 350 to over 650 kg. (Stirling 1988). Most modern bears, including the Brown or Grizzly Bear (U. arctos), the American Black Bear and Asiatic Black Bear (U. americanus, U. thibetanus), the Sun (Honey) Bear (Helarctos malayanus), and the Spectacled Bear (Tremarctos ornatus), are dietary generalists, ingesting a variety of concentrated energy sources such as fruits, nuts, insects, fish, carrion, and mammals. Mammals such as Moose (Alces alces) and Caribou (Rangifer tarandus) are usually only killed when they are easy to catch, such as when crippled or newly born. Nutritious, and easy to digest green vegetation is also eaten, especially when more concentrated energy sources are unavailable. Others bear are Giant Panda and Sloth Bear.