Brown bears enter a winter resting period between October and December. They usually dig a den which they may use for several consecutive years. Natural caves or rock fissures sometimes also serve as retreats. Before the winter sets in, the bears cushion their dens nicely with grass, leaves, ferns, moss and lichen. This cosy hole is ideal for dozing, because brown bears are not 'true' hibernators like, for example, hedgehogs. Although the frequency of their heartbeat and breathing slows down, the bears are easily woken and are able to defend themselves in the case of attack.   

Before their winter rest, bears must eat enough to form a decent layer of fat since they lose around a third of their body weight during this sleeping phase. Incidentally, the scientific community is divided about how to describe the bears' resting condition: as winter sleep, winter rest, torpor, hibernation – all these terms are in circulation. But it is always about saving energy during a time of scarce food.

It is often falsely assumed that bears are driven into their dens by the cold. But contrary to us humans, the bear is relatively insensitive to minus degrees, snow and frost. Bears use the rich autumn time when fruits containing plenty of fat and sugar such as nuts, beechnuts, seeds, berries, etc. can be harvested in order to gorge themselves fat. This layer of fat enables their survival in winter, under the condition of reduced energy consumption. For the latter, rest is an important precondition.

Bears go into torpor, which should not be confused with the deep hibernation of smaller mammals such as hedgehogs and marmots. During hibernation, the marmot reduces its metabolic rate to a tenth of its normal level: it breathes only once or twice per minute and its heart beats a maximum five times. Its body temperature drops to only three degrees Celcius. 

The bear, on the other hand, reduces its circulation, breathing and heartbeat only to a level at which it is able to defend its den at any time. If it were to reduce the temperature of its heavy body, which weighs several hundred kilogrammes, to three degrees Celsius, it would be unable to 'get into gear' without an external energy source. The exact manner in which it manages its energy balance, the shape of its temperature curves and its restricted kidney function –  this all largely remains a mystery.  

These questions are of great interest to human medicine. Taking into account that bears lie around for weeks without suffering a breakdown of bone or muscle mass, nor does the skin develop sores, a link can be made to one of the biggest challenges faced by geriatric medicine. And while kidney patients often need life-long dialysis, the bear can seemingly switch its kidney function 'on and off' when necessary.

The mating season for bears begins in May and June. During this period, the animals abandon their solitary way of life. Once a male has found a suitable female partner, he must proceed with caution –  first he must win her confidence. If the female does not accept his advances, he might get swiped by a paw. If she accepts him, the two roam together for a while, until they mate. Then they part ways and both look out for potential new partners. This way, the chance of fertilisation is bigger.  

Fertilised egg cells do not nest in the female's uterus until autumn (up to then they are dormant) and only develop once the bear has eaten enough to put on sufficient weight for the winter. This is the reason why most cubs are born at around the same time – approximately February – in the winter den. In lean years, the egg cells sometimes die because these young would probably not survive. 

Bear cubs are born after a gestation period of around 180-270 days. The newborn bears are naked, blind and toothless and are completely dependent on the mother. A litter usually consists of one to three cubs that weigh only 300 to 400 grammes at birth, and are around 30 centimetres long. The mother's milk, which is rich in fat, makes the cubs grow fast: after four months they already weigh four to five kilogrammes.  

In the next three years, they learn everything about a bear's life from their mother. They explore their surroundings with curiosity and make their own discoveries. But they always return to the mother who nurses them during the entire rearing phase. During this time, the mother animal is not ready for new young. This can sometimes cause a male intent on mating to kill her cubs in order to sire his own offspring. Only around half of the young survive the first three years. When rearing is over, the mother firmly drives away her offspring, especially the males. This serves to protect her from incest. Female young are allowed to remain in the territory for longer.

The brown bear, known as Ursus arctos, is a widely distributed species. Brown bears can be found in various regions of the world, including North America and Eurasia. Read 10 interesting facts about the giant mammal here.