Tiergarten Nürnberg

Polar bears are the earth's largest living predators on land. Being at the end of the food chain, they are often called "Kings of the Arctic". Observed more closely in the polar light and environment, they are kings without land subjects, hermits of the desert of pack ice which is haphazardly broken up, torn apart, and thrust together again by wind and water currents.
Because their main source of food, the ringed seal, leads a solitary life, polar bears are forced to become solitary creatures, too. Since the amount of food available varies greatly throughout the year, polar bears often have to fast for several months. When they get too hungry, both sexes can turn into cannibals.

Again and again, hunters have reported that young or weak polar bears were hunted, killed and eaten by stronger ones. Since the females are distinctly smaller than the males, they are probably victims more often. In 2006, researchers from the US scientific authority, Geological Survey (USGS), reported three cases of cannibalism which they put down to lack of food. They discovered the partially eaten body of a polar bear mother who had been dragged from her snow den by a male polar bear. In another place, they found a polar bear eating a cub he had presumably dug out of its den.

The half-eaten body of a female polar bear on Herschel Island, a Canadian island in the Beaufort Sea, was particularly revealing. Small paw prints in the snow led to the conclusion that the cub had been able to escape. So the polar bear had not particularly targeted the young, but killed from hunger.

Biologist always prick up their ears when they hear about an adult male animal attacking a helpless young animal. Not because they consider this behaviour to be deviant or even immoral, but because infanticide is quite common in a large number of mammals, right through to gorillas.

Each individual strives for greatest possible reproductive success. This behaviour seems to be determined by genetics, and this is why researchers talk of the "selfish gene". It is not the preservation of the species but the selfishness of genes, the passing on of one's own genetic information which is the driving force in the animal kingdom. If an adult male kills young of his species who are still being suckled, the female will be ready to mate again in the near future. This is also true for polar bears: a female who loses her young, will be "on heat" again in the same spring. In contrast, when she successfully rears her young, the interval between births is at least three years.

This "calculation" looks completely different from the female bear's point of view. She has already reproduced her genes and does not want to endanger her "success". Female bears with young therefore always give sexually mature males a wide berth. The adult male can, of course, also look for a childless mate, and therefore the danger of infanticide is greater in theory than in practice. In the wild, it can be difficult to determine whether the killing of a cub was motivated by hunger or the selfish gene. One thing is certain, though: adult male polar bears constitute a permanent danger to cubs.

Apart from the intimate relationship between mother and child, polar bears are inclined to be solitary. Nevertheless, some groups of polar bears do live together peacefully for a period. Polar bears congregate on beaches, when the ice in the Canadian Hudson Bay melts in the summer. During this season the animals are not competing for food or for sexual partners, so they can show their sociable side. This remains an exception, though.

Apart from the intimate relationship between mother and child, polar bears are inclined to be solitary. Nevertheless, some groups of polar bears do live together peacefully for a period. Polar bears congregate on beaches, when the ice in the Canadian Hudson Bay melts in the summer. During this season the animals are not competing for food or for sexual partners, so they can show their sociable side. This remains an exception, though.

Particularly during the mating season, between late March and late May, violent fights occur between adult males. The main weapons used in these fights are the canine teeth. Adult male polar bears have longer canine teeth than females, in relation to the size of their jaws. Since both sexes hunt in the same way, such differences can only be explained by the competition between males: polar bears with strong and long canine teeth will win more rival fights than others. This results in selection favouring stronger canine teeth.

Differences between the sexes are very common in the animal kingdom. They have different causes and functions. Polar bear males are significantly bigger and heavier than females, although this is a disadvantage when hunting nimble seals. This sexual dimorphism – the scientific term for the anatomical differences between males and females of a species – can be found in mammals in all species where the animals do not live in a monogamous relation, i.e. where the male mates with more than one female. Attentive observers can deduce certain facts about a species' social system just by comparing their body sizes.

Female polar bears only rear five cubs on average in the course of their lives. Since the cubs stay with their mother for about 2.5 years, only a third of the female population is available for mating in any one year. This increases the pressure of competition among males. Females mate successfully at the age of four to five years, but whether their first litter will survive into adulthood, is another story. Male animals become sexually mature at about five to six years of age. But generally they cannot assert themselves successfully against the older males at this stage so their actual reproductive age will be eight to ten years. By then they are fully grown. Competition among males is heightened by the fact that each female is only fertile for a maximum period of four weeks. In zoos, fertile periods of only 8 to 23 days have been observed.

Polar bears are polygamous. Nevertheless during the mating season, couples may form and stay together for a few days. Then the male looks for another partner. Sometimes, individual females also trail a train of up to seven males. There is little research on whether the female mates with different males or in the end prefers the strongest male.

Polar bears copulate several times in a row. Scientists assume that ovulation is only triggered by one or more copulations. Males have a penis bone (baculum) about 20 centimetres long, allowing intercourse to last from ten to forty minutes. It is possible that ovulation in females is stimulated by this penis bone. The fertilized egg "waits" at the end of the Fallopian tube in a 64-cell. It is only when the female has amassed sufficient fat reserves to be sure of surviving the pregnancy and very strenuous nursing period that the egg lodges in the womb in September/October.

Mathias Orgeldinger