Tiergarten Nürnberg

Chris Davies

"On ice they hunt, and the prey they hunt may well be humans." Still-hunting, stalking, diving after your prey – smashing, deceiving and playing "dead man": polar bears are intelligent, flexible predators who have learned to survive in a continuously changing, hostile environment. While tigers and lions only become "man-eaters" in exceptional cases, hungry male polar bears deliberately go out hunting for humans. As far as Homo sapiens is concerned, polar bears are the most dangerous mammal on earth.

Females with cubs avoid other polar bears. They only become as threat to humans if their young are in immediate danger. It is different for males, particularly adolescent males who are not yet very successful hunters and therefore always threatened by hunger. Out of 20 encounters with polar bears when humans were killed or injured, about two thirds are presumed to have been raids, i.e. the victims were not forewarned. This was reported by Canadian researchers who examined all incidents with polar bears in Manitoba and the North-West Territories between 1965 and 1985. During this period, six people were killed, 16 were injured, and at least 230 bears were killed.

There is hardly a travel log from the Arctic which does not report at least one dangerous encounter with Nanook. This is the name given by the Inuit to the "eternal vagabond", who "has no shadow". Hungry polar bears rip open tents and break into houses, they snatch at photographers who lean too far out of the bus window, and even try catching helicopters from the sky during lift-off.

But there are also encounters of a completely different kind, particularly on land when the animals fast and save energy. In 1981, Russian polar bear researcher, Savva Uspenski, presented his startled colleagues at a convention in Oslo with photographs from Northern Siberia showing a woman standing at the door of her house feeding a large male polar bear.

Nevertheless: "The only safe bear is a far-away bear." This is a statement from a children's book published by the polar bear emergency call programme in Canadian Churchill. The rules stated there, of course, also apply to adults: "Move away slowly … always looking at the bear." "Do not turn around and run." If the bear should be very close: "Curl up, with your hands behind your neck, and pretend to be dead."

Out on the ice, in Nanook's hunting grounds, there is only one rule: avoid any similarity to a seal! For polar bears are highly specialized seal hunters who live in continuous interaction with their prey. Without seals, there would be no polar bears. If there had not been seals, brown bears would never have dared step on the ice. If there had been no seals, the ancestors of Ursus maritimus would never have developed a white fur and transformed from land mammal to marine mammal and from omnivore to carnivore.

With an estimated population of up to seven million individuals, ringed seals are the most common seal species in the Northern Polar Sea. They reach an average length of 1.25 metres and weight of 65 kilograms. This makes them the smallest representative of the seal family. Nevertheless, polar bears prefer ringed seals to the larger species, such as hooded seals, bearded seals and harp seals.

Ringed seals feed mainly on fish and shellfish. They do not establish colonies, but live as loners. And they live almost exclusively on the ice, for on land – without the protection of a colony – they and their young would be easy prey.

In autumn, when the ocean slowly freezes over, all marine mammals face the same problem: they have to come up for air at regular intervals. Using their strong claws, ringed seals dig holes in the ice layer which can be up to 2-3 metres thick, maintaining these breathing holes all winter. Their breathing holes resemble upside-down funnels, only about 15 centimetres wide at the top so that bears cannot climb in. Ringed seals typically use three to four, sometimes even six breathing holes. In winter, the holes are usually covered with snow. Some are even linked with ice or snow lairs dug by the seals which offer protection from the extreme cold and omnipresent polar bears. The distance between these lairs can range up to 4.5 kilometres.

A typical ringed seal cave comprises several chambers. Between mid March and mid April, the females give birth to one pup, which weighs between four and five kilograms. Since the seal pups are rather inexperienced and cannot flee into the water, they are easy prey for polar bears. Unfortunately, pinnipeds have a strong and distinct body odour. Researchers assume that polar bears can smell a seal's breathing hole at a distance of at least one kilometre, even if it is hidden under a metre of snow.

Things get particularly dangerous, if continuous rain makes the snow roof of the maternity den melt. Then there can be veritable massacres: "The bears simply went from seal to seal, killing them all", observed polar bear researcher, Ian Stirling, in 1979 on the South-East coast of Baffin Island. Whenever Nanook has the choice, he will hunt on one-year-old thin ice with a thin covering of snow which affords very few opportunities for seals to hide.

In spring and early summer, the table is richly furnished for Ursus maritimus. At no other time of year do polar bears weigh more. But as early as mid summer, the young seals take to the open sea and Nanook has to think up other ways to survive.

Polar bears presumably have more hunting strategies than any other predator. In summer they resort to "aquatic stalking". The bears "spy out" the seals' resting places and breathing holes, swim or dive to where they assume their prey to be (diving time up to 72 seconds) and suddenly leap up onto the ice floe. Or they sneak up on water flows formed in the pack ice. When the white hunters crawl up, flat on their bellies, they are almost invisible in the ice grooves.

Camouflage and deception are very much part of the polar bears' business. Often, polar bears run over the ice against the wind and in a zig-zag line, using any cover available. Ian Stirling observed a female polar bear crawling through an ice groove, with her behind up in the air, pretending to be a small "iceberg". Inuits report that Nanook even pushes a block of snow, hiding behind it in order not to be recognized. Some observations would point to polar bears using tools: they allegedly use blocks of ice to break open snow dens or breathing holes. The white hunters may also drift in the water, completely motionless. By the time the seal notices that the "ice floe" has teeth, it is often too late.

The following hunting strategy seems particularly "devious" and almost unworthy of the "king of the Arctic": after breaking open the ringed seal's snow den using the sledge-hammer method (by standing on his hind legs and letting the top of his body crash down on the den's roof), the polar bear kills the seal pup, puts it aside and then closes up the hole in the roof with his body, head first. Thus, no sunlight will shine into the water. As soon as the mother seals dives close without a worry, to look after her young, she will be overwhelmed. After all, a nursing mother has a lot more fat than her pup.

Polar bears are able to convert 84% of the protein and 97% of the fat content of their prey into their own protein and fat. Fat plays a particularly important role, since water is set free when fats are "digested." If the bear obtained this water from its deeply frozen environment, by eating snow, for example, it would need to spend valuable energy on the melting process. Far north, survival depends mainly on the energy balance. An adult ringed seal supplies "fuel" for eleven days, with about 2 kilograms of seal fat securing the daily needs of an adult polar bear of average activity level.

It seems that polar bears are very good at estimating the nutritional value of their prey very accurately. In summer you can observe polar bears in Hudson Bay, happily wandering through a colony of geese without attacking the birds. Physiological experiments measuring the energy consumption of polar bears on a treadmill explained this strange behaviour: if a polar bear of 320 kilograms' weight needs more than 12 seconds to catch a snow goose, he uses up more calories hunting than the prey would provide. The most common hunting technique used by polar bears is still-hunting. In winter, when the oceans are frozen over, there is no other alternative; they have to wait in front of a breathing hole until a seal comes up for air. Usually, the bear lies on its belly, with its chin on an ice ledge – a comfortable and energy-saving position.

The white hunters have to keep absolutely still in order not to forewarn the seal. Experiments have shown that footsteps on the ice can be clearly heard under water from a distance of 400 metres. Sometimes, the bear waits for its chance for over an hour. Often in vain.

As soon as a careless seal comes up, things happen in a flash. The bear bites the seal, or hits it with his paw, grabs it and throws it on the ice where it kills the seal with several well-aimed bites in the head. Most attacks, however, are unsuccessful, and even in the high season, the hunt is only successful every four to five days.

If the prey was captured close to the water's edge, Nanook pulls it a few metres further onto the pack ice before starting to eat in haste. Generally, the first thing eaten is the high-calorie fat - there is always the fear that some bigger and stronger polar bear might come along. After eating, the bear washes itself until its fur is camouflage white again.

The polar bear’s fur is as white as that of a baby ringed seal. Ian Stirling does not believe in coincidence. Rather he sees the white camouflage colour as a further proof for co-evolution, i.e. the parallel and mutually influential development of polar bears and ringed seals in their habitat, sea ice.

This may be illustrated by a comparison with Antarctic Weddell seals, who do not have to fear any land predators. The fur of their young is grey, and in addition, the adult animals are noticeably larger and heavier than ringed seals, another indication of lack of hunting pressure. A small, agile seal can both hide more easily and flee faster than a bigger one.

While the Weddell seals of the Antarctic fall fast asleep on the ice, ringed seals are very attentive. No ringed seal would ever defecate at its resting place, while Weddell seal regularly do this. Being threatened by polar bears – according to Stirling – even determines the ringed seals' social behaviour. Ringed seals are widely distributed to make it as difficult as possible for their hunters to catch a number of them at once? A male can therefore claim no more than one female. On the other hand, Weddell seals are polygamous, with a bull surrounded by as many as eight females. This development of polar bears is rooted in their evolutionary history. Today's almost existential interaction between polar bears and their habitat on the pack ice does, however, not stop the animals from going on land every now and then. The white descendants of brown bears were spotted as far as 150 kilometres inland. It is in particular pregnant females who retire to their traditional denning areas on land.

The polar bear's favoured habitat remains the habitat of the ringed seal, ideally in the circumpolar pack ice belt near the 200 m depth line which does not completely freeze over, even in winter. Because of its biological diversity, Savva Uspenski called this region the "Arctic life belt".

But not every polar bear is born on the "icy side" of life and is able to hunt seals all year round. The populations in Hudson Bay, for example, have to cope with the fact that in summer the sea ice melts almost entirely. They have to build up sufficient fat stores in spring so that they can survive the seal-less summer. Pregnant females who retire to their birth den before the ice comes back, possibly have to face a fast lasting up to nine months. Nature can cope with this problem, too. No other mammal can manage to survive as long without food or water. The king of the Arctic is a master of fasting. After a short phase of 10-14 fast days, the animal can lapse into a state which physiologically is very similar to "hibernation" although the bear is still wandering about. In English, the term "walking hibernation" is used to describe this state. It can obviously be "switched on and off" by the polar bear, depending on the amount of food available, and even independent of the season.

From the outside you cannot see what particular physiological state a polar bear is in at any given time. A polar bear on land could be on a "fasting trip" or feeding off berries, mosses, lichens, grass, mussels and birds’ eggs in brown bear style. It is able to hunt birds and small mammals (ground squirrels, lemmings, voles) or rummage through rubbish heaps and food stores. In July 1778, 32 polar bears were observed fishing for salmon together with three black bears. Some individuals still hunt for fish today, but this is not typical behaviour for their species.

A polar bear on land is something of a caricature. With the exception of pregnant females, polar bears only seem to go on land if they absolutely have to. Whether they mainly fast when on land or whether terrestrial flora and fauna provide them with necessary food supplements, is still a topic of much debate among researchers.

Since polar bears, probably due to their massive legs and paws, use up twice as much energy in running as most other mammals, their appetite for game is limited. Just because of the danger of overheating, they are not able to hunt reindeers, caribous or muskoxen. But as ever, exceptions prove the rule.

The carcasses of marine mammals, however, are part of the white bears' normal diet. In extreme cases, up to 50 animals assemble around a dead whale for feeding together. There is evidence for hunting attacks on walruses and narwhals. Arctic foxes, scavengers usually suffered to deal with leftovers, may also suddenly end up on the polar bear's menu. Long-tailed ducks and other sea birds have to reckon with occasional underwater attacks. Sometimes, polar bears dive up to five metres deep for seaweed.

When Beluga whales get trapped by ice in winter, often several polar bears gather around the "breathing hole" which slowly freezes up. Again and again, they attack the helpless whales with their paws. In this and similar situations, polar bears are surplus killers. There are reports that on one occasion at least 40 Beluga whales were dragged up on the ice. None of the inhabitants of the Arctic, however big they might be, can be sure that they will not end up in a polar bear's stomach one day.

But the king of the northern polar sea is not invincible either. There is evidence for attacks by killer whales, Greenland sharks and walruses. And sometimes wolf packs hunt polar bear cubs on their way from the birth den to the coast. Some wolves distract the mother, while the others get the cub.

Mathias Orgeldinger