2011-12-30 / Columnists

The Sea Bear

Drawing On Science


Polar Bear,Ursus maritimus Polar Bear,Ursus maritimus The Polar Bear is the world’s largest non-aquatic predator living within the borders of the United States (Alaska), Canada, Russia, Norway and Greenland. It evolved some 100,000 - 250,000 years ago from brown (as in Grizzly) bear ancestors. In the course of the evolutionary changes from Grizzly to Polar, the Polar bear developed the “Roman nose,” a distinguishing feature of the smaller Black bear. (The Grizzly’s nose is concave or dished-in.) From its large, round posterior forward to its head with the somewhat pointed Roman nose, a Polar bear presents a distinct silhouette: long and tapered. It is highly adapted to living in the harsh, freezing environment of the arctic region both on the ice and in the water. In fact, its scientific name, Ursus maritimus, means sea bear, a name well suited to its swimming abilities in the arctic waters. The name was first used by a British naval officer, Commander C.J. Phipps, in 1774 when he observed Polar bears’ ability to swim.

A Polar bear has two layers of fur: a dense, insulating undercoat overlaid by guard hairs of various lengths. The fur shafts are hollow and pigment free so that they are transparent. This structure makes the fur appear white because the visible light spectrum is reflected and scattered. Prior to molting the fur may have a yellowish tinge due to the oils consumed when the bear feeds. A Polar bear appears whitest in sunlight, especially when its fur is cleanest just after the molting period beginning in the spring and ending in summer. The fur covers the bear’s black skin under which is a layer of fat some 4.5 inches thick. But it is the two layers of fur that actually insulate the animal by keeping its body warm so that it can survive in temperatures as cold as 50 degrees below zero Fahrenheit. A Polar bear’s small, round ears and compact tail are also factors in conserving heat, preventing too much of it from escaping. This fact is one of the major reasons why the bear is threatened: global warming (climate change) will result in the animal’s inability to survive excessive heat, i.e., the warmer weather will cause the bear to overheat.

The arctic ice is a requirement for Polar bears. They depend on the ice to feed and to use as a platform when they tire of swimming long distances. When hunting for seals a Polar bear, using its powerful senses of smell, sight and hearing, will locate a seal either on the surface of the ice or find an opening in the ice used by seals for breathing. This opening is known as a lead. The breathing holes are cut by the seals using the sharp claws on their fore flippers. A seal will surface every five to fifteen minutes for a breath of air. When an opening is located the Polar bear, a very patient animal, will lie in wait for many hours or even days for a seal to show itself. The bear will also stalk seals. When one is spotted on the surface of the ice the Polar bear will slowly crawl towards the unsuspecting prey. When it gets close enough it will pounce on the seal killing it immediately. The bear’s large, thick, curved, two-inch claws are used to grasp and hold prey as well as for defense. The bear’s side teeth are very sharp allowing it to shear chunks of meat from the unlucky seal. The canines are equally sharp, but longer, and are used for grasping and holding prey. If food is scarce, the bear will reduce its metabolic rate until such time that food becomes abundant again.

The Polar bear’s dependence on ice and ice flows is the second reason why global warming threatens the animal’s existence. Ice flows normally retreat in the summer so that the Polar bear must follow the ice in order to feed. This journey may cover hundreds of miles. Ice that has not melted will be very far from land and the open water between the ice and the shore is subjected to waves. This condition makes it even harder for the animal to swim from shore to the ice to seek food. When there is no sea ice during the summer the Polar bear must live on land where there is little or no food available; it is stranded until fall, especially in the southern portions of its range like Hudson Bay, Canada. It must endure this existence until the fall when the sea ice again returns. Coupled with summer melting, the excessive melting of arctic ice due to global warming means less available ice for the bear to rest on after a long swim and to locate and feed on prey. Under this condition the bear can drown or starve to death; examples of which have already been discovered. The US Geological Survey has predicted that climate change will result in the loss of two-thirds of the Polar bears’ population by the year 2050.

A Polar bear is able to travel on the ice because of the structure of its paws. The bear’s paw has a spread of some 12 inches, enabling it to distribute the bear’s weight evenly on the ice. When traveling on thin ice the animal will spread its legs and lower its bulk closer to the ice. This posture allows for even greater distribution of weight. Each paw has black footpads with soft bumps or papillae. Between the papillae are tufts of fur. The combination of papillae, fur and, of course, claws allows the Polar bear to take hold of the ice and walk, stalk, or run with ease. It prefers to walk slowly and leisurely and can run as fast as a horse for short distances. The 12-inch paws play an important role in swimming as well. Forepaws are used as paddles pulling the bear forward, while the hind paws act as rudders steering the animal in a chosen direction.

An adult male Polar bear weighs in at 775 to 1200 pounds; however, a few individuals have been found to weigh over 1200 pounds. Adult females reach 330 to 650 pounds. Most Polar bears are active throughout the year. Unlike their Brown and Black bear cousins, they do not hibernate (actually bears don’t really hibernate, but that’s another story) in a den.

A pregnant female will feed heavily in August and Sept-ember to gain as much weight as possible. She will soon scrape out a tunnel in a snowdrift at the end of which she prepares two chambers. She will then remain in a state of semi-hibernation giving birth, in early winter, to twin cubs, but one or three cubs are also possible. The family will emerge from the den in March or April at which time mom will begin to hunt for food.

Questions/comments? E-mail Steve: Drawingonscience@aol.com.

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