|Bear moving toward the forming pack ice at the western edge of Hudson Bay.|
C. Michael Hogan PhD.
November 18, 2008
U. maritimus spends so much of its annual time at sea that the species is often classified as a marine mammal; however, it is more commonly deemed the largest terrestrial carnivore.. There are nineteen genetically indistinguishable populations of this species, and they are known to interbreed. The Polar Bear is a champion swimmer, capable of an uninterrupted trip of 65 kilometers across the open Arctic Ocean or polynyas, and a much longer seasonal marine migration. This species is threatened by overharvesting and by climate change. In the short term, killing of U. maritimus is not sustainable at the present rates of legal and poaching takes; as far as the intermediate term, the IUCN has asserted that projected global warming will likely lead to extirpation of the species within 100 years.
DISTRIBUTION AND MIGRATION
U. maritimus has an extremely large home range, amounting to hundreds of thousands of square kilometers. The global circumpolar distribution is limited to the northern hemisphere and has been observed as high as 88 degrees N, whilst the southern extreme is constrained by the occurrence of pelagic ice pack.(Servheen) In the Bering Sea, the southern limits are the Pribilof Islands and St. Matthew Island. Iceland is the southern range extreme in the North Atlantic. The thirteen North American populations have a southern limit of Hudson Bay, southeast Labrador and St. James Bay; the northern limit of the terrestrial habitat range in North America is the shoreline of the Beaufort Sea.
Most of the species members are comprised by thirteen North American populations, for which there exists the greatest amount of scientific data. There are five recognizable Eurasian populations: Barents Sea, Tunu, Kara Sea, Laptev Sea and Chukchi Sea. The greatest amount of time spent at sea is in the biologically productive areas of the southern Arctic, (Derocher) where seals and lower members of the food chain concentrate, in contrast to the deepwater northern Arctic Sea.
The Polar Bear spends a large fraction of its life at sea, either in swimming or in resting/hunting on pack ice. Males generally are at sea for more days of the year than females and are correspondingly stronger swimmers. Northerly populations tend to be at sea to a greater extent than southerly populations. For example the Beaufort Sea and Chukchi Sea Polar Bears migrate north in summer to access the permanent pack ice, whereas the Hudson Bay population seeks land when its seasonal ice melts. Mothers migrate to the ice with cubs and males migrate singly.
Males may attain a length of 2.6 meters, with a maximum female length of 1.9 meter. Weights vary considerable by season, and adult males vary from 300 to 1002 kg, while females attain only half the typical male weight. The world record size belongs to a male at Kotzebue Sound, who weighed 1002 kg. Cubs typically weigh less than one kilogram at birth, but reach ten to fifteen kg in spring when leaving the den and 100 to 150 kg by age one. The neck and muzzle are elongated, which are well adapted for poking under ice to attack seal prey. Vibrissimae are simply vestigial and the large canine teeth are conical.
The architecture of U. maritimus follows Allen's rule, with stocky limbs and abbreviated ears to limit heat loss by minimizing surface area relative to body mass. The massive paws can reach 25 to 28 cm in width and exhibit numerous papillae and perimeter fur to aid in gripping ice. Seven cm long curved non-retractable claws adorn this five-toed bear to aid in gripping ice and attacking prey. A blubber layer of up to ten cm provides effective thermal insulation. Skin color is black and the hollow guard fur is a very pale yellowish-brown, allowing visual discrimination in snowscapes. The undercoat is a a dense underwool, to conserve heat both for swimming and terrestrial modes.
|Juvenile males in mock-fight preparing for the real combat that will ensue years later.|
Females are sexually mature by age four; although males are mature at a similar age, due to need for successful combat amongst peers, the male is unlikely to mate before age six. Mating typically occurs with advent of milder weather and may endure to mid June, often on ice sheets. In males the onset of elevated testosterone as well as lutropin and prolactin. A male will often herd a female to a remote area to protect her from other suitors. Copulation induces ovulation. Uterine embryo implantation is typically time delayed, a phenomenon thought to involve adaptation, with the female body "foretelling" the optimum time for birthing. Total gestation, including the time delay, amounts to 28 to 38 weeks.
The female bear accesses a maternity den as early as 13 weeks prior to parturition, with consequent litter sizes typically ranging from one to three. Maternity dens are often at some distance from the marine pack ice; for example on the Hudson Bay Plain, dens are chiefly found in the Black Spruce forests of Wapusk National Park, a location approximately 100 km from Hudson Bay. Most cubs are born between late November to early January, and complete weaning does occur until age two (slightly earlier in southern climes), although cubs are taught to hunt by their mother, and achieve part of their sustenance by carnivory prior to weaning The female breeds only every two to four years. The greatest cause of Polar Bear death is starvation, with resultant lifespans estimated to be 20 to 25 years. Cub mortality is ten to 30 percent, while adult mortality is eight to 16 percent per annum.
Winter retreat to an earth or snow den is a maternity strategy for pregnant Polar Bears, but is simply a fasting and hibernation strategy for other individuals. Metabolic rates then subside by 30 to 50 percent, (Hellgren) with a resulting low serum urea to creatinine ratio in hibernating U. maritimus. Ringed seals followed by Bearded seals are the preferred food, probably resulting in the high cholesterol levels (151-638) found (Bossart) in Polar Bears. Other marine mammals consumed include Harp seals, Hooded seals and Walrus. The iconic hunting mode is to wait on ice near a seal's breathing hole, and attack when the seal is near enough to the water surface. Even young Beluga whales and Narwal are occasionally taken. Swimming speeds of ten km/hr are achieved in aquatic attacks, while terrestrial pursuit manifests velocities of 40 km/hr. (Novak) The fastidious Polar Bear often rinses paws and face with water after dining.
|Polar Bear rolling in newfallen snow near the west coast of Hudson Bay|
When U. maritimus is constrained to land before sea pack ice formation, an assortment of terrestrial species are consumed comprising less than ten percent of the annual diet: grasses, sedges, mosses, broad-leaved herbs and berries such as Empetrum nigrum; in my November, 2008 observations at Churchill I also observed Polar Bears consuming dwarf birch and willow roots as well as beached kelp. More rarely Polar Bears will attack and eat Musk Ox or Rangifer tarandus. Occasionally Polar Bears will stalk and consume birds such as Branta canadensis, Branta leucopsis, Anser caerulescens and Clangula hyemalis. Infrequently this bear will invade bird nests and consume eggs and chicks of certain gull species such as Larus hyperboreus or auks such as Alle alle. Alopex lagopus and Corvus corax are common scavengers cleaning up carcass remains from Polar Bear kills of seals; while I didn't observe direct scavenging I noted the correlation of proximity, density and activity of these two scavengers at Churchill to the increasing presence and activity of U. maritimus in the annual migration to the forming marine pack ice.
Visual acuity is keen with ability to recognize a moving object at several kilometers. (Kolenosky) Olfactory perception is also keen with detection of submerged seals up to one kilometer distant. Many of the bears I observed in western Hudson Bay in November, 2008 presented a behavior of pointing the muzzle upwards and waving the head about in an evident attempt to enhance scents. This behavior was exaggerated when in proximity to other bears, and was evidently used to select which bears to avoid or approach. Snow rolling is another interesting behavior I observed during the November migration to ice. While the full basis for this activity is unclear, there is likely a correlation to the need for stretching, since the behavior was most common after bears had risen from a 24 to 48 hour sleep refuge during blizzard conditions. It is also possible that the activity is used to cool the bear down, since it often came after an active walk or run across the snowfield; it is well known that Polar Bears easily overheat after sustained exercise, especially at temperatures that are less than winter nadirs. Finally it is possible that the rolling motion is invoked as a scratch reflex or simply a sign of carefree joy. Other authors have noted this rolling behavior and linked it to the playfulness and social development of the species, especially when multiple bears participate in sequence. (Herrero)
The worldwide population of U. maritimus is 22,000 to 25,000 individuals, but there are several threats to this species, classified as vulnerable by the IUCN. Historically hunting of Polar Bears is documented to have existed since the 14th century, but taking by indigenous peoples is known to have occurred much earlier based upon archaeological data. Numerical takes are inventoried as early as 1784 in the Magdalena Fjord on Spitzbergen, where native Russian Pomors killed 150 Polar Bears.
Extensive hunting is conducted through illegal poaching and legal taking, the latter particularly by native peoples. For example, in 2005 Canada expanded its allowed take by Nunavut tribes to 518 bears.(CBC) The Nunavut conduct active lobbying to defend and expand this rate of taking, and much of the kill is for mercenary rather than tribal consumption purposes, based upon export sales and selling of hunting licenses by the Nunavut. As recently as 2005, Greenland began to permit limited recreational killing of U. maritimus for the first time. Russia has advanced its protection, but is not able to curtail some ongoing poaching. The USA has enacted strict legislation which not only bans import of Polar Bear products, but potentially allows secondary enforcement against actions that aggravate global warming. I have made population dynamics calculations that show the present level of poaching and legal kill exterminate about 60 percent of the adult female recruitment, after natural mortality is factored into the lengthy reproduction cycle. Clearly this level of take is not sustainable, without even factoring in the threat of climate change.
Global warming is a significant threat by shortening the pack ice seasonal endurance and hence the time available for seal hunting by U. maritimus. In addition, early melting of large sheets of drift ice can also expose Polar Bears to endangerment by forcing the fattened bear to navigate an unexpectedly early and more expansive water crossing to reach land or northern permanent ice. Finally the presence of poly-chlorinated biphenyls in Arctic waters is an accumulative toxin to bears, who consume at the highest end of the food chain, where this lipophilic toxin is concentrated. As a final note on climate change impacts, one must consider the threat of decreased freshwater flow from basins whose precipitation diminishes; this effect will mitigate against the rich estuarine concentrations of seals at locations such as the mouth of the Churchill River, where seals are drawn to the confluence of freshwater inflow. Furthermore, any reduction in freshwater inflow at these rich estuaries will impede ice formation in a sea now enriched with salts.
* C. Servheen, S. Herrero and B. Peyto (eds.) (1999) Bears. Status Survey and Conservation Action Plan. IUCN/SSC Bear Specialist Group, Gland, Switzerland & Cambridge, UK . ISBN 2-8317-0462-6
* Andrew E. Derocher, Nicholas J. Lunn and Ian Stirling (2004) Polar Bears in a Warming Climate, Integrative and Comparative Biology 44 (2): pages 163–176
* E.C. Hellgren (1998) Physiology of hibernation in bears, Ursus, International Association for Bear Research and Management, USA, vol. 10, pp. 467-477
* G.D. Bossart, T.H. Dierauf and D.A. Duffield Clinical Pathology. In: CRC Handbook of Marine Mammal Medicine, Second Edition
* R.M. Novak (1999) Walkers Mammals of the World Sixth Edition'', The Johns Hopkins University Press, Baltimore, USA
* G.B.Kolenosky (1987) Wild Furbearer Management and Conservation in North America: Polar Bear, Ontario Ministry of Natural Resources, Ontario, Canada, pages 474-485
* Stephen Herrero (2002) Bear Attacks: Their Causes and Avoidance, Globe Pequot, 304 pages ISBN 158574557X
304 pages* CBC News, 10 January 2005, Nunavut hunters can kill more polar bears this year