shaggygod.proboards.com/ Common Names. Polar bear, nanook, nanuq, nanuk, ice bear, sea bear, eisbar, isbj0rn, white bear.
Scientific Name. Ursus maritimus Phipps (1774) first described the polar bear as a species distinct from other bears and gave the name Ursus maritimus.
Subsequently, alternative generic names including Thalassarctos, Thalarctos, and Thalatarctos were suggested. Erdbrink (1953) and Thenius (1953) settled on Ursus (Thalarctos) maritimus, citing interbreeding between brown bears (Ursus arctos) and polar bears in zoos. Kurten (1964) described the evolution of polar bears based on the fossil record and recommended the name Ursus maritimus as adopted by Phipps (1774). Harington (1966), Manning (1971), and Wilson (1976) subsequently promoted use of the name Ursus maritimus, and it has predominated ever since.
istribution and Population. Polar bears occur only in the Northern Hemisphere. Their range is limited to areas in which the sea is ice covered for much of the year. Over most of their range, polar bears remain on the sea-ice year-round or visit land only for short periods. Polar bears are common in the Chukchi and Beaufort Seas north of Alaska. They occur throughout the East Siberian, Laptev, and Kara Seas of Russia and the Barent's Sea of northern Europe. They are found in the northern part of the Greenland Sea, and are common in Baffin Bay, which separates Canada and Greenland, as well as through most of the Canadian Arctic Archipelago. Because their principal habitat is the sea-ice surface rather than adjacent land masses, they are classified as marine mammals. In most areas, pregnant females come ashore to create a den in which to give birth to young. Even then, however, they are quick to return to the sea ice as soon as cubs are able. In some areas, notably the Beaufort and Chukchi Seas of the polar basin, many females den and give birth to their young on drifting pack ice (Amstrup and Gardner 1994).
Polar bears are most abundant in shallow-water areas near shore and in other areas where currents and upwellings increase productivity and keep the ice cover from becoming too solidified in winter (Stirling and Smith 1975; Stirling et al. 1981; Amstrup and DeMaster 1988; Stirling 1990; Stirling and 0ritsland 1995; Stirling and Lunn 1997; Amstrup et al. 2000). Despite apparent preferences for the more productive waters near shorelines and polynyas (areas of persistent open water), polar bears occur throughout the polar basin including latitudes >88°N (Stefansson 1921; Papanin 1939; Durner and Amstrup 1995).
Because they derive their sustenance from the sea, the distribution of polar bears in most areas changes with the seasonal extent of sea-ice cover. In winter, for example, sea-ice extends as much as 400 km south of the Bering Strait, which separates Asia from North America, and polar bears extend their range to the southernmost extreme of the ice (Ray 1971). Sea-ice disappears from most of the Bering and Chukchi Seas in summer, and polar bears occupying these areas may migrate as much as 1000 km to stay with the southern edge of the pack ice (Garner et al. 1990, 1994). Throughout the polar basin, polar bears spend their summers concentrated along the edge of the persistent pack ice. Significant northerly and southerly movements appear to be dependent on seasonal melting and refreezing of ice near shore (Amstrup et al. 2000). In other areas, for example, Hudson Bay, James Bay, and portions of the Canadian High Arctic, when the sea-ice melts, polar bears are forced onto land for up to several months while they wait for winter and new ice (Jonkel et al. 1976; Schweinsburg 1979; Prevett and Kolenosky 1982; Schweinsburg and Lee 1982; Ferguson et al. 1997; Lunn et al.1997).
Until the 1960s, the prevalent belief was that polar bears wandered throughout the Arctic. Some naturalists felt that individual polar bears were carried passively with the predominant currents of the polar basin (Pedersen 1945). Researchers have known for some time that is not the case (Stirling et al. 1980, 1984). However the advent of radio telemetry (Amstrup et al. 1986), including the use of satellites (Fancy et al. 1988;
Harris et al. 1990; Messier et al. 1992; Amstrup et al. 2000), detailed knowledge of polar bear movements was not available.
EVOLUTION. The polar bear appears to share a common ancestor with the present-day brown bear. It apparently branched off the brown bear lineage during the late Pleistocene. Kurten (1964) suggested that ancestors of the modern polar bear were “gigantic.” Although still the largest of the extant bears, the polar bear, like many other mammals, has decreased in size since the Pleistocene. Also, significant morphological changes have continued within the last 20,000–40,000 years, perhaps through the present (Kurten 1964). Stanley (1979) described the many recently derived traits of polar bears as an example of “quantum speciation.”
Evidence of polar bear evolution contained in the sparse samples of fossils has been strengthened recently by molecular genetics. Whereas traits of fossil teeth and bones from polar bears clearly indicate their brown bear origins, fossil remains include only a handful of specimens (Kurten 1964). Genetic data from extant bears can provide phylogenetic information unavailable in the fossil record. Shields and Kocher (1991) first analyzed mtDNA sequences and showed a close relationship between brown bears and polar bears. Cronin et al. (1991) then discovered that mtDNA of brown bears is paraphyletic with respect to polar bears. That is, the mtDNA of brown bears of the Alexander Archipelago in southeastern Alaska is more closely related to the mtDNA of polar bears than it is to the mtDNA of other brown bears. Cronin et al. (1991) reported that mtDNA sequence divergence between Alexander Archipelago brown bears and polar bears is only about 1%, whereas a divergence of about 2.6% separates polar bears from brown bears occurring elsewhere. Cronin et al. (1991) and Cronin (1993) emphasized that mtDNA sequence divergence trees are not species trees and that mtDNA is not, by itself, a good measure of overall genetic differentiation. Nonetheless, these relationships provide a compelling argument regarding the origin and evolution of polar bears.
Following the discovery of Cronin et al. (1991), others corroborated the finding of paraphyletic mtDNA in brown bears and polar bears. Talbot and Shields (1996a, 1996b) suggested that the Alexander Archipelago brown bears represent descendents of ancestral stock that gave rise to polar bears. This stock may have survived Pleistocene glaciers in an ice-free refugium in southeastern Alaska, isolated from brown bears in other Pleistocene refugia (Heaton et al. 1996). This island-dwelling ancestral stock apparently has remained isolated from the more recent mainland bears by broad ocean passages.
Talbot and Shields (1996b) found mtDNA sequence divergence rates similar to those reported by Cronin et al. (1991), and proposed that ancestors of the Alexander Archipelago brown bears diverged from the other mtDNA lineages of brown bears 550,000–700,000 years ago.The mtDNA sequence divergences also suggested that polar bears branched from the Alexander Archipelago ancestral stock of brown bears about 200,000–250,000 years ago, a date closely corresponding with that suggested in the fossil record (Thenius 1953; Kurten 1964).Shields and Kocher (1991) and Cronin et al. (1991) reported that the mtDNA nucleotide sequence divergence between brown and polar bears (grouped together) and black bears was 7–9%. Applying the substitution rate (6%/million years) for mtDNA genes reported by Talbot and Shields (1996a) to the sequence divergence reported by Cronin et al. (1991) suggests that brown bear ancestral stock diverged from that of black bears approximately 1.2–1.5 million years ago. This “molecular clock” estimate may be low. The fossil record suggests black bears diverged from the brown bear lineage 1.5–2.5 million years ago.
Cronin (1993) cautioned that mutation rates vary among genes as well as among taxa, and that conclusions based on “molecular clocks” must be viewed with caution and in the context of other evidence. For example, mtDNA sequences for two functional nuclear genes, K-casein and the DQß gene of the major his to compatibility complex, show polyphyletic relationships among the three species of bears (M. Cronin and S. Amstrup, unpublished data). That is, the DNA sequences do not resolve the relationships among the species. These functional genes are presumably under strong selection and do not diverge as rapidly as mtDNA. Nonetheless, the mtDNA analyses indicate that Alexander Archipelago brown bear derive from more ancient stocks and are more closely related to polar bears than are other members of the brown bear clan. These conclusions also corroborate the recent appearance of the polar bear in the fossil record and the more ancient roots of the black bear (Thenius 1953; Kurten 1964). All DNA evidence, regardless of some areas of uncertainty, corroborate conclusions from the fossil record that the polar bear is a recently derived species and is undergoing rapid evolution. The extreme arctic marine environment is undoubtedly exerting strong selection pressures for rapid adaptation.
FEEDING HABITS. The polar bear is more predatory than other bears and is the apical predator of the arctic marine ecosystem. Polar bears prey heavily throughout their range on ringed seals (Phoca hispida) and, to a lesser extent, bearded seals (Erignathus barbatus). Ringed seals apparently have been a principal food of polar bears for a significant portion of their co evolutionary history and ringed seal behaviors appear to be oriented around avoidance of polar bear predation. Stirling (1977) contrasted the behavioral ecology of ringed seals and Weddell seals (Leptonychotes weddelli). Steady predation pressure from polar bears may have led ringed seals to use sub nivian birthing lairs and to interrupt spring and summer basking with frequent periods of scanning their surroundings for predators. Weddell seals, on the other hand, evolved in the Antarctic system, where surface predators are absent. They give birth unsheltered on the surface of the sea ice, and they are so ambivalent about activities on the ice surface that human researchers often can walk right up to them for study purposes (Stirling 1977).
Although seals are their primary prey, polar bears also have been known to kill much larger animals such as walruses (Odobenus rosmarus) and belugas (Delphinapterus leucas) (Stirling and Archibald 1977; Kiliaan et al. 1978; Smith 1980, 1985; Lowry et al. 1987; Calvert andStirling1990).The heaviest prey may be taken mainly by large male polar bears (Stirling and Derocher 1990), and unusual circumstances may be required. Nonetheless, in some areas and under some conditions, alternate prey may be quite important to polar bear sustenance. Stirling and Øritsland (1995) suggested that in areas where the estimated numbers of ringed seals are proportionately reduced relative to numbers of polar bears, other prey species were being substituted.
Overall, polar bears are most effective predators of young ringed seals, perhaps because they are naive with regard to predator avoidance. In spring, polar bears may concentrate their predatory efforts on capture of new-born ringed seal pups (Smith and Stirling 1975; Smith 1980). In some areas, predation on pups is extensive. Hammill and Smith (1991) estimated that polar bears annually kill up to 44%of newborn seal pups if conditions are right. Throughout the rest of the year, polar bears take seals predominantly from the first two year classes (Stirling et al. 1977a; Smith 1980). Whereas abundance of ringed seals may regulate density of polar bears in some areas, polar bear predation may regulate density and reproductive success of ringed seals in other areas (Hammill and Smith 1991; Stirling and Øritsland 1995).
Polar bears apparently digest fat more easily than protein (Best 1984). They seem to prefer the fatty portions of seals (and presumably other animals) to muscle and other tissues. Stirling (1974) reported that polar bears often remove the fat layer from beneath the skin of freshly killed seals and consume it immediately. Because over half of the calories in a whole seal carcass may be located in the layer of fat between the skin and underlying muscle (Stirling and McEwan 1975), a bear that quickly consumes most of the fat available has maximized its caloric return in the minimal amount of time possible. This may be important to all but the largest polar bears because there is considerable competition for kills. Younger and smaller bears often are driven away from their kills by larger bears.
A high-fat and low-protein diet apparently serves polar bears physiologically as well. They are very efficient at recycling nitrogenous products of canabolism, and can use metabolic water released from fat metabolism (Nelson et al. 1983). Digestion of protein requires water, whereas digestion of fat releases water. In a cold environment, free water is available only at the energetic cost of melting ice and snow. The lipophilic habits of the polar bear minimize energy expended to obtain water in winter (Nelson 1981).
Polar bears tend not to cache prey animals they have killed like grizzly bears do (Stirling 1974; DeMaster and Stirling 1981; Stirling and Derocher 1990). This may be another reason why they consume the highest reward portion of their prey first. Although they have not been observed to cache, polar bears are surplus killers. Stirling and Derocher (1990) reported seeing a polar bear kill two seals with in an hour of feeding extensively on another seal. Neither of the latter two seals killed was eaten. Stirling and Øritsland (1995) also have reported surplus killing in polar bears. I once observed a young male polar bear still-hunting at a breathing hole on new autumn ice. There was a partially consumed seal nearby, and between that feeding site and where he was still-hunting were three freshly killed ringed seals stacked like cordwood. When my helicopter approached the bear to capture him, he abandoned his still-hunting site, ran to the pile of dead seals, and covered them with his body as if to protect his stash. This bear apparently had eaten his fill from the first seal but was continuing to hunt, catch, and stack seals despite a low probability that he would consume much of them.
An interesting adaptation to the carnivorous diet, and a difference between polar bears and other temperate and arctic bears, is that only the pregnant females enter dens for the entire winter. Other members of the population continue to hunt seals on the sea-ice throughout the winter. The year-around availability of seals allows denning in polar bears to be strictly a reproductive strategy (affording an acceptable environment for neonates), whereas in most bears it is largely a foraging strategy (avoiding the winter period of food unavailability).
Like other ursids, polar bears will eat human refuse (Lunn and Stirling 1985), and when trapped on land for long periods they will consume coastal marine and terrestrial plants and other terrestrial foods (Derocher et al. 1993). The significance of other foods to polar bears maybe limited, however (LunnandStirling1985; Derocher et al.1993). Over most of their range, polar bears have little opportunity to take foods of shoreline or terrestrial origin. Derocher et al. (1993) found that 31% of pregnant polar bears in the Hudson Bay area fed on berries before denning in autumn. The significance of this to their productivity was not known. Ramsay and Hobson (1991) and Hobson and Stirling (1997) differed in opinions of the value of supplemental terrestrial food. In general, the significance of terrestrial foraging to polar bears is poorly understood.
Clearly the value of alternate foods for polar bears depends on their richness and digestibility. Polar bears are poorly equipped to consume and digest most plant parts (Bunnell and Hamilton 1983), and it seems likely that except for fruiting bodies, plants will contribute little to their energy balance. Lunn and Stirling (1985) found that polar bears using human refuse at a dump maintained their weight or lost less weight than bears not using anthropogenic foods. Some bears using the dump even gained weight, but the supplemental food did not appear to confer a reproductive advantage (Lunn and Stirling 1985). Derocher et al. (2000) reported that some polar bears in Svalbard have become adept at catching reindeer (Rangifer tarandus). Considering the high digestibility of meat, it seems plausible that if readily available, reindeer could be an important alternate food of polar bears. Likewise, in the Beaufort Sea, dozens of polar bears each year have developed a habit of gathering at the butchering sites of bowhead whales (Balaena mysticetus) that are killed by local Native people. The value of this alternate food is apparently great, as nearly every bear seen near whale carcasses in autumn is obese.
Size & Weight. Male polar bears weigh about 375-600 kilograms (825-1320 pounds) while occasional individuals may reach 800 kilograms (1760 lb). They sometimes exceed 250 centimeters (10 feet) in length, measured in a straight line from the tip of the nose to the tip of the tail, although most male polar bears are a bit shorter. They are roughly twice the size and weight of adult females, which weigh 200 to 350 kilograms (440-750 lb) and achieve an adult body length of about 190-220 centimeters (up to about 7 ft). Females first breed at four to six years of age and most often give birth to two cubs in snow dens on land (some cubs are born in dens on the sea ice). Cubs stay with their mothers for two and a half years before weaning which means that unless cubs die prematurely, females do not breed more frequently than every three years. Both sexes live twenty to twenty-five years and sometimes to over 30 years. Their primary prey is ringed seals and, to a lesser degree, bearded seals.
In Hudson Bay, the mean scale weight of 94 males >5 years of age was 489 kg. The largest bear in that group was a 13-year-old, which weighed 654 kg (Kolenosky et al. 1992). The heaviest bear we have weighed in Alaska was 610 kg, and several animals were heavy enough that we could not raise them with our helicopter or weighing tripod. Some animals too heavy to lift have been estimated to weigh 800 kg (DeMaster and Stirling 1981). Females are smaller, with peak weights usually not exceeding 400 kg. Total lengths of males in the Beaufort Sea of Alaska ranged up to 285 cm. Such an animal may reach nearly 4 m when standing on its hind legs and is 1.7 m shoulder height when standing on all four legs. Chest girth for large males is close to 200 cm. Although smaller, females in the Beaufort sea were as long as 247 cm with chest girths up to 175 cm. Only prehistoric polar bears and the giant short-faced bear (Arctodus spp.) of the Pleistocene were of greater stature than today's polar bears (Kurten 1964; Stirling and Derocher 1990).
Manning (1971) suggested there is a cline in size of polar bears across the Arctic. Size increases, he suggested, with distance from east Greenland across the Nearctic to the Chukchi Sea between Alaska and Russia. Manning (1971) also suggested that polar bears from Svalbard may be larger than those from east Greenland. A cline in size across the Palearctic also might occur, but samples from the Russian Arctic are inadequate to confirm it (Manning 1971).
The hypothesized cline was based on measurements made from skulls housed in museums around the world. Unfortunately, the sources of skulls in the various collections were not similar. Of particular note was that many of the skulls originating in the Chukchi Sea may have been donated by trophy hunters. These hunters worked over the ice in teams of aircraft (Tovey and Scott 1957) and were quite effective in killing a great number of the largest polar bears (Amstrup et al. 1986). Another potential problem is that ages of bears in the sample were estimated only by class or life stage. Hence, older bears from one locale might have been compared to younger bears (of the same age class) in another.
Potentially non standardized collection methods prevent any meaningful conclusions about relative sizes of polar bears from different locales. Also, if there is a cline in skull sizes around the world, it appears that body sizes and weights of polar bears do not follow a similar cline. The largest bears for which actual scale weights are known have come from the Hudson and James Bay areas of Canada and from the Beaufort Sea of Alaska, not from the Chukchi Sea. That observation, too, may be subject to some bias, as the most prolonged and intensive polar bear studies have been conducted in Hudson Bay and the Beaufort Sea. Greater numbers of captures in those locations may have increased the probability that very large bears were included in the sample.
Despite their large adult sizes, the young of polar bears are among the most altricial (undeveloped) of eutherian mammals (Ramsay and Dunbrack 1986). Newborn polar bears weigh only 600-700 g. They are blind, only lightly furred, and totally helpless (Blix and Lentfer 1979). Mother polar bears when giving birth commonly weigh over 300 kg, and can weigh 400 kg (Ramsay 1986). If only a single cub is born, the ratio of maternal to neonate weights could be between 400 and 500 to 1. Even with the more common two-cub litter, the ratio of maternal to neonate mass is extraordinarily large (Ramsay and Dunbrack 1986). Cubs grow very fast after birth. In Alaska, they average 13 kg on emergence from the den in late March or early April, with maximum weights of 22 kg. Cubs continue to grow rapidly through their first summer on the sea-ice and some weigh over 100 kg as they approach 1 year of age.
DENNING. Across most of their range, pregnant female polar bears excavate dens in snow and ice in early winter (Harington 1968; Lentfer and Hensel 1980; Ramsay and Stirling 1990; Amstrup and Gardner 1994). They give birth in those dens during midwinter (Kostyan 1954; Harington 1968; Ramsay and Dunbrack 1986) (see section on reproduction), and emerge from dens when cubs are approximately 3 months old. Because neonates are so altricial, the period of denning is essential to their early survival. Recognizing it as a critical phase in the polar bear life cycle, scientists have devoted much attention to aspects of maternal denning.
Denning on the Pack Ice. Although most maternal denning appears to occur on coastlines of main lands and islands, Amstrup and Gardner (1994) discovered that 53% of the dens of polar bears radio-collared between 1981 and 1991 were on drifting pack ice. They also found that 4% were on land-fast ice adjacent to shore. Lentfer and Hensel (1980) recognized the occurrence of dens on pack ice, but suggested that it was limited to bears that could not make it to shore to den. Harington (1968) concluded that denning on ice was not preferred, and Messier et al. (1994) reported no maternal denning on pack ice, although some “shelter denning” on pack ice was observed. The discovery that half of the bears in the Beaufort Sea may den on drifting sea ice, therefore, was not expected.
Claws. The claws of polar bears are shorter and more strongly curved than those of brown bears. They also are larger and heavier than those of black bears (Ursus americanus).They appear to be very well adapted to clambering over blocks of ice and snow and especially to securely gripping prey animals. The claws are normally black, but rarely may, like polar bear fur, lack pigment.
Skull and Dentition. Polar bears share the general ursid dental formula: I 3/3, C 1/1, P 4/4, M 2/3. The first premolars are vestigial and occur in a long diastema or gap between the functional canine and molari form teeth. That gap allows the powerful canines to penetrate deeply into the bodies of seals and other prey without interference from adjacent cheek teeth. Although polar bears apparently evolved from brown bears <250,000 years ago, their teeth have changed significantly from the brown bear form. The cheek teeth are greatly reduced in size and surface area, and the carnassials are more pronounced than in brown bears, reflecting the predatory lifestyle. The teeth of polar bears are well suited to the tasks of grabbing and holding prey and shearing meat and hide. They no longer are as suited to grinding grasses and other vegetation as are those of brown bears. The canine teeth of males are larger and heavier, relative to the size of the jaw, than those of females (Kurten 1955), and the molar arcade of males is longer than in females (Larsen).
Pelage. Polar bears are completely furred except for the tip of the nose. Pelage density is more even than in other ursids, which are often more sparsely furred ventrally and in axillary and groin areas. Even the pads of the feet of polar bears may be covered with hair, especially in late winter. Fur red foot pads may provide a more secure purchase on the slippery sea ice surface and add another layer of insulation between the bear’s foot and the substrate of ice and snow. Under the fur, pads of the feet of polar bears are made up of the same cornified epidermis characteristic of the pads of other bears (Storer and Tevis 1955; Ewer 1973).
The skin of polar bears is uniformly black. Hence, if polar bears lose hair due to physical trauma or disease, they appear from a distance to have black patches on their bodies. Polar bear fur appears white when it is clean and in even sunlight. Because it actually is without pigment, however (Øritsland and Ronald 1978; Grojean et al. 1980), bears may take on the yellow-orange hues of the setting and rising sun and the blue of sunlight filtered through clouds and fog. They appear the whitest right after molting. In spring and late winter, however, many polar bears are “off-white” or yellowish because of oils from their prey and other impurities that have attached to and been incorporated into their hair.
The molt appears to be some what variable, but begins by late April and May. The molt appears to be complete by late summer, and bears captured in autumn have notably shorter coats than those captured in spring. The pelt is thick with a dense under fur and guard hairs of various lengths. Polar bear fur may have a high propensity to take on the colors of environmental impurities because the guard hairs have a hollow medulla (or core) where impurities may lodge. In zoo environments, some species of algae can enter the hollow cores of guard hairs and result in a pronounced “greening” of the fur (Lewin and Robinson 1979).
in a pronounced “greening” of the fur (Lewin and Robinson 1979). Lavigne and Oritsland (1974) noted that polar bears effectively absorb ultraviolet (UV) light, and suggested that could be useful in remote-sensing surveys to enumerate them. The discovery that polar bears appear to absorb UV light led to much speculation about their ability to capture the energy in that light. Popular and scientific reports claimed that the ability to absorb energy in the UV spectrum was an adaptation to help maintain body heat in the rigorous Arctic environment (Anonymous 1978; Grojean et al. 1980; Lopez 1986; Mirsky 1988).Suddenly, the hollow hairs of polar bears, adept at catching algae and other contaminants (LewinandRobinson1979) also were endowed with the powers of optic fibers to funnel UV light to the skin .According to this theory, the skin was black to better absorb such energy without damage. Capturing this high-frequency electromagnetic energy would be a great adaptation for polar bears. This ability has attained the status of an Arctic legend, and contributed to the mystique surrounding the great white bears of the north.
Unfortunately, this supposed adaptation has no basis in fact. Lavigne (1988) and Koon (1998) established unequivocally that the hair of polar bears, although transparent in the visible spectrum, absorbs UV light. If the hair of polar bears absorbs UV light, it does not efficiently transmit UV light. As UV light moves down the shaft of the hair, its energy is absorbed, preventing significant energy from being transmitted to the skin.
REPRODUCTION. Reproduction in the female polar bear is similar to that in other ursids. They enter a prolonged estrus between March and June. In the polar basin, the peak of estrus as evidenced by turgidity of the vulva and vaginal discharge seems to be in late April and early May. Ovulation is thought to be induced by coitus (Wimsatt 1963; Ramsay and Dunbrack 1986; Derocher and Stirling 1992). Implantation is delayed until autumn, and total gestation is 195–265 days (Uspenski 1977), although during most of this time, active development of the conceptus is arrested. Young are born by early January (see below), but stay within the shelter of the den until March or early April (Amstrup and Gardner 1994). Litters of two cubs are most common over most of the polar bear range. Litters of three cubs are seen sporadically across the Arctic, and were most commonly reported in the Hudson Bay region (Stirling et al. 1977b; Ramsay and Stirling 1988; Derocher and Stirling 1992). Young bears will stay with their mothers until weaning most commonly in early spring when the cubs are 2.3 years of age. Female polar bears undergo a lactational anestrus and are available to breed again after weaning. Therefore, in most are as, the minimum successful reproductive interval for polar bears is 3 years (see below).
Newborn polar bears have hair, but are blind and weigh only 0.6 kg (Blix and Lentfer 1979). The growth of cubs is very rapid, and they may weigh 10–12 kg by the time they emerge from the den in the spring. After leaving the den, the rapid growth continues, and cubs may increase their weight by an order of magnitude between den exit and their & first birthday (S. C. Amstrup, unpublished data). Cubs can double their weight between their first and second birthdays. Cubs receive an especially rich milk from their mothers. The milk of polar bears typically has a higher fat content than that of other bears, and in general the milk of bears is richer in fat and protein than the milk of other carnivores (Jenness et al. 1972). Polar bear milk is more similar to that of pinnipeds than it is to milk of most terrestrial mammals (Jenness et al. 1972; Ramsay and Dunbrack 1986). Although polar bears may nurse cubs through their second birthday, some females apparently stop allowing cubs to suckle sometime after their first birthday. The contribution to growth from milk during the second year of life is much lower than during the first year (Arnould and Ramsay 1994). Arnould and Ramsay (1994) noted that fat content begins to decline fairly early in lactation, but the biggest differences are between the first and the second year of the cubs’s lives. Mean fat content of milk provided to cubs of the year was 31.2 ± 1.6%, whereas the fat content of milk fed to yearlings was 18.3 ± 2.4%. The energy contribution from milk is a significant contributor to the observed rapid growth of cubs and comes at a significant cost to mother bears (Arnould and Ramsay 1994).