Polar bear (Ursus maritimus) COSEWIC assessment and status report: chapter 2

2. Species Information

2.1 Name and Classification

Class

Mammalia

Order

Carnivora

Family

Ursidae

Subfamily

Ursinae

Scientific name

Ursus maritimus Phipps (1774), no subspecies

Common names

Polar bear, Ours blanc, Nanuk, Nanuq, Wapusk

Gentry (2001) provides an historical summary of the nomenclature of the polar bear. In brief, Linnaeus(1758, p. 47) first referred to the polar bear as “Ursus maritimus albus-major, articus ,” but he did not consider the polar bear as a distinct species from the brown bear (Ursus arctos). Phipps (1774, p. 185) was first to describe the polar bear as its own species. Gentry (2001) writes that, based on the rules of nomenclature, the appropriate authority and date of the specific name of Ursus maritimus is Phipps (1774) and not Linnaeus (1758), as is sometimes observed.Alternative generic names have included Thalassarctos, Thalarctos, and Thalatarctos; however, since the 1960s most authors have used the name Ursus maritimus.

2.2 Morphological Description

Polar bears are large ursids most comparable in size and shape to the brown bear, from which they differ because they lack the characteristic shoulder hump, have a smaller and less dish-shaped head, a longer rostrum, and an elongated neck. Compared to brown bears, the grinding surfaces of the cheek teeth of polar bears are more serrated, which is an adaptation to a more carnivorous diet.The claws of the polar bear are smaller and sharper than those of the brown bear, and the forepaws are enlarged making them useful for paddling in water, collapsing roofs of subnivian birth and haul-out lairs of seals, digging through or climbing on snow and ice, and dispatching seals. Unlike other bear species, pads of the paws of the polar bear are entirely furred, which may function to help insulate the feet or improve traction on ice and snow. Polar bear skin is black, which improves absorption of solar energy. Translucent hair makes the pelage appear white, especially right after moulting, although it may appear yellow or off-white during summer. Fur of the polar bear will often reflect the colours of the sky and snow, which provides camouflage while hunting. Sexual dimorphism is pronounced. Male polar bears can weigh up to 800 kg and reach 2.8 m in length from nose to tail (DeMaster and Stirling 1981). Females are smaller, usually not exceeding 400 kg and 2.5 m (Amstrup 2003).

2.3 Evolution and Genetic Description

Fossil evidence suggests that polar bears evolved from brown bears sometime within the last 400,000 years (Thenius 1953; Kurtén 1964). This finding is supported by data on molecular genetics that suggests divergence of the species occurred as recently as 200,000–250,000 years ago. In particular, the polar bear has been linked to a clade of brown bears that occur today in the Alexander Archipelago of southeast Alaska (Cronin et al. 1991; Talbot and Shields 1996a,b). Cronin et al. (1991) reported that mitochondrial DNA (mtDNA) of polar bears and brown bears of the Alexander Archipelago differ by only 1%; however, divergence of more than 2.5% separates polar bears from brown bears occurring elsewhere. Brown bears are thought to have survived in refugia in the southern Alexander Archipelago during the Late Wisconsin glacial maximum of 20,000 years ago (Heaton and Grady 1993); however, since the polar bear evidently separated from this clade long before the first of the Wisconsin glaciations (commencing ca. 70,000 years ago), it is unlikely that polar bears evolved specifically in the Alexander Archipelago.

The close relationship between the polar bear and brown bear is highlighted by instances of hybridization in the wild (e.g., a hybrid bear was recently harvested near Banks Island; personal communication of Gau [2006]). Successful, interspecific matings in captivity have been observed for many years, with clear evidence of first- and second-generation fertility in offspring (Martin 1876, 1882; Kowalska 1962, 1965, 1969).

The molecular ecology of polar bears has most recently been studied by Paetkau et al. (1995, 1999), Crompton (2004), and Saunders (2005). As discussed in more detail in Section 3.3, there are varying degrees of genetic differences between most identified subpopulations of polar bears in Canada (Figures 1 and figure2; Tables 1 and table2); however, these differences do not identify them as genetic subunits or subspecies. Paetkau (1999) states that, with respect to identified management subpopulations across the circumpolar range of the polar bear: “no genetic discontinuities [have been] found that would be consistent with evolutionarily significant periods of isolation between groups.”

Since the Late Pleistocene, notable morphological change has occurred in the polar bear. In particular, there has been a trend toward a decrease in body size, fossil polar bears being much larger than they are today (Kurtén 1964). Polar bears evolved to take advantage of killing seals from a sea-ice platform, particularly ringed seals (Pusa hispida [prior to 2003 was under the genus Phoca, see IUCN 2006]) and bearded seals (Erignathus barbatus). Many of the physical traits of polar bears can be viewed as adaptations to hunting arctic seals.

Figure 1. Circumpolar distribution of the polar bear. Abbreviations of delineated subpopulations include Viscount Melville Sound (VM), Norwegian Bay (NW), Kane Basin (KB), Lancaster Sound (LS), Baffin Bay (BB), Davis Strait (DS) Southern Hudson Bay (SH), Western Hudson Bay (WH), Foxe Basin (FB), Gulf of Boothia (GB), M’Clintock Channel (MC), Southern Beaufort Sea (SB), and Northern Beaufort Sea (NB). Source: IUCN/SSC Polar Bear Specialist Group (2006).

Figure 1. Circumpolar distribution of the polar bear.

 

Figure 2. Canadian subpopulations of polar bears. Abbreviations of delineated subpopulations include Viscount Melville Sound (VM), Norwegian Bay (NW), Kane Basin (KB), Lancaster Sound (LS), Baffin Bay (BB), Davis Strait (DS) Southern Hudson Bay (SH), Western Hudson Bay (WH), Foxe Basin (FB), Gulf of Boothia (GB), M’Clintock Channel (MC), Southern Beaufort Sea (SB), and Northern Beaufort Sea (NB). Source: Taylor et al. (2001) and PBTC (2007).

Figure 2. Canadian subpopulations of polar bears.

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