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< > APPEARANCE/ MORPHOLOGY: HEAD AND NECK with literature reports for the Polar bear - Ursus maritimus: Use sub-contents list below, or simply scroll down the page to view findings.

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HEAD AND NECK - Editorial Comment

Editorial Comment

(Editorial Overview Text Replicated on Overall Species page - Ursus maritimus - Polar bear

GENERAL HEAD STRUCTURE:
Adult:
Polar bears have a long neck and a relatively small head with a smooth facial profile and small ears. As with other bears, the vibrissae are vestigial. The skull is massive and the tympanic bullae are not inflated.
Newborn: The ears of newborn cubs are hairless and closed, lying pointing towards the back of the head.

DENTITION: 
Adult:
Polar bears have the dental formula i 3/3, c 1/1, p 2-4/2-4, m2/3, total 34 - 42. The canine teeth are massive and conical, reaching up to 5 cm (2 inches) long (outside the gum) while the molariform teeth have cusps elevated further than those of other bears.
Newborn: --

EYES:
Adult:
The eyes are small and dark brown.
Newborn:
The eyes are closed at birth.

(References are available in detailed literature reports below)

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General Head Structure

Adult

SUMMARY: Polar bears have a long neck and a relatively small head with a smooth facial profile and small ears. As with other bears, the vibrissae are vestigial and the lips are mobile and protrusible. The skull is massive and the tympanic bullae are not inflated.
  • The head is relatively small and flat and the neck long. (B147)
  • The ears are small. (B285.w4)
  • Bears have small ears. (B424)
  • In bears the vibrissae are vestigial. (B399.3.w3)
  • The neck is elongated and the facial profile is smooth. (B406.37.w37)
  • The lips of bears are mobile and protrusible. (B425)
Skull:
  • Bears have a massive skull. The tympanic bullae are not inflated. (B147)
  • Skull length: males 353 - 412 mm; females 311 - 380 mm. (D244)
  • Skull lengths vary between populations, with body size. There is a cline in size, with the largest polar bears being found in the Bering Straight and the smallest in East Greenland. For example, in East Greenland, males had a skull length (condylobasilar length) of 369 +/- 2.6 mm, a skull width (zygomatic breadth) of 228 +/-2.3 mm and skull height (maxilla - supraorbital) of 100 +/- 0.9 mm and for females length 326 +/- 2.3 mm, width 193 +/- 2.1 mm and height 86 +/- 0.9 mm. In southern Alaska, measurements for males were length 407 +/- 4.3 mm, width 257 +/- 3.9 mm and height 116 +/- 1,5 mm, and for females length 344 +/- 3.5 mm, width 202 +/- 3.0 mm and height 91 +/- 1.3 mm. (B406.37.w37)
  • The skull is large, but skulls of large Ursus arctos - Brown bear are larger than those of large polar bears. Compared with Ursus arctos - Brown bear, the skull is proportionally narrower across the palate between the second molars, and the ratio of condylobasilar length to zygomatic width is greater: 1.63 for 150 polar bears versus 1.59 for 279 Ursus arctos - Brown bear. The skull height is lower and they lack the pronounced brow ridge, having a "Roman nose" appearance. (B490.27.w27)

Neonate / Young

SUMMARY: The ears of newborn cubs are hairless and closed, lying pointing towards the back of the head.
  • The ears of newborn cubs are hairless and closed, lying pointing towards the back of the head. In two hand-reared cubs, the ears of both cubs opened fully when the cubs were 45 days old.(J23.11.w3)

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Dentition

Adult

SUMMARY: Polar bears have the dental formula i 3/3, c 1/1, p 2-4/2-4, m2/3, total 34 - 42. The canine teeth are massive and conical, reaching up to 5 cm (2 inches) long (outside the gum) while the molariform teeth have cusps elevated further than those of other bears.
  • i 3/3, c 1/1, pm 4/4, m 2/3 x2 = 42. (B147, B490.27.w27)
  • i 3/3, c1/1, p2-4/2-4, m2/3, total 38 - 42. (D244)
  • i 3/3, c 1/1, p 2-4/2-4, m 2/3, total 34 - 42. (B406.37.w37)
  • The dental formula is 3/3, 1/1, 4/4, 2/3 = 42. (B471.4.w4)
  • Incisors: unspecialised. (D244)
  • Canines: elongated, conical, slightly hooked. (D244)
  • Carnassials: weakly developed. (D244)
  • Premolars: the first premolars are generally rudimentary; the fourth upper premolar lacks a third root. (D244)
  • The canines are large and heavy, particularly in males. The first premolars are vestigial. There is a long diastema between the canines and the functional cheek teeth and the molar arcade is longer in males than in females. Compared to Ursus arctos - Brown bear, the cheek teeth are smaller, with reduced surface area and more pronounced carnassials. Overall, the teeth are adapted for grabbing and holding prey, and shearing meat and hide, rather than grinding vegetation. (B490.27.w27)
  • Compared with other bears, the canines are large and the molariform teeth sharp. (B180.w4)
  • The massive, conical canines can be 5 cm (2 ins.) long outside the gums. (B406.37.w37)
  • The molariform teeth have cusps elevated further than those of other bears. (B406.37.w37)
  • The dentition of bears is designed for crushing food. Compared to more typical carnivores, the carnassials are reduced in size, P4 has rounder, smaller cusps than in canids (Canidae - Dogs, foxes (Family)), while M1 and M2 are expanded and bunodont. (B471.4.w4)

Neonate / Young

  • In hand-reared cubs, the first incisors started to appear at 36 days. Lower incisors started to appear before upper incisors, but upper canines appeared before lower canines. (J23.11.w3)
    • Male cub: day 36, lower left incisor; day 38, lower right incisor; day 48, upper left incisor; day 51, upper left and right canines; day 53, upper right incisor. (J23.11.w3)
    • Female cub: day 36, lower left and right incisors; day 45, upper left canine; day 47, upper right canine; day 51, upper left incisor; day 53, lower left and right canines; day 73, three upper and lower left and right molars present. (J23.11.w3)
  • In a hand-reared cub, the canines were noted to be erupting at 47 days and more teeth were visible at 60 days. (J23.11.w4)

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Eyes

Adult

SUMMARY: The eyes are small and black; the polar bear can pull its well-developed nictitating membrane across the eye.
  • Bears have small eyes. (B424)
  • Small. (B147)
  • Black. (B285.w4)
  • Bears have a well developed nictitating membrane; polar bears can pull the nictitating membrane all the way across the eye. (B399.4.w4)
  • The eyes are dark brown. (V.w100)

Neonate / Young

SUMMARY: The eyes are closed at birth.
  • The eyes are closed at birth. (B288.w11, D244, D247.6.w6, D251.8.w8, J23.4.w1, J23.14.w3)
  • Eyes began to open at 26 days and were fully open at 33 days in one cub; in another cub they started to open at 32 days and were fully open at 39 days. Both cubs were definitely able to see at 58 days. (J23.11.w3)
  • The eyes of a hand-reared cub started to open at 36 days and were fully open at 42 days; the cub was noted to be focusing at 47 days. (J23.11.w4)

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Authors & Referees

Authors

Dr Debra Bourne MA VetMB PhD MRCVS (V.w5)

Referees

Andrew Derocher (V.w100)

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