December is Polar Bear Nativity Month

Polar bears are generally out of sight at this time of year and will be for several more months. Pregnant females will be snug in maternity dens giving birth and all others will be out on the sea ice looking for seals to eat – if they can find them in the dark.

In most areas of the Arctic, December is when polar bear cubs are born, although in southern regions (like Western and Southern Hudson Bay), some may be born in late November and in the far, far north, a few may be born as late as early February.

The actual “date of birth” for polar bear cubs is often back-calculated from when they emerge with their mothers in the spring, because they are born well away from our prying eyes in the dark of the Arctic winter, deep with a snow or soil den dug for that purpose (see previous post here). So our knowledge of the “true” dates of birth in various regions is limited. We have some evidence from native Canadian hunters prior to 1968, when it was both legal and common practice in Canada for Inuit to hunt bears in their dens (Van de Velde et al. 2003), and from a few scientific research expeditions (Amstrup and Gardner 1994; Harington 1968; Ramsay and Stirling 1988; Derocher et al. 1992).

Polar bear cubs, like all bears, are born tiny and rather undeveloped (see Figs 1 and 2 below). Their eyes do not open until about one month after birth. By the time they are 63 days old (two long months after birth, see Toronto Zoo photo here ), their eyes are wide open and they are well furred. Keep in mind, for perspective, that domestic dogs are born after a 63 day gestation period and their eyes open at about 12 days.

Mothers nurse the tiny white newborns for three to four months  after their birth before the family emerges from the den in March-April. The cubs continue to nurse for many months after the family leaves the den.

Figure 1. Toronto Zoo newborn, one day old. This cub was hand-reared. More images here

Figure 1. Toronto Zoo newborn, one day old. This cub was hand-reared.More images here

 Figure 2. Toronto Zoo newborn, 14 days old. This cub was hand-reared. More images here

Figure 2. Toronto Zoo newborn, 14 days old. This cub was hand-reared. More images here

Polar bears, like all bears, have what is called “delayed implantation.” This means that after the polar bear female mates in the spring (during April to May in most areas), her fertilized eggs divide a few times and then stop. The eggs resume dividing (and grow into embryos) about the time that the female crawls into her maternity den for the winter, about September-October.

If a pregnant female has spent the summer on land, she has already fasted for 2-4 months by the time she enters the maternity den. This fast will continue throughout the winter, even while she supports her growing embryos and then nurses the newborn cubs for several months. She will not eat again until the family emerges from the maternity den in the spring (usually in April), when she will feed on the first newborn seals she can catch.

In summary, it appears that most of the world’s polar bears are born in December, with most probably born within a week to 10 days either side of Christmas Day (December 25) – approximately Dec. 15-Jan. 5. That’s a ball park range that probably captures the bulk of polar bear births worldwide.

I do believe that’s something to celebrate.

Amstrup, S.C. and Gardner, C. 1994. Polar bear maternity denning in the Beaufort Sea. Journal of Wildlife Management 58:1-10.

Derocher, A.E., Stirling, I., and Andriashek, D. 1992. Pregnancy rates and serum progesterone levels of polar bears in western Hudson Bay. Canadian Journal of Zoology 70:561-566. [added 12/13/2012, left out by mistake]

Harington, R. C. 1968. Denning habits of the polar bear (Ursus maritimus Phipps). Canadian Wildlife Service Report Series No. 5., Ottawa

Ramsay, M. A. and Stirling, I. 1988. Reproductive biology and ecology of female polar bears (Ursus maritimus). Journal of Zoology London 214:601-634.

Van de Velde (OMI), F., Stirling, I. and Richardson, E. 2003. Polar bear (Ursus maritimus) denning in the area of the Simpson Peninsula, Nunavut. Arctic 56:191-197.

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