Polar Bears In Depth
Reproduction
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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 195265 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 areas, the minimum successful reproductive interval for polar bears is 3 years (see below).
Newborn polar bears have hair, but are blind and weigh only 0.6kg (Blix and Lentfer 1979). The growth of cubs is very rapid, and they may weigh 1012 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and cubs may increase their weight by an order of magnitude between den exit and their first birthday (S. C. Amstrup, unpublished data).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).
The exact timing of birth may vary across the range of polar bears. Harington (1968) reported births as early as 30 November with a median date of 2 December. Derocher et al. (1992), reported, based on progesterone spikes in the blood of pregnant bears and the implied date of implantation, that births of Hudson Bay bears probably occur from mid-November through mid-December. Messier et al. (1994) suggested that polar bears give birth by 15 December. In contrast, many pregnant female polar bears in the Beaufort Sea did not enter dens until late November or early December (Amstrup and Gardner 1994; S. C. Amstrup, unpublished data). Unless those bears were giving birth immediately on den entry, a later date of birth can be assumed. One captive female in Barrow, Alaska gave birth on 27 December, corroborating that assumption (Blix and Lentfer 1979). Similarly, LØnØ (1972) reported that implantation of the conceptus into the uterus of the polar bear began in November, around the peak of den entry in the Beaufort Sea. The timing of implantation, and hence that of birth, is likely dependent on body condition of the female. Condition of the female, in turn, depends on a variety of environmental factors. The interaction between environmental and physiological factors that control births is clearly an area in need of further research.
Newborn polar bears have hair, but are blind and weigh only 0.6kg (Blix and Lentfer 1979). The growth of cubs is very rapid, and they may weigh 1012 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).
The exact timing of birth may vary across the range of polar bears. Harington (1968) reported births as early as 30 November with a median date of 2 December. Derocher et al. (1992), reported, based on progesterone spikes in the blood of pregnant bears and the implied date of implantation, that births of Hudson Bay bears probably occur from mid-November through mid-December. Messier et al. (1994) suggested that polar bears give birth by 15 December. In contrast, many pregnant female polar bears in the Beaufort Sea did not enter dens until late November or early December (Amstrup and Gardner 1994; S. C. Amstrup, unpublished data). Unless those bears were giving birth immediately on den entry, a later date of birth can be assumed. One captive female in Barrow, Alaska gave birth on 27 December, corroborating that assumption (Blix and Lentfer 1979). Similarly, LØnØ (1972) reported that implantation of the conceptus into the uterus of the polar bear began in November, around the peak of den entry in the Beaufort Sea. The timing of implantation, and hence that of birth, is likely dependent on body condition of the female. Condition of the female, in turn, depends on a variety of environmental factors. The interaction between environmental and physiological factors that control births is clearly an area in need of further research.