The very low reproductive rate of polar bears means that there must be a high rate of survival to maintain population levels. In fact, polar bears "defer" reproduction in favor of survival when foraging conditions are difficult (Derocher et al. 1992). A complete reproductive effort is energetically expensive for polar bears. So, when energetically stressed, female polar bears will forgo reproduction rather than increase risks by incurring the energetic costs of the reproductive process. The reproductive cycle lends itself to convenient early termination if that is appropriate (Ramsay and Dunbrack 1986; Derocher and Stirling 1992). Many radio-collared female polar bears in the Beaufort Sea region entered dens and then abandoned them early without cubs (Amstrup and Gardner 1994). Others lost cubs shortly after emerging from their den and bred again that same spring. Bears leaving dens early may have resorbed their fetuses or may have experienced a pseudopregnancy (Derocher et al. 1992). In any event, they did not complete a full reproductive cycle.
Breeding takes place in the early spring, long before the female can assess whether she will secure sufficient resources to bring her pregnancy to fruition. After fertilization, if she has been able to secure sufficient reserves, birth and rearing can follow pregnancy with some reasonable probability of success. Polar bears, however, also are equipped to abandon a reproductive effort if reserves are insufficient. Because implantation is delayed many months (Wimsatt 1963), and because neonates are so undeveloped (Blix and Lentfer 1979; Ramsay and Dunbrack 1986), early stages of reproduction are relatively inexpensive. Termination of the reproductive process, through abortion or resorbtion of the fetus or failure to nurse after birth, costs a female relatively little (Derocher et al. 1992). The biggest maternal investment begins with postpartum lactation (Ramsay and Dunbrack 1986). Even after emergence from the den, however, it is not unusual for females in poor condition to lose their cubs (Amstrup and Gardner 1994; Amstrup and Durner 1995). Polar bears that terminate a pregnancy and leave their dens early or lose their cubs in early spring usually breed again, preparing them for an opportunity to successfully rear cubs the following year if conditions improve.
In the Hudson Bay region during the 1980s, the survival rate of more than 200 cubs from spring through the ice-free period of autumn was 44% (Derocher and Stirling 1996). Although less mortality was thought to occur after the ice returned in autumn, first-year survival clearly was lower than the 48% reported by Larsen (1985). The body mass of cubs was a significant determinant of survival during early life that included the ice-free period of food deprivation. The mass of cubs, of course, is at least partly dependent on the mass of their mother. Survival of Hudson Bay cubs (N > 400) from their first to their second autumn was 35% (Derocher and Stirling 1996). Annual survival of yearlings ranged from 43% to 53%. The survival estimates Derocher and Stirling (1996) calculated for cubs >1 year old were derived from bears that were actually captured.
Because many in that age class were independent of their radio-collared mothers, they were not recaptured or reobserved, and their fate was not known. Hence, these must be considered minimum survival values, and likely are below the actual values.
In the Beaufort Sea, survival of cubs was approximately 65% from den exit to the end of their first year of life. Yearlings fared much better, with 86% surviving to weaning (Amstrup and Durner 1995). Observations of the young of radio-collared females substantiate the observation from Hudson Bay that most cub mortality comes early in the period after emergence from the den (Amstrup and Durner 1995; Derocher and Stirling 1996), but depart radically from the very minimal yearling survivals observed there. Derocher and Stirling (1996) suggested that a heavy harvest accounted for much of the yearling mortality in Hudson Bay. Nonetheless, only 15% (44% � 35%) of the cubs produced were confirmed to survive through their second autumn. This contrasts with the survival of 56% (65% � 86%) of Beaufort Sea bears surviving until weaning, 5 months beyond their second autumn. If actual values are close to the minimums reported there, the differences in survival could more than compensate for the apparent reproductive differences between bears in the Beaufort Sea and Hudson Bay.
Tait (1980) hypothesized that brown bear females may choose to abandon a single cub on the chance that they might enhance fitness by breeding again and giving birth to twin or larger litters. That concept has resulted in much discussion and debate about parental care and investment in young. Whether or not it makes sense mathematically, such a strategy apparently does not prevail among polar bears. In the Hudson Bay area, single cubs may actually survive at a somewhat higher rate than cubs from larger litters (Derocher and Stirling 1996). Furthermore, deaths of dependent young in the Beaufort Sea were independent of litter size, and cubs were lost at similar rates whether as whole litters or portions of litters (Amstrup and Durner 1995). Parental investment in single polar bear cubs is not different from investment in litters of two or more. Single cubs are often much heavier than twin cubs (S. C. Amstrup, unpublished data), and survival of cubs appears to be heavily dependent on their weight (Derocher and Stirling 1992).
Estimating survival rates of independent polar bears has been an even greater challenge than estimating survival of dependent young. Eberhardt (1985) argued that survival of adult marine mammals must be in the high 90% range for their populations to be sustaining. However, early estimates of survival in polar bears derived by mark and recapture methods were much lower (Amstrup et al. 1986). More recent estimates derived from Hudson Bay, where the intensity of marking exceeds all other study areas, still have ranged between 0.86 and 0.90 (Derocher and Stirling 1995a; Lunn et al. 1997). Only by relying on radiotelemetry monitoring of individual animals have estimates in line with Eberhardt's (1985) theory been developed. Amstrup and Durner (1995) estimated that survival of adult females in the prime age groups may exceed 96%. Although that estimate fits well with population dynamics theory, the fact that it is much higher than estimates derived by other methods suggests added work on polar bear survival is necessary.
Causes of natural mortalities among polar bears are largely unknown. Because polar bears spend most of their time on drifting seaice, dead animals are likely to go undiscovered and cause of death for animals that are discovered is seldom discernible. Therefore, we are forced to extrapolate from a very few observations to understand natural mortality patterns and causes. Accidents involving unskilled young must be a common cause of natural death in the harsh arctic environment (Derocher and Stirling 1996). Starvation of independent young as well as very old animals must account for much of the natural mortality among polar bears. Age-specific differences in hunting success rates have been reported by Stirling and Latour (1978) for polar bears of the central Canadian High Arctic. Cubs of any age spent little time hunting, and were not effective at taking seals in the spring of the year. During summer, the success rate of 2-year-olds was similar to that of adults, although they spent much less time hunting. Young of the year and yearlings were less successful than adults. Cubs abandoned prior to the normal weaning age of 2.5 years likely have poor survival (Stirling and LaTour 1978). That conclusion is corroborated by the dearth of observations of independent bears <2 years old in all populations except Hudson Bay. Also, age structure data show that subadults aged 2�5 years survive at lower rates than adults (Amstrup 1995), probably because they are still learning hunting and survival skills. I once observed a 3-year-old subadult that weighed only 70 kg in November. This was near the end of the autumn period in which Beaufort Sea bears reach their peak weights (Durner and Amstrup 1996), and his cohorts at that time weighed in excess of 200 kg. This young animal apparently had not learned the skills needed to survive and was starving to death. As they age, polar bears that avoid serious injury must simply get too old and feeble to catch food, and thus literally die of old age. Local and widespread climatic phenomena that make seals less abundant or less available also can significantly affect polar bear populations (Stirling et al. 1976; Kingsley 1979; DeMaster et al. 1980; Amstrup et al. 1986).
Injuries sustained in fights over mates or in predation attempts also may lead to natural mortalities of polar bears. Some injuries are immediately fatal. I have seen three instances where a bear has killed another and consumed it. Broken teeth and even broken jaws may frequently result from fighting and failed predation attempts. In brown and black bears, such injuries commonly are not life-threatening. L. Aumiller (Alaska Department of Fish and Game, pers. commun.) has observed several brown bears at Alaska's McNeil River Sanctuary with jaws that had broken and healed in a variety of distorted conformations. D. Garshelis (Minnesota Department of Natural Resources, pers. commun.) captured a 2-year-old black bear with a missing lower jaw. The jaw and all lower teeth were destroyed by gunshot wounds that had largely healed when Garshelis examined the bear in its winter den. The bear was radio-tracked through the following spring and summer and killed by a hunter the following autumn as a normal-size 3-year-old. Brown bears and black bears often survive on a diet including plant parts, fish, insects, small animals, and carrion. A videotape made by the hunter revealed how ingenious the young Minnesota black bear was in feeding without a lower jaw. These and other observations of injured brown and black bears (D. Moody, Wyoming Game and Fish Department, Pers. Commun.; M. Haroldson, USGS Interagency Grizzly Bear Study Team, pers. commun.) suggest they regularly survive with severely damaged mouth-parts, perhaps because of their great adaptability and the small particle size of most of their foods.
FIGURE 27.10. Broken upper jaw of a large male polar bear captured in the Beaufort Sea in autumn 1999. Because they rely on strong jaws to catch and hold large prey animals, such injuries are probably fatal to polar bears. SOURCE: Photo by Steven C. Amstrup. Click image to enlarge.
Injuries to polar bear's mouth parts also may not be immediately fatal, but they probably are deadly in the long run. Despite capture of thousands of polar bears worldwide, confirmed observations of mended jaws or survival of polar bears with broken jaws are rare or lacking. The long penetrating canine teeth are the polar bear's most important trophic appendage and are critical to holding and killing large prey. Polar bears usually cannot switch to a diet of smaller food particles, and a broken jaw may simply reduce hunting efficiency below the survival threshold. I captured an emaciated but very large male polar bear one autumn when he should have been near his maximum weight. His weight was less than half that of similar-size males at that time. He seemed to be fit and his teeth were in excellent shape. On examination, however, we discovered that his maxilla was broken through (Fig. 27.10), and there was a pronounced gap in his palate. The front portion of his upper jaw was attached only by the skin and musculature of his lips. His ability to bite and hold large prey was seriously compromised. How this injury was sustained is not clear. He has not been recaptured, and given the bear's lean state just before the harshest season of the year, I suspect he did not survive the winter.
In addition to trauma of various kinds, an array of maladies occurs at low frequencies in polar bears just as they do with other wild and domestic mammals including humans. For example, a very large male in the Beaufort Sea died of gastric dilatation and volvulus (Amstrup and Nielsen 1989). This is a condition in which the alimentary organs, including the stomach and much of the intestine, rotate around the mesentaries that support those structures in the abdominal cavity. Blood supply is cut off, resulting in edema, shock, and rapid death. This is a phenomenon common in large, deep-chested dogs and in bears in zoos. Another bear apparently died as a result of occlusion of the bile duct by numerous large gall-stones (S. C. Amstrup unpublished data).
Reported diseases and parasites of polar bears are few. In 21 years of research in Alaska, I have not seen any evidence of ectoparasites. In an extensive review of ursid parasites, Rogers and Rogers (1976) found that seven endoparasites had been reported in polar bears. Only Trichinella spp., however, had been observed in wild animals. The three species of nematode and three species of cestodes that had been reported in captive polar bears had not occurred in the wild. Trichinella can be quite common in polar bears and has been observed throughout their range. Concentrations of this parasite in some tissues can be high, but infections are not normally fatal (Rausch 1970; Dick and Belosevic 1978; Larsen and Kjos-Hanssen 1983; Taylor et al. 1985). Arctic foxes (Alopex lagopus) are common carriers of rabies and they routinely interact with polar bears. However, only one instance of rabies has been confirmed in a polar bear (Taylor et al. 1991). Although polar bears are not immune to diseases and parasites, they seem to be plagued by fewer of these problems than most terrestrial mammals.
Male polar bears, like males of other ursids, will kill and eat dependent cubs (Hansson and Thomassen 1983; Larsen 1985; Taylor et al. 1985; Derocher and Wiig 1999). Although this activity does not account for a large percentage of the mortality, it is a curious cause of death in young bears. A male bear that kills cubs fathered by another probably confers some survival advantage to cubs he fathered by eliminating possible competitors for resources. Also, female bears undergo a lactational anestrus. By killing her cubs, a male interrupts that anestrus, and theoretically could breed with the female, inducing her to have his cubs rather than the cubs of some other male. Infanticide, therefore, is a mechanism by which males can increase their relative fitness.
To increase his fitness in this manner assumes that male bears recognize their own cubs. Clearly, with all of the risks to a conceptus that occur between breeding and emergence of cubs onto the sea-ice in spring, there is no selective advantage to a male if he kills cubs he fathered a year before. For the benefits of infanticide to be maximized, the male also must have some reasonable assurance of being around when the female comes back into estrus. In terrestrial bears with limited home range sizes and the ability to defend definable territories, it may be reasonable for a male bear to keep track of a female during the several days between loss of her cubs and onset of estrus. For polar bears, with no territories or other restrictions on movements, the likelihood of a male remaining with a female for that period seems small. In two cases of infanticide I observed in the Beaufort Sea, the male and female involved were already separated by dozens of kilometers the day after the cubs were killed, and they were going in opposite directions. In one case, the male and female were >200 km apart 2 weeks after the male killed her cubs. At least in that case of infanticide, it seems very unlikely that breeding was the goal of the male.
Polar bears will eat the flesh of their own kind, and often a bear that kills another will eat it. The killing of young cubs is probably not motivated by predatory instincts. Small cubs provide a very limited amount of energy, especially considering the risk of injury to a predatory male imposed by the defending female. Males that kill cubs may not even consume them (Derocher and Wiig 1999; S.C. Amstrup, unpublished data), perhaps due to their limited energy value. In terrestrial bears, harassment, or infanticide by large males may be a mechanism of densitydependent population regulation (McCullough 1981; Young and Ruff 1982; Stringham 1983). Derocher and Wiig (1999) also speculated that infanticide may be a density-dependent phenomenon, increasing in frequency with population size. Harassment of subadults by adult males at scavenging sites (Smith 1980) also may be an important regulating factor among polar bears. Infanticide has been detected more often in the Svalbard area than in other parts of the polar bear range where relative densities may be lower (Taylor et al. 1985). In all areas, however, frequencies of infanticide and cannibalism appear to be low enough that understanding their significance to population regulation is difficult. Infanticide in polar bears may be nothing more than an atavistic trait carried over from their terrestrial ancestors, and quantitative effects male polar bears have on their population are unknown.
|
