Polar Bears In Depth
Survival
Page 5 of 5
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 Males that kill cubs may not even consume them (Derocher and Wiig 1999; S.C. Amstrup, unpublished data), perhaps due to their limited energy value.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.
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.