Abstract
BackgroundPolar bears (Ursus maritimus) of the Beaufort Sea enter hyperphagia in spring and gain fat reserves to survive periods of low prey availability. We collected information on seals killed by polar bears (n = 650) and hunting attempts on ringed seal (Pusa hispida) lairs (n = 1396) observed from a helicopter during polar bear mark-recapture studies in the eastern Beaufort Sea in spring in 1985–2011. We investigated how temporal shifts in ringed seal reproduction affect kill composition and the intraspecific vulnerabilities of ringed seals to polar bear predation.Principal FindingsPolar bears primarily preyed on ringed seals (90.2%) while bearded seals (Erignathus barbatus) only comprised 9.8% of the kills, but 33% of the biomass. Adults comprised 43.6% (150/344) of the ringed seals killed, while their pups comprised 38.4% (132/344). Juvenile ringed seals were killed at the lowest proportion, comprising 18.0% (62/344) of the ringed seal kills. The proportion of ringed seal pups was highest between 2007–2011, in association with high ringed seal productivity. Half of the adult ringed seal kills were ≥21 years (60/121), and kill rates of adults increased following the peak of parturition. Determination of sex from DNA revealed that polar bears killed adult male and adult female ringed seals equally (0.50, n = 78). The number of hunting attempts at ringed seal subnivean lair sites was positively correlated with the number of pup kills (r2 = 0.30, P = 0.04), but was not correlated with the number of adult kills (P = 0.37).Conclusions/SignificanceResults are consistent with decadal trends in ringed seal productivity, with low numbers of pups killed by polar bears in spring in years of low pup productivity, and conversely when pup productivity was high. Vulnerability of adult ringed seals to predation increased in relation to reproductive activities and age, but not gender.
Highlights
Reproduction can incur considerable survival tradeoffs, including increased risk of predation
Polar bears of the Beaufort Sea primarily feed on ringed seals (Pusa hispida), and occasionally bearded seals (Erignathus barbatus), both of which reproduce and mate between late March and late May
Ringed seals accounted for 90.2% (406/450) of kills of known species, while bearded seals accounted for the remaining 9.8% (44/450)
Summary
Reproduction can incur considerable survival tradeoffs, including increased risk of predation. Polar bears (Ursus maritimus) are obligate carnivores, and enter a period of hyperphagia during spring, facilitated by the reproduction and mating cycle of their prey [8,9,10]. Hyperphagic behaviour in spring allows polar bears to increase their mass before the onset of the open water season [9], when reduced prey availability can result in the onset of a fasting physiological state similar to hibernation in other bear species [18,19,20,21]. Polar bears (Ursus maritimus) of the Beaufort Sea enter hyperphagia in spring and gain fat reserves to survive periods of low prey availability. We collected information on seals killed by polar bears (n = 650) and hunting attempts on ringed seal (Pusa hispida) lairs (n = 1396) observed from a helicopter during polar bear mark-recapture studies in the eastern Beaufort Sea in spring in 1985–2011. We investigated how temporal shifts in ringed seal reproduction affect kill composition and the intraspecific vulnerabilities of ringed seals to polar bear predation
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