We studied behavioral and ecologic effects of differential predation pressure on moose (Alces alces gigas) in the eastern Talkeetna Mountains, Alaska, during 1996–1998. Annual mortality from grizzly bear (Ursus arctos) and wolf (Canis lupus) predation was higher for solitary yearling moose than for dependent yearlings (those with adults) and solitary adults. We tested hypotheses that solitary yearlings have heightened levels of vigilance and greater variation in vigilance responses and spend more time near protective cover than do dependent yearlings or adults. Proportion of time spent vigilant did not differ between solitary and dependent yearlings, and no differences were found between solitary yearlings and adults. However, variation in vigilance increased with distance to protective cover for yearlings but not for adults. Only at distances ≥20 m from protective cover did variation in vigilance differ between solitary yearlings and adults. Increased variation in vigilance at distances far from protective cover, locations where putative vulnerability to predators is greatest, may account for increased mortality among solitary yearlings. Mean distance to protective cover was not different between solitary and dependent yearlings or between solitary yearlings and adults. Because hypotheses regarding differences in vigilance and use of protective cover between age and social status categories generally were not supported, other factors may be involved in promoting high vulnerability of independent yearlings to predation. Smaller body size and maternal defense are unexplored alternatives for explaining differential mortality between yearlings with and without mothers.
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