Abstract 42 – Summer diet and energetic balance in Arctic coastal grizzly bears relative to polar bears

Anthony Pagano, Karyn D. Rode, Kerry L. Nicholson, Nicholas J. Lunn, David
McGeachy, William B. Leacock, Charles T. Robbins

Climate warming is increasing polar bear land use across much of their range. While on land
polar bears are without access to their primary seal prey and have been shown to lose body
mass potentially making them vulnerable to starvation. Yet, in some areas, polar bear land use
overlaps with Arctic grizzly bear habitat, where terrestrial foods are sufficient to sustain grizzly
bears. We measured the energy expenditure, changes in body mass, behavior, diet,
movements, and activity of 12 Arctic grizzly bears on the North Slope of Alaska over 17 – 22
days between August to September to compare to similar measures collected from 20 polar
bears on land in Manitoba, Canada. Across 4 age and sex classes, Arctic grizzly bears largely
gained mass over 3 weeks (mean = 0.2 kg/day, range: -0.20 – 0.65 kg/day), while polar bears
lost mass with the exception of one individual (mean = -0.9 kg/day, range: -1.7 – 1.6 kg/day).
On average, Arctic grizzly bears moved 2.8× greater distances. The mean mass-specific energy
expenditure of two Arctic grizzly bears was 2.7× greater than the mean energy expenditure of
20 polar bears on land and similar to the mean energy expenditure of polar bears on the spring
sea ice. Grizzly bears consumed waterfowl, vegetation, berries, fish, moose, and caribou, while
polar bears consumed waterfowl, vegetation, berries, seal, and beluga. Our findings indicate
that the smaller body size of Arctic grizzlies (50% lower body mass than the polar bears we
sampled) allows them to more efficiently forage on terrestrial foods relative to polar bears.
Nevertheless, although terrestrial foods largely compensated for the energy expended to
acquire them, they provided relatively small energetic surpluses during a period when grizzlies
are typically building fat deposits in advance of winter hibernation. These findings reinforce the
hypothesis that most terrestrial foods within the polar bear’s range are inadequate to prolong
the period they can survive on land.