Polar bear energetic and behavioral strategies on land with implications for surviving the ice-free period

Abstract

Declining Arctic sea ice is increasing polar bear land use. Polar bears on land are thought to minimize activity to conserve energy. Here, we measure the daily energy expenditure (DEE), diet, behavior, movement, and body composition changes of 20 different polar bears on land over 19-23 days from August to September (2019-2022) in Manitoba, Canada. Polar bears on land exhibited a 5.2-fold range in DEE and 19-fold range in activity, from hibernation-like DEEs to levels approaching active bears on the sea ice, including three individuals that made energetically demanding swims totaling 54-175 km. Bears consumed berries, vegetation, birds, bones, antlers, seal, and beluga. Beyond compensating for elevated DEE, there was little benefit from terrestrial foraging toward prolonging the predicted time to starvation, as 19 of 20 bears lost mass (0.4-1.7 kg•day^-1). Although polar bears on land exhibit remarkable behavioral plasticity, our findings reinforce the risk of starvation, particularly in subadults, with forecasted increases in the onshore period.

Fig. 1. Map of polar bear movements derived from GPS-enabled video camera collars.

Capture (light points) and recapture (dark points) locations and GPS movement paths of 20 polar bears (8 adult females (green lines), 4 subadult females (orange lines), 3 subadult males (yellow lines), and 5 adult males (purple lines)) dosed with doubly-labeled water and equipped with GPS-enabled video camera collars on land near Churchill, Manitoba, Canada. (Inset) Image from a GPS-equipped video camera collar on an adult male polar bear near Churchill, Manitoba, Canada (datetime in GMT). Source data are provided as a Source Data file.

Fig. 2. Contrasting movements, activity, behavior, diet, and energy expenditure of a subadult female and an adult male polar bear on land.

a A subadult female (X33939) monitored for 19 days and b an adult male (X32415) polar bear monitored for 21 days on land near Churchill, Manitoba, Canada. Movements were derived from GPS location data at 5-min intervals. Movements are color-coded to show where bears spent ≥50% of time resting, walking, swimming, eating, or a mix of these behaviors. Feeding events were derived from the video-camera footage. Behaviors and activity levels were derived from tri-axial accelerometer data. The bear shading (orange) reflects the overall mean activity expressed as a % total time spent active (non-resting). Also shown is the mean daily energy expenditure derived using doubly-labeled water, a representative image derived from the video-camera collar (subadult female: swimming toward a beluga carcass, adult male: resting; datetimes in GMT), and the cumulative daily distance moved derived from the GPS location data. Source data are provided as a Source Data file.

Fig. 3. Differences in energy expenditure, movements, activity, and changes in body mass of polar bears on land by age and sex class.

Data are summarized from 8 adult female (6 pregnant (green points), 2 non-pregnant (light green points)), 5 adult male, 4 subadult female, and 3 subadult male polar bears on land near Churchill, Manitoba, Canada. a Daily energy expenditure derived using doubly-labeled water, b total distance moved based on GPS location data, c accelerometer-derived activity expressed as a % total time spent active (non-resting), and d percent body mass change between initial capture and recapture 19–23 days later. Boxplots indicate the median, 1st and 3rd quartiles, maximum within 1.5× the inter-quartile range, and minimum within 1.5× the inter-quartile range. Numbers next to boxplots indicate the median values by sex and age class. Points represent raw values per individual. Source data are provided as a Source Data file.

Fig. 4. The effects of activity and body mass on the energy expenditure of polar bears on land.

Daily energy expenditure (DEE) of 8 adult female (6 pregnant (green points), 2 non-pregnant (light green points)), 5 adult male, 4 subadult female, and 3 subadult male polar bears on land near Churchill, Manitoba, Canada over 19–23 days. a Multiple linear regression in relation to age and sex class and activity derived from tri-axial accelerometer data expressed as a % total time spent active (non-resting). b The allometric regression (solid line) of DEE with mean body mass compared to the DEE of female polar bears on the spring sea ice in the Beaufort Sea (white points and dashed line), predicted DEE of male polar bears on land in western Hudson Bay based on changes in body composition (white squares), predicted basal metabolic rates (BMR; dotted line), and the average energetic cost of hibernation in Holarctic bears (dash-dot line). Source data are provided as a Source Data file.

Fig. 5. Percent time engaged in active (non-resting) behaviors by polar bears on land near Churchill, Manitoba, Canada.

a Behaviors derived from tri-axial accelerometer data recording continuously at 16 Hz from 20 polar bears, and b behaviors derived from video footage from 18 polar bears recorded during daylight hours. Source data are provided as a Source Data file.

Fig. 6. Hourly changes in polar bear activity rates while on land near Churchill, Manitoba, Canada.

a Mean (±SE) hourly movement, b mean activity (±SE), and c mean eating (±SE) of 8 adult female, 5 adult male, 4 subadult female, and 3 subadult male polar bears. Movement rates were derived from GPS location data. The percent time active (i.e., non-resting) or eating were derived from tri-axial accelerometer data recorded continuously at 16 Hz. Dotted vertical lines are the mean timing of sunrise and sunset based on each bear’s GPS locations and gray shaded areas represent dark periods between sunset and sunrise. Times are Central Daylight Time (CDT). Source data are provided as a Source Data file.

Fig. 7. Images from GPS-equipped video camera collars on polar bears on land near Churchill, Manitoba, Canada.

An adult female polar bear eating: a berries (Vaccinium uliginosum), b a waterfowl egg, and c holding a seal carcass. An adult male polar bear eating: d a gull, e a microtine rodent, and f chewing on bones from a beluga skeleton. Datetimes in GMT.

Fig. 8. Predicted date of starvation in polar bears on land.

Predicted date of starvation of 8 adult female (6 pregnant (green points), 2 non-pregnant (light green points)), 5 adult male, 4 subadult female, and 3 subadult male polar bears on land near Churchill, Manitoba, Canada based on their body composition and daily energy expenditure relative to the mean date of sea ice freeze up in western Hudson Bay (dark dashed line) 2013–2022. a Date of starvation derived from the bear’s initial body composition, b date of starvation derived from the bear’s recapture body composition. Boxplots indicate the median, 1st and 3rd quartiles, maximum within 1.5× the inter-quartile range, and minimum within 1.5× the inter-quartile range. Numbers next to boxplots indicate the median values by sex and age class. Points represent raw values per individual. Source data are provided as a Source Data file.