Correlates of seasonal change in the body condition of an Arctic top predator

Abstract

Climate‐driven sea ice loss has led to changes in the timing of key biological events in the Arctic, however, the consequences and rate of these changes remain largely unknown. Polar bears (Ursus maritimus) undergo seasonal changes in energy stores in relation to foraging opportunities and habitat conditions. Declining sea ice has been linked to reduced body condition in some subpopulations, however, the specific timing and duration of the feeding period when bears acquire most of their energy stores and its relationship to the timing of ice break‐up is poorly understood. We used community‐based sampling to investigate seasonality in body condition (energy stores) of polar bears in Nunavut, Canada, and examined the influence of sea ice variables. We used adipose tissue lipid content as an index of body condition for 1,206 polar bears harvested from 2010–2017 across five subpopulations with varying seasonal ice conditions: Baffin Bay (October–August), Davis Strait and Foxe Basin (year‐round), Gulf of Boothia and Lancaster Sound (August–May). Similar seasonal patterns were found in body condition across subpopulations with bears at their nadir of condition in the spring, followed by fat accumulation past break‐up date and subsequent peak body condition in autumn, indicating that bears are actively foraging in late spring and early summer. Late season feeding implies that even minor advances in the timing of break‐up may have detrimental effects on foraging opportunities, body condition, and subsequent reproduction and survival. The magnitude of seasonal changes in body condition varied across the study area, presumably driven by local environmental conditions. Our results demonstrate how community‐based monitoring of polar bears can reveal population‐level responses to climate warming in advance of detectable demographic change. Our data on the seasonal timing of polar bear foraging and energy storage should inform predictive models of the effects of climate‐mediated sea ice loss.

Transient benefits of climate change for a high‐Arctic polar bear (Ursus maritimus) subpopulation

“…Polar bear body condition fluctuates in response to spatiotemporal variation in environmental conditions and can exhibit long‐term trends (Stirling, 2002). In general, polar bear body condition improves in spring in annual ice regions as more prey become available (Galicia, Theimann, & Dyck, 2020; Hammill & Smith, 1991; Pilfold, Derocher, Stirling, Richardson, & Andriashek, 2012; Smith, 1980; Stirling & Archibald, 1977; Stirling & Øritsland, 1995). The BCS used in this study is closely correlated with quantitative condition indices for polar bears (Laidre et al., 2020; McKinney et al., 2014; Stirling, Richardson, et al, 2008) and has been found to be repeatable in blind comparisons (Stirling, Thiemann, et al, 2008).…”

The acute physiological response of polar bears to helicopter capture

“…We derived activity and body temperature data from recaptures in October and polar bears may respond differently to helicopter operations and immobilization during other seasons. For example, polar bears are typically in reduced body condition during April–May when spring captures are performed (Whiteman et al 2018, Galicia et al 2019), which could help prevent elevated body temperatures because they are displacing less body mass when running. Alternatively, their fur is likely denser during April–May than during late summer (Frisch et al 1974), which could increase the likelihood of high body temperatures.…”

Influence of sea ice dynamics on population energetics of Western Hudson Bay polar bears

“…Similarly, Derocher and Stirling (1994 ) found that an adult female’s condition in the previous year was a strong determining factor for reproductive success in WH. In other polar bear populations, lower body condition has been associated with time lags in breakup date and the duration of the ice-free period ( Galicia et al , 2019 ; Laidre et al , 2020 ). The observed decline in solitary adult female energy and the relationship with the lagged open-water period suggests that females may not be able to recover from declines in stored energy that have occurred in previous years, which has the potential to accumulate over time and affect lifetime reproductive success.…”