The stress of Arctic warming on polar bears

Abstract

Arctic ecosystems are changing rapidly in response to climate warming. While Arctic mammals are highly evolved to these extreme environments, particularly with respect to their stress axis, some species may have limited capacity to adapt to this change. We examined changes in key components of the stress axis (cortisol and its carrier protein—corticosteroid binding globulin [CBG]) in polar bears (Ursus maritimus) from western Hudson Bay (N = 300) over a 33 year period (1983–2015) during which time the ice‐free period was increasing. Changing sea ice phenology limits spring hunting opportunities and extends the period of onshore fasting. We assessed the response of polar bears to a standardized stressor (helicopter pursuit, darting, and immobilization) during their onshore fasting period (late summer–autumn) and quantified the serum levels of the maximum corticosteroid binding capacity (MCBC) of CBG, the serum protein that binds cortisol strongly, and free cortisol (FC). We quantified bear condition (age, sex, female with cubs or not, fat condition), sea ice (breakup in spring–summer, 1 year lagged freeze‐up in autumn), and duration of fasting until sample collection as well as cumulative impacts of the latter environmental traits from the previous year. Data were separated into “good” years (1983–1990) when conditions were thought to be optimal and “poor” years (1991–2015) when sea ice conditions deteriorated and fasting on land was extended. MCBC explained 39.4% of the variation in the good years, but only 28.1% in the poor ones, using both biological and environmental variables. MCBC levels decreased with age. Changes in FC were complex, but more poorly explained. Counterintuitively, MCBC levels increased with increased time onshore, 1 year lag effects, and in poor ice years. We conclude that MCBC is a biomarker of stress in polar bears and that the changes we document are a consequence of climate warming.

Long‐term increases in pathogen seroprevalence in polar bears (Ursus maritimus) influenced by climate change

“…Additionally, Boonstra et al (2020) found a shift in the stress axis of polar bears between 1983-1990 and 1991-2015. In our study, we found a similar temporal pattern in the change in the seroprevalence of T. gondii, F. tularensis and B. bronchiseptica. We do not have direct evidence for the virulence of these pathogens in polar bears, and therefore cannot associate pathogen exposure with survival.…”

“…These indices subsequently stabilized in the mid‐2000s (Lunn et al, 2016 ). Additionally, Boonstra et al ( 2020 ) found a shift in the stress axis of polar bears between 1983–1990 and 1991–2015. In our study, we found a similar temporal pattern in the change in the seroprevalence of T. gondii , F. tularensis and B. bronchiseptica .…”

Of mammals and milk: how maternal stress affects nursing offspring

“…Chronic stress also inhibits corticosteroid‐binding globulin production by the liver, further increasing free GC levels. Evidence from several species suggests that milk GC concentrations reflect blood GC concentrations, though they rapidly equilibrate, and so once the stressor ceases and plasma levels decrease, there is a parallel decrease in milk GC levels (modified from Boonstra et al 2020)…”

The acute physiological response of polar bears to helicopter capture

“…For example, cortisol and glucose were not influenced by helicopter operations, despite capture stress generally causing their increase (Malisch et al 2018). This may reflect that the hypothalamic‐pituitary‐adrenal stress response is not graded during helicopter captures (i.e., it is more of an off or on response) or that cortisol can be regulated by other mechanisms such as altering concentration of the binding protein (Chow et al 2010, 2011; Boonstra et al 2020). In addition, our assessment of peak values of activity and body temperatures did not consider the effect of different durations between initial captures and recaptures, as 15 of 17 datasets encompassed the same duration (i.e., Aug–Oct).…”