Abstract
ABSTRACTThe Arctic is warming at approximately twice the global rate, with well-documented indirect effects on wildlife. However, few studies have examined the direct effects of warming temperatures on Arctic wildlife, leaving the importance of heat stress unclear. Here, we assessed the direct effects of increasing air temperatures on the physiology of thick-billed murres (Uria lomvia), an Arctic seabird with reported mortalities due to heat stress while nesting on sun-exposed cliffs. We used flow-through respirometry to measure the response of body temperature, resting metabolic rate, evaporative water loss and evaporative cooling efficiency (the ratio of evaporative heat loss to metabolic heat production) in murres while experimentally increasing air temperature. Murres had limited heat tolerance, exhibiting: (1) a low maximum body temperature (43.3°C); (2) a moderate increase in resting metabolic rate relative that within their thermoneutral zone (1.57 times); (3) a small increase in evaporative water loss rate relative that within their thermoneutral zone (1.26 times); and (4) a low maximum evaporative cooling efficiency (0.33). Moreover, evaporative cooling efficiency decreased with increasing air temperature, suggesting murres were producing heat at a faster rate than they were dissipating it. Larger murres also had a higher rate of increase in resting metabolic rate and a lower rate of increase in evaporative water loss than smaller murres; therefore, evaporative cooling efficiency declined with increasing body mass. As a cold-adapted bird, murres' limited heat tolerance likely explains their mortality on warm days. Direct effects of overheating on Arctic wildlife may be an important but under-reported impact of climate change.
Highlights
Climate change is warming the Arctic at approximately twice the global rate (Anisimov et al, 2007; McBean et al, 2005)
When we compared the maximum Ta tolerated by murres with Mb prior to heat tolerance trials, larger birds had lower heat tolerance limits, with maximum tolerated Ta decreasing with increasing Mb (Fig. 1; r2=0.63, F1,8=16.5, P=0.0036)
Conclusions and ecological implications Recent heat waves in the Gulf of Alaska were associated with the mass mortality and reproductive failures of several colonies of common murres (Uria aalge) (Piatt et al, 2020)
Summary
Climate change is warming the Arctic at approximately twice the global rate (Anisimov et al, 2007; McBean et al, 2005). In the Canadian Arctic, mean annual air temperature has increased by ∼2.3°C from 1948 to 2016, and could increase by an additional 7.8° C by 2100 under high-emission scenarios (Zhang et al, 2019). This warming may have severe effects on cold-adapted homeothermic endotherms (i.e. organisms that actively maintain relatively constant body temperatures through metabolic heat production). In birds, which maintain their core body temperature at levels higher than in mammals (41°C versus 37°C), heat waves have caused mass mortality events (McKechnie et al, 2012) and reproductive failures (Bolger et al, 2005; Boersma and Rebstock, 2014). As larger birds have proportionally less surface area to volume ratios, and less surface to dissipate heat, they may be even more sensitive to heat stress
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