Disentangling environmental drivers of metabolic flexibility in birds: the importance of temperature extremes versus temperature variability

Abstract

Examining physiological traits across large spatial scales can shed light on the environmental factors driving physiological variation. For endotherms, flexibility in aerobic metabolism is especially important for coping with thermally challenging environments and recent research has shown that aerobic metabolic scope [the difference between maximum thermogenic capacity (Msum) and basal metabolic rate (BMR)] increases with latitude in mammals. One explanation for this pattern is the climatic variability hypothesis, which predicts that flexibility in aerobic metabolism should increase as a function of local temperature variability. An alternative explanation is the cold adaptation hypothesis, which predicts that cold temperature extremes may also be an important driver of variation in metabolic scope. To determine the thermal drivers of aerobic metabolic flexibility in birds, we combined data on metabolic scope from 40 bird species sampled across a range of environments with several indices of local ambient temperature. Using phylogenetically‐informed analyses, we found that minimum winter temperature was the best predictor of variation in avian metabolic scope, outperforming all other thermal variables. Additionally, Msum was a better predictor of latitudinal patterns of metabolic scope than BMR, with species inhabiting colder environments exhibiting increased Msum over their counterparts in warmer environments. Taken together, these results suggest that cold temperature extremes drive latitudinal patterns of metabolic scope via selection for enhanced thermogenic performance in cold environments, supporting the cold adaptation hypothesis. Temperature extremes may therefore be an important selective pressure driving macrophysiological trends of aerobic performance in endotherms.

Difference in plasticity of resting metabolic rate – the proximate explanation to different niche breadth in sympatricFicedulaflycatchers

“…Differences in RMR might be particularly relevant during periods of exponential growth (i.e., between approximately 3 and 10 days old), such as when we measured it here. The estimates of RMR that we report here are approximately two times as high as those that have been reported for other bird species that are between 10 and 20 g (0.84 ml/min ± 0.2; Stager et al., ), and substantially higher than what has previously been reported in pied flycatchers (i.e., 0.608 ml/min; Moreno & Carlson, , 0;.887 ml/min; Bushuev et al., ). However, these estimates were all in adult, or nearly fledged nestlings, while our study was on growing nestlings, specifically during the period of nestling exponential growth.…”

Does metabolism constrain bird and mammal ranges and predict shifts in response to climate change?

“…Physiological measurements indicate metabolic constraints and adaptations. Maximum cold-induced metabolic rate (summit metabolism, M sum ) is greater in cold environments (Wiersma, Muñoz-Garcia, Walker, & Williams, 2007) and is phylogenetically conserved (Stager et al, 2015;Swanson & Garland, 2009). Observations that metabolic scope, the extent to which M sum is elevated over BMR, increases poleward are explained by two related hypotheses: The Climate Variability Hypothesis (Ghalambor, Huey, Martin, Tewksbury, & Wang, 2006;Janzen, 1967;Stevens, 1989) proposes that variable climates at high latitudes and altitudes select for greater flexibility in metabolic rate.…”

How extreme is extreme? Demographic approaches inform the occurrence and ecological relevance of extreme events

“…), and vary with latitude (Stager et al. ), thresholds in survival related to weather extremes may be geographically and taxonomically idiosyncratic and therefore, difficult to predict (Buckley and Huey ). Consequently, we suggest future studies should focus on characterizing the demographic response function to both high and low temperature extremes across broad geographic gradients.…”