Reaction Norms in Natural Conditions: How Does Metabolic Performance Respond to Weather Variations in a Small Endotherm Facing Cold Environments?

Abstract

Reaction norms reflect an organisms' capacity to adjust its phenotype to the environment and allows for identifying trait values associated with physiological limits. However, reaction norms of physiological parameters are mostly unknown for endotherms living in natural conditions. Black-capped chickadees (Poecile atricapillus) increase their metabolic performance during winter acclimatization and are thus good model to measure reaction norms in the wild. We repeatedly measured basal (BMR) and summit (Msum) metabolism in chickadees to characterize, for the first time in a free-living endotherm, reaction norms of these parameters across the natural range of weather variation. BMR varied between individuals and was weakly and negatively related to minimal temperature. Msum varied with minimal temperature following a Z-shape curve, increasing linearly between 24°C and −10°C, and changed with absolute humidity following a U-shape relationship. These results suggest that thermal exchanges with the environment have minimal effects on maintenance costs, which may be individual-dependent, while thermogenic capacity is responding to body heat loss. Our results suggest also that BMR and Msum respond to different and likely independent constraints.

Body temperature maintenance acclimates in a winter-tenacious songbird

“…The comparable magnitude of response to these two different temperature treatments contrasts with previous work showing that wild juncos and other birds modulate M sum with environmental temperature across the winter (Swanson & Olmstead, 1999). Taken together, these findings suggest that M sum might be coarsely adjusted, rather than fine-tuned, to environmental temperature, and that there may be limits to their flexibility in response to temperature variation (Petit and Vézina, 2014). Dissecting the relative contribution of subordinate phenotypic traits to M sum -e.g., pectoralis muscle size, hematocrit, or cellular metabolic intensity (Liknes & Swanson, 2011;Swanson, 1990b;Swanson et al, 2014)-will illustrate how birds build this phenotype and which traits (if any) may be limiting its flexibility.…”

Repeatable inter‐individual variation in the thermal sensitivity of metabolic rate

“…Previous studies reported that metabolic reaction norms to temperature can vary among individuals (Careau et al 2014) or populations and also depend on other environmental factors such as food availability and latitude (Giebelhausen and Lampert 2001, Lovegrove 2005, Petit and Vézina 2014, as well as covary with behaviour and life-history traits (Moe et al 2009, Mathot et al 2018. Altogether, these studies already improved our understanding of the factors driving variations in the thermal reaction norm of metabolic rate.…”

Global patterns of seasonal acclimatization in avian resting metabolic rates

“…It is striking, for instance, that within both a north-temperate species (Black-capped Chickadee; Poecile atricapillus) and a southern subtropical species (White-browed Sparrow-weaver; Plocepasser mahali) the direction of winter changes in BMR varies among populations from winter decreases to winter increases ( A related question that remains largely unanswered, but which is potentially important for understanding observed patterns in intra-and interspecific variation in seasonal metabolic adjustments, concerns the extent to which seasonal changes within populations are flexible and are modulated by environmental conditions. Data for several north-temperate species reveal that Msum closely tracks short-term fluctuations in temperature, suggesting a proximate influence of temperature (Swanson and Olmstead 1999;Petit and Vezina 2014). But the intraspecific variation in seasonal BMR adjustments highlighted in the preceding paragraph raises the possibility that not just the magnitude, but also the direction of seasonal metabolic adjustments may vary among seasons.…”