Body temperature maintenance acclimates in a winter-tenacious songbird

Abstract

<jats:title>ABSTRACT</jats:title><jats:p>Flexibility in heat generation and dissipation mechanisms provides endotherms the ability to match their thermoregulatory strategy with external demands. However, the degree to which these two mechanisms account for seasonal changes in body temperature regulation is unexplored. Here we present novel data on the regulation of avian body temperature to investigate how birds alter mechanisms of heat production and heat conservation to deal with variation in ambient conditions. We subjected Dark-eyed Juncos (<jats:italic>Junco hyemalis</jats:italic>) to chronic cold acclimations of varying duration and subsequently quantified their metabolic rates, thermal conductance, and ability to maintain normothermia. Cold-acclimated birds adjusted traits related to both heat generation (increased summit metabolic rate) and heat conservation (decreased conductance) to improve their body temperature regulation. Increases in summit metabolic rate occurred rapidly, but plateaued after one week of cold exposure. In contrast, changes to conductance occurred only after nine weeks of cold exposure. Thus, the ability to maintain body temperature continued to improve throughout the experiment, but the mechanisms underlying this improvement changed through time. Our results demonstrate the ability of birds to adjust thermoregulatory strategies in response to thermal cues and reveal that birds may combine multiple responses to meet the specific demands of their environments.</jats:p>

The architecture of phenotypic flexibility within a complex trait: an empirical case study using avian thermogenic performance

“…Ultimately, our treatments lasted up to two months and birds were exposed to a fairly extreme temperature stimulus such that the phenotypic responses shown here are not likely to have been constrained by time or insufficient severity of the cue. Additionally, junco M sum plateaued within one week of cold acclimation (Stager et al, 2020). As a result, any discrepancies between our findings and those shown in wild birds may follow from the fact that winter acclimatization likely involves the combination of several environmental cues.…”

“…We assayed M sum and resting metabolic rate (RMR) using open-flow respirometry at three sampling points: capture, before and after acclimations (referred to as pre- and post-acclimation, respectively). Data for pre- and post-acclimation measures are published in (Stager et al, 2020). We assayed RMR in the evening on the day of capture and M sum the following morning using methods identical to those detailed for pre-acclimation assays (see Stager et al, 2020 for details).…”

“…Data for pre- and post-acclimation measures are published in (Stager et al, 2020). We assayed RMR in the evening on the day of capture and M sum the following morning using methods identical to those detailed for pre-acclimation assays (see Stager et al, 2020 for details). In brief, birds were placed in a modified 1-L plastic Nalgene container for metabolic trails.…”

“…The methods for capture, acclimation, and metabolic assays have been previously described (Stager et al, 2020). Briefly, we captured adult juncos near the end of the breeding season in Missoula County, Montana, USA (~47.0°N, -113.4°W) in 2016 and 2017.…”

“…Juncos overwinter at high latitudes across North America and show increases in peak thermogenesis (the maximum metabolic rate under cold exposure; M sum ) and cold tolerance in winter (Swanson, 1990). We exposed juncos to temperature treatments of varying duration (from one to nine weeks) and previously reported that cold-acclimated juncos increased their M sum and the ability to maintain their T b during acute cold exposure (Stager et al, 2020). Here we add 20 additional organ- and tissue-level phenotypes for these same individuals to explore the degree to which flexibility in subordinate physiological traits contributed to thermogenic flexibility.…”