Social hierarchy reveals thermoregulatory trade-offs in response to repeated stressors [RESEARCH ARTICLE]

Joshua K. Robertson, Gabriela F. Mastromonaco, and Gary Burness

Coping with stressors can require substantial energetic investment, and when resources are limited, such investment can preclude simultaneous expenditure on other biological processes. Among endotherms, energetic demands of thermoregulation can be immense, yet our understanding of whether a stress response is sufficient to induce changes in thermoregulatory investment is limited. Using the black-capped chickadee as a model species, we tested a hypothesis that stress-induced changes in surface temperature, a well-documented phenomenon across vertebrates, stem from trade-offs between thermoregulation and stress responsiveness. Because social subordination is known to constrain access to resources in this species, we predicted that surface temperature and dry heat loss of social subordinates, but not social dominants, would fall under stress exposure at low ambient temperatures ("Ta"), and rise under stress exposure at high Ta, thus permitting a reduction in expenditure toward thermoregulation. To test our predictions, we exposed four social groups of chickadees to repeated stressors and control conditions across a Ta gradient (ndays/treatment/group=30), whilst remotely monitoring social interactions and surface temperatures. Supporting our hypothesis, we show that: 1) social subordinates (n=12), who fed less than social dominants and alone experienced stress-induced mass-loss, displayed significantly larger changes in surface temperature following stress exposure than social dominants (n=8), and 2) stress-induced changes in surface temperature significantly increased heat conservation at low Tas and heat dissipation at high Tas among social subordinates alone. These results suggest that chickadees adjust their thermoregulatory strategies under stress when resources are limited by ecologically relevant processes.

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