Social hierarchy reveals thermoregulatory trade-offs in response to repeated stressors

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 also 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 (T_s), 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 T_s and dry heat loss of social subordinates, but not social dominants, would fall under stress exposure at low ambient temperatures (T_a), and rise under stress exposure at high T_a, thus permitting a reduction in total energetic expenditure toward thermoregulation. To test our predictions, we exposed four social groups of chickadees to repeated stressors and control conditions across a T_a gradient (n=30 days/treatment/group), whilst remotely monitoring social interactions and T_s 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 T_s following stress exposure than social dominants (n=8), and (2) stress-induced changes in T_s significantly increased heat conservation at low T_a and heat dissipation at high T_a among social subordinates alone. These results suggest that chickadees adjust their thermoregulatory strategies during stress exposure when resources are limited by ecologically relevant processes.