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.