Heterothermy as a mechanism to offset energetic costs of environmental and homeostatic perturbations

Environmental and biotic pressures impose homeostatic costs on all
organisms. The energetic costs of maintaining high body temperatures (Tb)
render endotherms sensitive to pressures that increase foraging costs. In
response, some mammals become more heterothermic to conserve energy. We
measured Tb in banner-tailed kangaroo rats (Dipodomys spectabilis) to test
and disentangle the effects of air temperature and moonlight (a proxy for
predation risk) on thermoregulatory homeostasis. We further perturbed
homeostasis in some animals with chronic corticosterone (CORT) via
silastic implants. Heterothermy increased across summer, consistent with
the predicted effect of lunar illumination (and predation), and in the
direction opposite to the predicted effect of environmental temperatures.
The effect of lunar illumination was also evident within nights as animals
maintained low Tb when the moon was above the horizon. The pattern was
accentuated in CORT-treated animals, suggesting they adopted an even
further heightened risk-avoidance strategy that might impose reduced
foraging and energy intake. Still, CORT-treatment did not affect body
condition over the entire study, indicating kangaroo rats offset decreases
in energy intake through energy savings associated with heterothermy.
Environmental conditions receive the most attention in studies of
thermoregulatory homeostasis, but we demonstrated here that biotic factors
can be more important and should be considered in future studies.