Energy savings due to the use of shallow body temperature reduction in overwintering Northern Cardinals

Abstract

Northern Cardinals (Cardinalis cardinalis) are able to endure drastic seasonal variations in ambient temperature. Many endotherms in these conditions utilize heterothermy (e.g., torpor) to conserve energy by adjusting their body temperatures according to changing environmental conditions. Previous research shows that cardinals reduce energy expenditure during winter nights. By examining whole-animal function, we asked whether this reduced metabolism was a result of decreased activity or an induced state of torpor. We measured body mass, percent fat content, metabolic rate ( $$\dot{V}{\text{O}}_{2}$$ ) across a range of ambient temperatures and body temperature (T b) using data loggers during both summer (August to early September) and winter (December to January) conditions. We hypothesized that: (a) daily winter Tb fluctuations would reveal an induced torpor, and (b) alterations of insulation would play a significant role in thermoregulation. Although insulation in the form of fat stores was higher in the winter, there was no seasonal difference in whole body conductance. We found no evidence for torpor, but found a slight depression in overall circadian temperature rhythms for the winter animals compared to summer animals, resulting in a predicted 10–16 % savings in daily energy expenditure. Our findings support recent work showing that thermoregulatory mechanisms are not as fixed as previously thought and, while fat deposits may function to increase insulation, they are more likely important for fuel storage. These data identify subtle changes in the homeostatic set point for body temperature and show that these slight alterations can have significant impacts on daily energy expenditure in wild birds.