Meeting Abstract
Reaction norms reflect an organism’s capacity to adjust its phenotype to environmental changes and allows for identifying trait values associated with physiological limits. However, shapes of reaction norms are usually unknown, especially in natural conditions, and this constrains our capacity to predict how organisms may cope with extreme climatic events such as those associated with climate change. Small endotherm species living year-round at high latitudes face highly seasonal environments and are therefore good models to study reaction norms of traits thought to influence fitness. We repeatedly measured basal (BMR, minimal maintenance costs) and summit (Msum, cold endurance) metabolic rates in 183 Black-capped chickadees (Poecile atricapillus) over a year to determine, for the first time in a free-living endotherm, reaction norms of these parameters across the natural range of weather variations. BMR was not related to weather parameters but varied consistently between individuals. Msum did not differ between individuals but was related to minimal temperature following a Z-shape curve, with a linear increase between 22°C and -10°C and was related to absolute humidity following a U-shape relationship, attaining its lowest point at a humidity level of 2.6 g.m-3. These findings suggest 1) that thermal exchanges with the environment likely have minimal effects on physiological maintenance costs, which may be individual-dependent and that 2) thermogenic capacity does not differ between individuals but is strongly affected by weather conditions. Our results bring support to the hypothesis that parameters of metabolic performance respond to different and likely independent constraints.
