Body size and environment influence both intraspecific and interspecific variation in daily torpor use across hummingbirds

1. Torpor, or a regulated drop in body temperature and metabolic rate, allows animals to inhabit energetically costly environments, but among torpor-using species, we have a poor understanding of how plasticity in torpor use relates to the experienced environment.

2. To better understand the ecology of daily torpor, we completed the largest study to date on the intraspecific variation of daily torpor use in hummingbirds by exposing 149 individuals of two hummingbird species to ambient or experimentally cooled temperatures in a field setting.

3. The smaller species, a latitudinal migrant, used daily torpor frequently under ambient conditions. The larger species, an elevational migrant, also used daily torpor regularly, but further increased the frequency of daily torpor use when experiencing colder temperatures and prior to migration – indicating a facultative adaptation.

4. To place our results within a broader phylogenetic context, we combined our experimental results with a m...