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
meta-analysis, including 31 species and all major hummingbird clades, and
found a broad taxonomic pattern in which smaller hummingbirds are more
likely to use daily torpor than their larger counterparts. Smaller
hummingbirds may be physiologically constrained, requiring nearly obligate
daily torpor use, while larger hummingbirds are physiologically more
flexible and can facultatively respond to changing environmental
conditions. 5. Our results reveal how physiological traits, such as the
frequency and depth of daily torpor, can provide a mechanism to understand
how hummingbird species have established and persisted across broad
environmental gradients.