Ecological drivers and evolutionary consequences of torpor in Andean hummingbirds

Daily torpor allows endotherms to save energy during energetically stressful (e.g., cold) conditions. Although studies on avian torpor have mostly been conducted under laboratory conditions, information on the usage of torpor in the wild is limited to few, predominantly temperate-zone species. We studied torpor under seminatural conditions from 249 individuals from 29 hummingbird species across a 1,920 m elevational gradient in the western Andes of Colombia using cloacal thermistors. Small birds used torpor more often than large birds, but only at low ambient temperatures, where torpor was prolonged. We also found effects of proxy variables for body condition and energy expenditure on the use of torpor, its characteristics, and impacts. Our results suggest that judicious deployment and subtle modifications of torpor, as well as phylogenetic variation in propensity for torpor, have allowed only certain clades of hummingbirds to colonize and persist in the harsh climate of the high Andes.