Extreme and variable torpor among high-elevation Andean hummingbird species

Abstract

Torpor is thought to be particularly important for small endotherms occupying cold environments and with limited fat reserves to fuel metabolism, yet among birds deep torpor is both rare and variable in extent. We investigated torpor in hummingbirds at approximately 3800 m.a.s.l. in the tropical Andes by monitoring body temperature ( T b ) in 26 individuals of six species held captive overnight and experiencing natural air temperature ( T a ) patterns. All species used pronounced torpor, with one Metallura phoebe reaching a minimum T b of 3.26°C, the lowest yet reported for any bird or non-hibernating mammal. The extent and duration of torpor varied among species, with overnight body mass ( M b ) loss negatively correlated with both minimum T b and bout duration. We found a significant phylogenetic signal for minimum T b and overnight M b loss, consistent with evolutionarily conserved thermoregulatory traits. Our findings suggest deep torpor is routine for high Andean hummingbirds, but evolved species differences affect its depth.