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

Abstract

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. 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. 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. 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. 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. A free Plain Language Summary can be found within the Supporting Information of this article.

Abiotic conditions shape spatial and temporal morphological variation in North American birds

“…S3C, Table S2), indicating that, contrary to the general associations found between body size and temperature over space, pressures unrelated to thermoregulation dominate over this gradient [potentially reflecting lower resource availability at higher elevations ( 31 )]. Species with wide elevational gradients may therefore rely on a variety of behavioral adaptations, including altitudinal migration ( 32 ) and even nightly torpor ( 33 ), to cope with lower temperatures at higher elevations.…”

Tropical hummingbird pollination networks are resistant to short-term experimental removal of a common flowering plant

“…An alternative possibility is that hummingbirds relied on physiological adaptations, such as a torpor and/or insectivory, to maintain body condition despite large declines in nectar availability – resulting in the stable body mass measurements we observed in recaptured hummingbirds. Torpor is an energy-saving adaptation characterized by declines in body temperature and metabolic rate (Hainsworth, Collins, and Wolf 1977; Spence and Tingley 2021), and even tropical hummingbirds inhabiting low-to-mid elevations appear to possess this capability, including the green hermit (Krüger, Prinzinger, and Schuchmann 1982; Bech et al 1997; Shankar et al 2020; Schuchmann and Prinzinger 1988). Additionally, hummingbirds are highly insectivorous and, although they cannot completely replace nectar with insects (Brice 1992; Stiles 1995), they may supplement their diet with arthropods when nectar becomes scarce (Young 1971; Kuban and Neill 1980; Montgomerie and Redsell 1980; Hazlehurst and Karubian 2018).…”

Cool birds: first evidence of energy-saving nocturnal torpor in free-living common swifts Apus apus resting in their nests

“…Following the general definitions of torpor in the literature, torpor is often characterized by an approximately 50–95% decrease in MR for individuals [ 8 , 9 , 43 ]. Although we are aware that we are measuring groups of birds and that there is no clear threshold for torpor events, we used a relative reduction in resting MR by approximately 50% in our study as a conservative guideline for torpor events, accompanied by a substantial T nest drop [ 43 ]. We know from video monitoring that adults arrive near sunset at the nest and start resting a few minutes later after feeding the young.…”