![]() ![]() This use of shallow torpor can show a variety of patterns, either stabilizing or oscillating up and down (alternative pink dashed lines). (C) An individual that starts the night normothermic, then uses ‘shallow’ torpor, potentially because the species has a very high minimum body temperature of only 4–5☌ below normothermic levels (e.g. (B) An individual that starts the night normothermic, then transitions into deep torpor, where body temperature drops with ambient temperature, minimizing the difference between minimum body temperature and ambient temperature (e.g. ![]() (A) A normothermic individual, with minimal circadian reductions in night-time body temperature (e.g. ![]() Both these states are characterized by lower body temperatures than the 1–2☌ drop below resting daytime body temperature that occurs following a circadian rhythm in normothermic sleep (‘nocturnal’ or ‘rest-phase’ hypometabolism Geiser, 2021 Walker et al., 1983).Ī schematic depiction of body temperature (colored lines) relative to ambient temperature (black dashed line) at night: in sleep, shallow torpor and deep torpor. Arctic ground squirrels, hummingbirds, −2 to 18☌ Fig. 1B). fasted doves, body temperature 28–36☌ Fig. 1C), while others use ‘deep’ torpor, in which body temperature is low (more than 20☌ below normothermic body temperature, e.g. Depending on their minimum torpid body temperature, some animals only use a ‘shallow’ form of torpor (e.g. These animals are often described as having species-specific minimum torpid body temperatures (between −2 and 29.6☌ Barnes, 1989 Hainsworth and Wolf, 1970 McKechnie and Lovegrove, 2002 Richter et al., 2015 Ruf and Geiser, 2015). Torpor is a fascinating ability – torpid animals save energy by lowering their metabolic rate and body temperature. Torpor is an energy-saving strategy documented in over 200 species of birds and mammals ( Boyles et al., 2020 Ruf and Geiser, 2015). The presence of a heterothermic spectrum in these bird species indicates a capacity for fine-scale physiological and genetic regulation of avian torpid metabolism. Depending on the species, they used shallow torpor for 5–35% of the night. All three species used both deep and shallow torpor, often on the same night. We infrared imaged three hummingbird species that are known to use deep torpor, under natural temperature and light cycles, to test whether they were also capable of shallow torpor. Although the literature hints that some bird species (mousebirds and perhaps hummingbirds) can use both shallow and deep torpor, little empirical evidence of such an avian heterothermy spectrum within species exists. Deep torpor occurs in three avian orders, but the trade-offs of deep torpor in birds are unknown. However, deep torpor in mammals can increase predation risk (unless animals are in burrows or caves), inhibit immune function and result in sleep deprivation, so even for species that can enter deep torpor, facultative shallow torpor might help balance energy savings with these potential costs. arctic ground squirrels, hummingbirds) enter deep torpor, dropping their body temperature by 23–37☌, while others can only enter shallow torpor (e.g. Many endotherms use torpor, saving energy by a controlled reduction of their body temperature and metabolic rate.
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