Torpor Use in Ruby-throated Hummingbirds (Archilochus colubris): Effects of Energy Stores, Seasonality, Temperature, and Age-Sex Class
Year: 2023
Type: dissertation
Abstract: Animals utilize various behavioral and physiological strategies to balance energy gain and energy expenditure despite seasonally changing environmental and ecological constraints. Torpor, a state of reduced metabolism, is a versatile energy conservation strategy used by many mammals and birds. Hummingbirds are known for their ability to use daily torpor to cope with energetic challenges they face throughout the annual cycle. Due to their small size and use of hovering flight, hummingbirds sustain extremely high metabolic rates and carry limited energy stores. Additionally, their body masses and endogenous fat stores vary daily and seasonally, in rhythm with environmental and ecological conditions. Thus, hummingbirds are a uniquely powerful system in which to interrogate whether the rules that govern torpor are constant or subject to individual, environmental, ecological, and seasonal contexts. My dissertation investigates torpor in ruby-throated hummingbirds (Archilochus colubris) by evaluating how the sensitivity of torpor to endogenous energy stores differs with respect to nocturnal metabolic fueling strategy, breeding, migration and wintering life history stages, nighttime air temperature, and age-sex class. First, I find that hummingbirds that perform evening hyperphagia quickly convert this sugar to fat, which they subsequently use to fuel normothermy. Second, I find that in the summer breeding period, birds use torpor to survive energy emergencies when their fat levels reach a critically low threshold, but shift to a routine fat-sparing strategy in the fall migratory period. Thirdly, I find that on colder nights in the summer, birds enter torpor more often and at higher fat levels, but in the winter, they use torpor frequently, irrespective of fat content and air temperature. Finally, I find that birds that experience natural daytime conditions in the summer use torpor more when leaner, and that developing juveniles use torpor less often than adults. These findings suggest that by reducing nightly energy expenditure through torpor, hummingbirds can maintain seasonally appropriate energy levels despite the various energetic constraints they face throughout the annual cycle. Thus, I propose that hummingbird torpor is governed by an adipostat mechanism sensitive to a fat threshold set relative to immediate (air temperature, night length) and future (seasonal, life history stage) resource availability, energy requirements, and biomechanical constraints.
Source: TSpace (University of Toronto)
Author: Eberts, Erich Robert
Institution: University of Toronto
Language: English
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7132922605