Warm bodies in cold places: thermoregulation, activity, and energy expenditure of boreal homeotherms in winter

Environments change over time and, as the environment changes, animals have to change too, in order to maintain function and stay alive. The ways in which animals change, and by how much, determines their tolerances for environmental variation and defines the times (when) and the places (where) they can be successful. The capacity of endotherms to maintain relatively constant and elevated body temperature (i.e., endothermic homeothermy) by adjusting both physiological and behavioural traits in response to environmental variation is considered a critical evolutionary step in the adaptive radiation of birds and mammals. But, the challenges of studying physiology in the field have, to date, limited available data on the extent and significance of body temperature, metabolic, and behavioural variation of free-ranging endotherms in response to the numerous and interacting ways that natural environments can vary. More recently, the advent of miniaturized, animal-borne data loggers (i.e., biologgers) has revolutionized the study of animal physiology, especially as it relates to behaviour, ecology, and natural history, by enabling better quantification of physiology outside of the lab, behavioural variation in the absence of an observer, and environmental variation as it is experienced by an organism. In this thesis, I document variation in the body temperature, heart rate, activity, and daily energy expenditure of two prey species – North American red squirrels (Tamiasciurus hudsonicus) and snowshoe hares (Lepus americanus) – and their shared predator – Canada lynx (Lynx canadensis) – at the same time (i.e., winter), in the same place (i.e., northern boreal forest), to investigate how these coexisting species adjust to and tolerate the cold, dark, and resource-limited boreal winters. A direct comparison of squirrels and hares (Chapter 2) demonstrates that, while both species are considered to be homeotherms, characterized by high and stable body temperature, they differ substantially in fine-scale body temperature variation, including responses to environmental variation, with hares defending higher and more constant body temperatures via mainly heart rate variation, and squirrels having lower and more variable body temperature alongside greater behavioural variation. Analogous data on lynx, a medium-sized carnivore (Chapter 3), reveals dampened physiological and behavioural responses to environmental variation, and unexpected intraspecific variation in the energetic strategies of lynx; more active, larger lynx have reduced resting costs and, as a result, have lower daily energy expenditure than less active, smaller lynx. Finally, in Chapter 4, I show that correlations among biologged traits (i.e., body temperature, heart rate, and activity) are both timescale and species dependent, and that, at least for boreal endotherms in winter, these traits are not always strongly predictive of daily energy expenditure. Together, this thesis 1) highlights the challenges and usefulness of biologging devices in exploring physiological and behavioural responses of free-ranging homeotherms to natural environmental variation, 2) provides insight into both broad-scale patterns of energy expenditure and fine-scale energetic flexibility and homeostatic regulation, and 3) demonstrates the ways in which natural history, functional niches, and the ecology of a species interact to define their responses to environmental variation and, potentially, the ways that they coexist

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