Heat tolerance and cold indifference in the short-beaked echidna, Tachyglossus aculeatus: body temperature management in a mammalian constitutional eurytherm.

Short-beaked echidnas, Tachyglossus aculeatus, are heterothermic, prototherian mammals whose distribution extends across Australia and parts of New Guinea. They are reputed to be poor physiological thermoregulators in the heat as they lack the evaporative cooling mechanisms associated with other mammals and can tolerate ambient temperatures in excess of 35 C to 37 C for only short periods in laboratory experiments. However, in the field in western Queensland, they routinely tolerate ambient temperatures in some of their daytime retreats that exceed 35 C for several hours and peak at 42 C. In other, and colder, situations, except when incubating, body temperatures of echidnas vary on a daily basis, increasing with activity and decreasing with inactivity. They appear to be fully coordinated even with body temperatures as low as 20 C. This suggests that the classical model of mammalian thermoregulation, assuming as it does, the maintenance of high and stable body temperatures, is of little relevance to nonincubating echidnas. The primary aim of this project was to investigate physiological adjustments that allow echidnas to tolerate ambient temperatures in the field that would be lethally high in laboratories. The secondary aim of this project was to develop simple heat budget modelling that could be used to analyse physiological adjustments and responses in animals with significant thermal inertia, endothermy and heterothermy in complex, dynamically fluctuating (field) thermal environments. The third aim of the project was to investigate the relevance of traditional concepts about mammalian thermoregulation to echidnas within the context of their heterothermy. The first aim was addressed by experiments conducted under field conditions where metabolic rate, evaporative heat loss, thermal conductance and body temperature were determined in response to ambient temperature fluctuations similar to those experienced by echidnas in the wild. It was found that echidnas in these conditions utilised the low ambient temperatures of the early morning to lose body heat passively to their environment which then gave them scope to absorb heat as ambient temperatures increased during the day. In this way, resting echidnas tolerated ambient temperatures that peaked in the range 30 C to 49 C as long as their body temperatures remained below 33 C: slightly higher than the modal body temperatures of foraging echidnas. Because body temperatures rose only slowly in these relatively large animals, echidnas were frequently unperturbed by experimental conditions until after ambient temperatures had reached their daytime peak. The second aim of the project was addressed by developing integrated parameter, a priori heat budget modelling to predict body temperature responses of animals in general to combined variations of metabolic heat production, thermal conductance and other physiological factors in dynamically fluctuating (field) thermal environments. This modelling was found to predict well the body temperature fluctuations in a wide range of ectotherms and endotherms of various sizes in various environments. When applied to