The energetics and patterns of torpor in free ranging Tachygiossus aculeatus from a warm-temperate climate

The echidna (Tachyglossus aculeatus), a monotreme mammal, is known to show patterns of both short-term torpor and hibernation as well as a daily oscillation in body temperature (Tb) of about 4°C. This thesis provides the first comprehensive examination of Tb patterns in free-ranging echidnas from a relatively warm climate by describing the patterns and metabolic correlates of Tb. Both ecological and physiological approaches were combined to determine the energetic significance of hibernation in these individuals in the wild, relating the outcomes to changes in body mass (Mb) and reproduction. Field metabolic rates were estimated using predictive equations developed in the laboratory based on variables measurable in the field, including heart rate (HR), Tb and observations on activity. In the course of the thesis, several important concepts are addressed in light of the results that have broad implications beyond this species. Such concepts include whether patterns of torpor and non-torpor have different metabolic support, and whether basal metabolic rate is applicable or useful for species with constantly changing Tb. Tb loggers, and Tb and HR radio-transmitters were implanted into eight free-ranging echidnas. Body mass (Mb) and reproductive status were determined monthly from late 1997 until early 2000. Respirometry experiments were conducted on each echidna in the laboratory to develop predictive equations for field metabolic rate. The best equations for each animal were achieved when data from resting echidnas was treated separately from active data (mean r = 0.70). Heart rate explained more than 60% of the variation in metabolic rate in all but one of the equations. Patterns of Tb, including daily torpor, short-term torpor and hibernation, were similar to, but more variable than those exhibited by echidnas in cooler climates (Tasmania and Kosciuszko). Hibernation at this study site was not an obligate behaviour since not all echidnas hibernated in each hibernation season. Non-hibernating echidnas gained or maintained Mb (-3 to 17%) over the hibernation period (May to July), while those that did hibernate lost 5 to 11% Mb. Patterns of hibernation varied both within and among animals, ranging from the classic mammalian pattern (arousals of < 48 h) to bouts separated by 18 days at high Tb. Female echidnas tended to hibernate for longer than males. Patterns indicative of short-term torpor were observed well outside the hibernation season as well as within it and thus represent more than simply test-drops into hibernation. Ambient factors appeared to have only a minor direct influence on Tb patterns. All of the measures of Tb and HR used to look for possible differences in the metabolic support for Tb patterns ranged in a continuum between what is surely non-torpor to what is surely torpor, e.g., minimum Tb and HR ranged from 5.8 to 36.6°C and 3 to 64 bpm, respectively; the duration of Tb cycles ranged from 0.3 to 14.5 days; and the minimum HR during single-day cycles ranged from values equal to those during long hibernation bouts (7 bpm) to values well above (64 bpm). These results, and the variations in the length or arousals in hibernation, suggest that there is a gradation of the same underlying mechanisms from non-torpor to short-term torpor and hibernation in the echidna. The continuums in pattern highlight the frustrations of applying terminology based on pattern rather than mechanisms, necessitating application of arbitrary limits to identify torpor. Annual energy savings from Tb cycles of longer than one day (e.g., hibernation bouts) were much lower than in other hibernators (1% to 27%). Annual savings accounted for less than 68% of the cost of reproduction in males and 15 -16% in reproductive female echidnas. Most of the energy savings were attributable to a reduction in daily activity rather than a reduction in Tb per se. It was estimated that marginal energy savings could be achieved during daily oscillations in Tb only if the cost of re-warming was at least partially substituted by heat produced through activity or exogenous heat sources. The echidna is thought not to hibernate for the traditional reasons of cold stress or famine. It is proposed that the timing of torpor and hibernation is dependent on a suite of intrinsic and perhaps extrinsic factors, with the duration of hibernation dependent upon the trade-off between life time reproductive success and the energetic benefits of hibernation. This thesis is based on the following papers, currently in preparation... Kuchel, L.J., Grigg, G.C. and Beard, L.A. Body temperature patterns and torpor in free-ranging echidnas (Tachyglossus aculeatus) from a warm-temperate climate, (in prep) Kuchel, L.J. and Grigg, G.C. Is there evidence for physiological differences between daily oscillations in Tb, long and short term torpor in echidnas {Tachyglossus aculeatus): insights from a field study using heart rate telemetry, (in prep) Kuchel, L.J. Estimates of energy expenditure during different Tb patterns, including hibernation, in free-ranging echidnas from a warm-temperate climate, (in prep)  .. .and the following oral presentations Cold, gluttony and a touch of debauchery: why do echidnas hibernate? The joint New Zealand Ecological Society and Ecological Society of Australia Annual Meeting. The University of Otago, Dunedin, New Zealand, 24-27th November, 1998. Echidna reproduction: fact, folklore and field observations. The 3rd Zoology Postgraduate Conference, Springbrook, Australia, 16-18th October, 1998. Torpor in echidnas from a relatively mild climate. Guest seminar. The University of New England, Armidale, Australia. September, 2002. Torpor cannot be distinguished from non-torpor in free-ranging echidnas. The sixth International Congress of Comparative Physiology and Biochemistry, Mt. Buller, Australia. 2-7th February, 2003. Developing predictive equations for field metabolic rate in echidnas: correlations between heart rate, Tb, VO2 and activity. Monotreme Conference, Tasmania, Australia. 13-16th February, 2003. Hibernation in an egg-laying mammal from a not-so-cold climate: insights from a field study using heart rate telemetry. Invited Speaker, Comparative Physiology Seminar Series, the University of British Columbia, Canada. 27th October, 2003.

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