Echidnas occur throughout Australia. They exhibit daily fluctuations in body temperature (T b ) and use torpor to various degrees throughout much of their range. Echidnas elsewhere are commonly diurnal except during hot weather. This study used temperature-sensitive radio-transmitters to investigate the activity patterns and temperature relations of echidnas in the relatively hot, dry climate of south-west Queensland with respect to temperature and photoperiod.During activity, echidnas were characterised by rising, but not necessarily high, T b s. Activity was seen only within an ambient temperature range (as measured in a nearby Stevenson Screen: T ss ) of 9-33°C so that activity was seen during the day and at night during the cool weather but only at night in summer. Echidnas used caves, burrows and logs when inactive. T b s of inactive echidnas declined except when affected by rising ambient temperatures, as determined within these shelters (T a ). In summer, T b s of echidnas in these shelters changed little or rose with increasing T a to levels even higher than in active echidnas.Torpor was used by echidnas for periods up to nine days during winter and occasionally for up to one day during summer. Due to the difficulty of identifying the occurrence of torpor from T b alone in warm conditions, the possibility that echidnas utilise torpor for less than one day remains inconclusive. Nevertheless, at least five bouts of torpor were identified in four (of eight) echidnas during winter/spring and two bouts of torpor from two echidnas in summer.Z O 0 1 0 8 0 A c t i v i t y p a t t e r n s i n e c h i d n a s P . H . B r i c e e t a l .

Abstract(1) Echidnas occur throughout the hot arid zone of Australia yet laboratory studies have concluded that they are ill equipped physiologically to manage T a higher than 35°C. (2) Consequently, it is generally assumed that echidnas must rely on behavioural thermoregulation, being nocturnal in hot weather and seeking less extreme microclimates during the day.(3) By monitoring T b of echidnas in the field and relating these to T a within their day time shelters in Western Queensland during summer, this study showed that echidnas are able to tolerate T a of 35-40°C in hollow logs for up to 10 h. (4) Further, as T b remains < T a in these situations, echidnas may have physiological mechanisms for dealing with the heat after all.

The three extant genera of the Monotremata have evolved, probably from a pre-Cretaceous Gondwanan origin, independently of the Theria to display a variety of ancestral and derived features. A comparison of their thermoregulation reveals a diversity of physiology that might represent both plesiomorphic and apomorphic elements within this mosaic. In the tachyglossids, the echidnas Tachyglossus and Zaglossus, body temperature is often labile, rising as a result of activity and allowed to decline during inactivity. This daily heterothermy, which is not necessarily torpor, may combine with typical mammalian hibernation to provide substantial energy economy in a wide variety of often unproductive habitats. Only when incubating do free-ranging echidnas display classic mammalian thermoregulation, the facultative nature of which suggests echidna-like physiology as an example of a protoendothermic stage in the evolution of endothermy. Similarly, physiological response to heat in Tachyglossus, at least, may be plesiomorphic, relying on the cyclic loss of heat stored during activity. Tachyglossids neither exhibit a panting response nor spread saliva to facilitate evaporative cooling and Tachyglossus, though not Zaglossus, lacks functional sweat glands. By contrast, the only extant ornithorhynchid, the platypus Ornithorhynchus, does not utilise heterothermy of any kind and maintains its body temperature more tightly than several semiaquatic eutherians. Although not necessarily required, it responds to heat via sweating, but not panting or saliva spreading. The classic nature of ornithorhynchid thermoregulation stands in marked contrast to the more diverse thermoregulatory responses shown by the tachyglossids, making it difficult to determine which aspects of monotreme thermoregulation are plesiomorphic and which are apomorphic.

Various conceptual or qualitative models have been proposed to explain the evolution of homeothermic endothermy from heterothermic ectothermy. Assessment of the feasibility of these models may benefit from quantitative analyses of hypotheses deriving from them. In this study, a quantitative approach was applied to a recent conceptual model which proposed a stepwise progression from ectothermy to endothermy in a medium sized vertebrate "evolving" in a warm, mesic thermal environment. By assuming that selection acted to maximise time available for activity, this study showed that endogenous heat, whether from activity or shivering thermogenesis, increased time within nominal preferred body temperatures only if low values of thermal conductance associated with insulation were already in place. Further, this study demonstrated that benefits were enhanced if the insulation was by-passable and associated with a wide range of thermal conductance. Because aerobic scope was incorporated as a constraint to thermogenic capacity, increases in standard metabolic rate were relatively ineffective until they were substantial and, at one stage, plesiomorphic daily torpor emerged, at least in the circumstances modelled. Also emerging from the modelling were the thermal neutral zone and basal metabolic rate characteristic of homeothermic endotherms. Additionally, benefits from rudimentary turbinal scrolls were quantified for this modelled, hypothetical animal in this particular environment. This initial exploratory study demonstrated how quantifying aspects of proposed models for the evolution of endothermy can provide insight into their feasibility. In so doing, this quantitative modelling highlighted avenues for further enquiry and demonstrated one tool for addressing debate about the evolution of endothermy.

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