Thermoregulation in monotremes: riddles in a mosaic

Abstract

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.

The heart rate - breathing rate relationship in aquatic mammals: A comparative analysis with terrestrial species

“…Owing to the scope of the study, preference was given to HR data collected in medium-size and larger species because differences in f between terrestrial and aquatic mammals begin to be apparent at M > 5-10 kg (Mortola and Limoges, 2006). Data were excluded if they originated from animals under anaesthesia, in hibernation or torpor; in addition, Monotremes and Marsupials were excluded because of their characteristically low body temperature and metabolic rate (Dawson, 1989;Brice, 2009). In case of multiple values or ranges of values, means were calculated.…”

Modelling mammalian energetics: the heterothermy problem

“…Basoendotherms (sensu [36]) such as tenrecs (Fig. 1d) and echidnas show the highest level of variability with rest-phase T b closely following T a during most times of the year [52][53][54][55][56].…”

Reexamining Echidna Physiology: The Big Picture forTachyglossus aculeatus acanthion

“…"proto-endotherms", Grigg et al 2014;Lovegrove 2012b), but it has also been considered to be a derived character related to their overall low-energy lifestyle (Brice et al 2009). Short-beaked echidnas are primarily myrmecophagous; their ant/termite diet has a low energy density, poor digestibility and low-energy feeding behaviour (McNab 1984;Redford and Dorea 1984;Cooper and Withers 2004b), and a low BMR is characteristic of myrmecophagous mammals in general (Cooper and Withers 2002).…”

“…Short-beaked echidnas have a low body temperature (T b ) and basal metabolic rate (BMR) at thermoneutrality, but the extent of their thermoregulatory capability over a wide range of T a , and how this reflects their evolutionary history and prototherian phylogenetic position, remains unclear (Brice 2009). Early physiological studies (de Miklouho Maclay 1883; Sutherland 1896) suggested that short-beaked echidnas are physiologically 'primitive' due to their low and variable T b , reflecting an intermediate position between the 'lower' reptiles and 'higher' mammals.…”

“…The ubiquitous distribution of the short-beaked echidna throughout Australia and its accessibility for scientific research has made this species a focus for studies of monotreme physiology. The many and varied physiological studies of the short-beaked echidna, conducted over a long period of time using a variety of methodologies, have resulted in a confusing array of interpretations and conclusions concerning the primitiveness or otherwise of its physiology (Brice 2009). Added to this confusion is the recognition of five subspecies of shortbeaked echidna, distinguished by geographical distribution and anatomical characteristics, which appear to also vary physiologically (Augee 1978;Nicol 2015).…”