Michael L. Augee

The evolution of endothermy and its diversity in mammals and birds.

Many elements of mammalian and avian thermoregulatory mechanisms are present in reptiles, and the changes involved in the transition to endothermy are more quantitative than qualitative. Drawing on our experience with reptiles and echidnas, we comment on that transition and on current theories about how it occurred. The theories divide into two categories, depending on whether selection pressures operated directly or indirectly on mechanisms producing heat. Both categories of theories focus on explaining the evolution of homeothermic endothermy but ignore heterothermy. However, noting that hibernation and torpor are almost certainly plesiomorphic (=ancestral, primitive), and that heterothermy is very common among endotherms, we propose that homeothermic endothermy evolved via heterothermy, with the earliest protoendotherms being facultatively endothermic and retaining their ectothermic capacity for “constitutional eurythermy.” Thus, unlike current models for the evolution of endothermy that assume that hibernation and torpor are specialisations arising from homeothermic ancestry, and therefore irrelevant, we consider that they are central. We note the sophistication of thermoregulatory behavior and control in reptiles, including precise control over conductance, and argue that brooding endothermy seen in some otherwise ectothermic Boidae suggests an incipient capacity for facultative endothermy in reptiles. We suggest that the earliest insulation in protoendotherms may have been internal, arising from redistribution of the fat bodies that are typical of reptiles. We note that short‐beaked echidnas provide a useful living model of what an (advanced) protoendotherm may have been like. Echidnas have the advantages of endothermy, including the capacity for homeothermic endothermy during incubation, but are very relaxed in their thermoregulatory precision and minimise energetic costs by using ectothermy facultatively when entering short‐ or long‐term torpor. They also have a substantial layer of internal dorsal insulation. We favor theories about the evolution of endothermy that invoke direct selection for the benefits conferred by warmth, such as expanding daily activity into the night, higher capacities for sustained activity, higher digestion rates, climatic range expansion, and, not unrelated, control over incubation temperature and the benefits for parental care. We present an indicative, stepwise schema in which observed patterns of body temperature are a consequence of selection pressures, the underlying mechanisms, and energy optimization, and in which homeothermy results when it is energetically desirable rather than as the logical endpoint.

The influence of thyroid hormones on the structure and function of mitochondrial membranes

Abstract In rat liver mitochondria, membrane lipid unsaturation increases, the temperature limits of the membrane phase transition decrease and the E a of succinate oxidase increases following thyroidectomy. All three parameters change in the opposite direction within 12 h after thyroxine treatment. It is suggested that many effects of thyroid hormones can be explained by changes in membrane structure and function.

Home range and movements of the quokka Setonix brachyurus (Macropodidae: Marsupialia), and its impact on the viability of the metapopulation on the Australian mainland

The home range and movements of the quokka Setonix brachyurus, a medium-sized macropodid marsupial, were investigated using radio telemetry. Fifty-eight quokkas from five remnant mainland populations in the northern jarrah (Eucalyptus marginata) forest of Western Australia were radio-collared and monitored for up to 2 years between 1998 and 2000. Mean (± SE) home-range sizes were 6.39 ± 0.77 ha and core ranges averaged 1.21 ± 0.12 ha. Male core home ranges were larger than those of females although not when corrected for body mass. Nocturnal ranges were larger than diurnal owing to nocturnal departures from the swamp. Ranges shifted to the edge of swamps in winter, as the swamps became inundated following rain, and toward the centre in autumn as the swamps dried. Quokka populations are thought to exist below carrying capacity at all sites and this probably caused the lack of dispersal observed in this study. We hypothesize that predation is suppressing population booms which would otherwise drive dispersal and thereby maintain metapopulation dynamics. Without dispersal to rescue unpopulated patches, we conclude that the original quokka metapopulation has collapsed and ameliatory measures are required.

Body Temperature and Diurnal Activity Patterns In the Platypus (Ornithorhynchus anatinus) During Winter

Using implanted radiotransmitters, we monitored body temperatures in five platypuses ranging freely in the Thredbo River in Australias southern alps between April and October 1988, where the water gets as cold as any that a platypus is likely to encounter. Activity pattern showed a distinct daily cycle. No evidence of hibernation or even brief periods of torpor was found, all individuals maintaining body temperatures close to 32 degrees C throughout the winter (mean s.d., 32.08 0.75 degrees C, range 29.2 34.6 degrees C, n = 2237). No differences were found between the means or the variances of body temperatures of animals during day-time rest in stream-bank burrows and those during night-time foraging in winter at temperatures as low as 1.0 degrees C.

Mortality and survivorship of the quokka (Setonix brachyurus) (Macropodidae : Marsupialia) in the northern jarrah forest of Western Australia

The potential for the quokka (Setonix brachyurus (Quoy & Gaimard, 1830)), a threatened macropodid marsupial, to increase in abundance following the initiation of predator control was investigated by determining the cause of deaths of radio-collared individuals. Predation was identified as a major cause of death followed by road kills. The non-parametric Kaplan-Meier method modified for staggered entry of individuals was used to estimate survivorship. Although males and females were affected differently by each cause of mortality, their overall survivorship did not differ significantly. Individuals alive at the beginning of the 25-month study had a 61% chance of surviving to the end. This represented an 81% chance of surviving for 1 year. There was no significant difference in survivorship between adults and juveniles. Current rates of adult and juvenile survivorship should allow population recovery, although none has been evident. Pouch young mortality is hypothesised to have inhibited the anticipated quokka population increase since the initiation of predator control. The observed expulsion of pouch young by females when threatened may be a primary predator avoidance strategy.

Seasonal and temperature-related changes in mitochondrial membranes associated with torpor in the mammalian hibernator Spermophilus richardsonii

Abstract Seasonal variations in the thermal response of liver mitochondrial membranes from Richardsons ground squirrels (Spermophilus richardsonii) were determined by measuring succinate-cytochrome c reductase activity and spin label motion over a temperature range of 2 °C to 35 °C. For seven summer animals from the field the Arrhenius-type plots for enzyme activity and spin label motion were biphasic indicating a transition in structure and function at 22 + 2.3°C and 23 ± 1.9°C , respectively; typical of homeothermic mammals. For 12 winter animals maintained at 19°C, the transition in structure and function was lowered to 12 ± 1.1°C and 13 ± 1.4°C , respectively. The transition for 5 of 11 winter animals which were kept at 4°C and maintained normal activity and body temperature was similar to animals maintained at 19°C, while for the other six the transition was further lowered to less than 4°C. The transition for seven winter animals which were in deep hibernation was less than 4°C. The results for liver mitochondria show that lowering of the transition in membrane structure and function occurs as a two-stage process of about 10 deg. C for each stage and that the lowering is a requisite for hibernation rather than a response to the low-body temperatures experienced during hibernation.

Heat tolerance of monotremes

Abstract 1. 1.|Echidnas do not sweat, pant, or lick in response to high ambient temperature ( T a ) Platypus produce copious sweat, presumably under adrenergic control. 2. 2.|At a thermoneutral T a (25°C) echidna body temperature ( T b ) is 32.2 C. When T a rises above this level, T b invariably rises, and T b levels above 38°C are generally lethal. 3. 3.|At normal T b 32°C the oxygen consumption of echidnas is about 33% of that predicted for eutherian mammals. When echidna T b is experimentally raised to eutherian level (38°C), the oxygen consumption is 70% of that predicted for eutherian mammals.

Thermal response of liver mitochondrial membranes of the heterothermic bat (Miniopterus schreibersh) in summer and winter

c reductase and no change in the temperature coefficient of the order parameter of spin label over the temperature range 7-40:C. For the mouse these enzymes showed an increase in E, below about 24~C and a change in molecular ordering at the same temperature. membranes of bats collected in summer and winter. 4. The thermal response of the bat liver mitochondrial membranes is consistent with the heterothermic behaviour of this species. Key Word lndex--Arrhenius activation energy: activity/torpor: summer/winter; Mmiopterus schreibersii; Mus musculus.