Gordon C. Grigg

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.

Does dingo predation control the densities of kangaroos and emus

The density of red kangaroos in the sheep country of the north-west corner of New South Wales is much higher now that it was last century. It is also much higher than the present density across the dingo fence in the adjacent cattle country of South Australia and Queensland. The picture is similar for emus. Farther east, about halfway along the New South Wales–Queensland border, no difference in density between the two States could be detected for red kangaroos, grey kangaroos or emus. We examine and discard several hypotheses to account for the density contrasts in the west and the lack of them farther east, deeming it unlikely that the pattern reflects environmental gradients, or differences in plant composition and growth, hunting pressure or availability of water. Instead, we favour this hypothesis: that the past and present patterns of density are attributable directly to predation by dingoes, which can hold kangaroos at very low density in open country if the dingoes have access to an abundant alternative prey.

Evidence for Endothermic Ancestors of Crocodiles at the Stem of Archosaur Evolution

Physiological, anatomical, and developmental features of the crocodilian heart support the paleontological evidence that the ancestors of living crocodilians were active and endothermic, but the lineage reverted to ectothermy when it invaded the aquatic, ambush predator niche. In endotherms, there is a functional nexus between high metabolic rates, high blood flow rates, and complete separation of high systemic blood pressure from low pulmonary blood pressure in a four‐chambered heart. Ectotherms generally lack all of these characteristics, but crocodilians retain a four‐chambered heart. However, crocodilians have a neurally controlled, pulmonary bypass shunt that is functional in diving. Shunting occurs outside of the heart and involves the left aortic arch that originates from the right ventricle, the foramen of Panizza between the left and right aortic arches, and the cog‐tooth valve at the base of the pulmonary artery. Developmental studies show that all of these uniquely crocodilian features are secondarily derived, indicating a shift from the complete separation of blood flow of endotherms to the controlled shunting of ectotherms. We present other evidence for endothermy in stem archosaurs and suggest that some dinosaurs may have inherited the trait.

Respiratory control in the lungfish, Neoceratodus forsteri (krefft)☆

1. Respiratory control has been studied in the lungfish, Neoceratodus forsteri by measuring ventilation (Ve), oxygen uptake (VO2), per cent O2 extraction from water, breathing rates of branchial and aerial respiration and changes in blood gas and pulmonary gas composition during exposure to hypoxia and hypercarbia. 2. Hypoxic water represents a strong stimulus for compensatory increase in both branchial and aerial respiration. Water ventilation increases by a factor of 3 or 4 primarily as a result of increased depth of breathing. 3. The ventilation perfusion ratio decreased during hypoxia because of a marked increase in cardiac output. Hypoxia also increased the fraction of total blood flow perfusing the lung. Injection of nitrogen into the lung evoked no compensatory changes. 4. It is concluded that the chemoreceptors eliciting the compensatory changes are located on the external side facing the ambient water or in the efferent branchial blood vessels. 5. Elevated pCO2 in the ambient water depressed the branchial respiration but stimulated aerial respiration. 6. It is suggested that the primary regulatory effect of the response to increased ambient pCO2 is to prevent CO2 from entering the animal, while the secondary stimulation of air breathing is caused by hypoxic stimulation of chemoreceptors located in the efferent branchial vessels.

Trends In The Numbers Of Red Kangaroos And Emus On Either Side Of The South Australian Dingo Fence: Evidence For Predator Regulation?

Most of Australia’s sheep rangelands are enclosed by a dingo-proof fence. Within these rangelands, where dingoes (Canis lupus dingo) are rare, red kangaroos (Macropus rufus) are considered to be food limited because their numbers respond to fluctuations in pasture biomass that are driven by highly variable rainfall. Outside this region, where dingoes are common, kangaroo densities are generally substantially lower, suggesting that dingoes are an important limiting factor. However, it is unclear whether dingoes can regulate kangaroo populations. In this study, red kangaroo and emu (Dromaius novaehollandiae) numbers were monitored for varying periods during 1978–92 by aerial survey on both sides of the dingo fence in three areas in the north of the South Australian pastoral zone. Densities of red kangaroos and emus were lower outside the fence, although the disparity varied between areas and over time. The similarity in the environments on both sides of the fence and the marked step in kangaroo density at the fence are consistent with dingoes strongly limiting these prey populations. In the north-east of the pastoral zone, where kangaroo and emu densities are greatest, the contrast in density across the fence was most pronounced. Furthermore, the trends in density over time differed across the fence. Outside the fence, red kangaroos and emus remained at low densities following drought as dingo numbers increased. Inside the fence, red kangaroo and emu populations showed a ‘typical’ post-drought recovery. The data therefore suggest that, in some situations, dingoes may not simply limit red kangaroo and emu populations, but also regulate them. For this to occur, predation rate would need to be density dependent at low prey densities. The availability of alternative prey, and the reduction in the numbers of all prey during drought may provide the mechanism.

Time constants of heating and cooling in the eastern water dragon, Physignathus lesueruii and some generalizations about heating and cooling in reptiles

In keeping with other reptiles, core temperature of the lizard Physignathus lesueurii responds more rapidly to a step function increase in temperature than to a corresponding decrease. 2. Observations on twelve species (five families) of non-Chelonian reptiles heating and cooling in air and water show that strongly predictable relationships exist between thermal time constants and body size. Chelonia show a different pattern. 3. These observations are compared with the predictions of a simple model which, although not sufficiently complex to simulate physiological changes, provides insight into the relative importance of the physical and biological factors which underlie the observed relationships.

A Double-Survey Estimate Of Population Size From Incomplete Counts

We sought to estimate the number of crocodile nests on the Liverpool River, Northern Territory, Australia. Two methods of survey were available, aerial survey and ground survey, in each of which sightings could be mapped. Hence, those nests which were detected by both methods could be identified. These counts were used to demonstrate the method by which an estimate of total number could be calculated. The assumptions and limitations are discussed.

KANGAROOS AND CLIMATE: AN ANALYSIS OF DISTRIBUTION

(1) The distributions of three partially sympatric kangaroo species, Macropus giganteus (Shaw), M. fuliginosus (Desmarest) and M. rufus (Desmarest), were analysed to determine their climatic characteristics. (2) M. giganteus occupies areas only where rainfall either has little seasonal trend or where rainfall in summer exceeds winter rainfall. (3) M. fuliginosus is found in areas of uniform or winter rainfall. (4) Seasonality of rainfall has little influence on the distribution of M. rufus. Instead, its distribution reflects interaction between mean annual precipitation and mean annual temperature. (5) The extent of sympatry and allopatry appears to be determined by the independent reaction of each species to specific and differing climatic stimuli rather than by biological interaction between species.

Respiratory properties of blood and pattern of gas exchange in the lungfish Neoceratodus forsteri (Krefft).

Blood respiratory properties and gas exchange patterns have been studied in the lungfish Neoceratodus. O2 - Hb dissociation curve reveals a high affinity for O2 (P50 of 11 mm Hg at PCO2 3.5 mm Hg). No Root effect but a pronounced Bohr effect was discernible. Temperature exerted only a minor influence on the affinity for O2. The CO2 dissociation curves conform to the shape observed in other classes of vertebrates and show a steep initial portion. The buffering capacity of the blood was low being 13.3 mMol/1/pH. An increased buffering power upon reduction of Hb was apparent. During rest in well oxygenated water, gill breathing prevailed entirely. Pulmonary arterial blood assumed to represent mixed arterial blood characteristically showed a Pco2 averaging 3.5 mm Hg. PaO2 was 40 mm Hg corresponding to an O2 saturation of 95%. The pulmonary venous blood showed gas partial pressures in equilibrium with those in pulmonary arterial blood and air, documenting the minor importance of the lung as a gas exchanger during these conditions. When the fish was artificially exposed to air PaO2 diminished to 10-15 mm Hg with a concurrent increase in PaCO2 to 20 mm Hg in 30 min. While the lung performed well as an O2 absorber during air exposure it was totally inefficient in eliminating CO2. The data indicate that Neoceratodus is unfit to live out of water for any length of time.

Temperature-induced changes in the oxygen equilibrium curve of the blood of the brown bullhead, Ictalurus nebulosus☆

The affinity of blood for oxygen is dependent on temperature, which would seem to present a disadvantage to those fishes which encounter large seasonal temperature changes. Considering the well-known acclimatory abilities of many fishes, it would seem reasonable to propose the occurrence of seasonal modification of blood oxygen equilibria to compensate for changes in temperature. 2. In Ictalurus nebulosus, blood from one group of fish acclimated at 24 degrees C showed a consistently higher oxygen affinity compared with a group acclimated at 9 degrees C, when measured at the same temperature. This shift, accompanying thermal metabolic acclimation, minimizes the effect of temperature on oxygen affinity. 3. The shift did not persist when dilute solutions of hemoglobin were studied. 4. No changes with acclimation were seen in the multiple hemoglobin pattern nor in blood pH. 5. The erythrocyte, rather than the plasma, appears to be the site of modification. 6. Some large changes in erythrocyte potassium accompanied thermal acclimation, but the relation of this to the shift in oxygen affinity is unknown.