Gavin S. Bedford

Seasonal Changes in Thermoregulation by the Frillneck Lizard, Chlamydosaurus Kingii, in Tropical Australia

The frillneck lizard Chlamydosaurus kingii is an arboreal lizard that is a conspicuous component of the reptile fauna of the wet-dry tropics of northern Australia during the wet season. During the dry season, however, they are secretive, and a previous study revealed that during this season they remain perched in trees and have field metabolic rates only 28% of the wet season levels. Body temperatures (Tbs) of the lizards were measured by radio telemetry throughout the day during the wet and dry seasons. The midday Tbs during the wet season were high (grand mean = 36.70C) and typical for heliothermic lizards, but the dry season midday Tbs were significantly lower (grand mean = 32.80C). Microclimatic data and physical characteristics of the lizards were used in a biophysical model to calculate the operative temperatures (Te) of lizards in the shade, in the sun on a horizontal plane, and normal to the sun at each hour of the day for the two seasons. The Tes revealed the physical possibility for the lizards to achieve much higher Tbs during the dry season than were measured. Thus, the lower Tbs in the dry season represent a shift in preference rather than an inability to attain a high Tb during the cooler dry season. Inspection of the Tbs and Tes revealed that although the lizards remained cooler in the dry season, they did not thermoregulate at the lowest possible Tbs. During both seasons the lizards basked in the sun early and late in the day, but during the dry season the lizards stopped intensive basking at a Tb -40C lower than in the wet season. An index of the extent to which the lizards exploit the available thermal environment indicates that they thermoregu- late carefully in both seasons. Tbs were also measured in a laboratory thermal gradient during both seasons, and the Tbs selected during the dry season were significantly lower than those selected in the wet season. This suggests that the seasonal shift in thermal preference is an acclimatization response or an endogenous seasonal cycle rather than a response to a simple thermal cue. The lower Tbs in the dry season result in a conservation of energy and water during a season when these resources are relatively scarce. However, the fact that the lizards do not thermoregulate at the lowest possible Tbs suggests that the dry season Tbs represent a compromise between conservation of resources and the ability to perform other functions such as escape predators and/or digest food.

Physiological ecology of frillneck lizards in a seasonal tropical environment

The frillneck lizard, Chlamydosaurus kingii, is a conspicuous component of the fauna of the wetdry tropics of northern Australia during the wet season, but it is rarely seen in the dry season. Previous studies have demonstrated that during the dry season the field metabolic rate (FMR) is only about one-quarter of the wet-season rate, and one factor involved in this seasonal drop is a change in the behavioural thermoregulation of the species such that lower body temperatures (Tbs) are selected during dry-season days. Here we examine other factors that could be responsible for the seasonal change in FMR: standard metabolic rates (SMR) and activity. Samples from stomach flushing revealed that the lizards in the dry season continued to feed, but the volume of food was half as much as in the wet season. SMR in the laboratory was 30% less in the dry season. During the dry season, the energy expended by the lizards is 60.4 kJ kg-1 day-1 less than during the wet season. Combining laboratory and field data, we determined the relative contribution of the factors involved in this energy savings: 10% can be attributed to lower nighttime Tb, 12% is attributable to lower daytime Tb, 12% is attributable to decreased metabolism, and the remaining 66% is attributable to other activities (including e.g. locomotion, reproductive costs, digestion). Calculations indicate that if FMR did not drop in the dry season the lizards would not survive on the observed food intake during this season. Seasonal analysis of blood plasma and urine indicated an accumulation of some electrolytes during the dry season suggesting modest levels of water stress.

Energetic consequences of metabolic depression in tropical and temperate-zone lizards

One response of ectothermic animals to periods of inactivity is inverse acclimation, or metabolic depression, which results in the conservation of energy. Most studies of metabolic depression and acclimation have involved temperate-zone species, and the information from tropical species has been largely restricted to laboratory studies that failed to demonstrate thermal acclimation of metabolism. Recently, metabolic depression has been shown in several species of reptiles from the wet-dry tropics of northern Australia during the dry season. We review existing data on the energy budgets of temperate and tropical species during periods of inactivity and make calculations of energy saved due to metabolic depression across a range of temperatures. Because tropical species experience relatively high temperatures during periods of inactivity, they have a greater potential for energy savings, any enhancement of their metabolic depression is disproportionately advantageous with respect to energy savings, and in some species metabolic depression is probably essential for survival. Thus, we would expect metabolic depression to be well developed in some tropical reptiles. The lack of thermal acclimation in laboratory studies indicates that environmental parameters other than temperature (such as food or water) may initiate metabolic depression in tropical species. Higher temperatures, however, magnify the energy savings accomplished by metabolic depression.

Digestive Efficiency in Some Australian Pythons

Abstract The digestive efficiencies of seven taxa of Australian pythons were examined with respect to body temperature, meal size, and passage time. The hair of rodents was not digested by the snakes. Digestive efficiencies were not influenced by body temperature, and the values ranged from 89% to 98% if hair was included and from 99.3% to 99.8% if hair was excluded from the analyses. The digestive efficiency of different food types can be made more comparable by taking into account the known indigestible components. Passage time was significantly longer at 24 C than at 30 or 33 C. Passage time was not related to relative meal size.

Standard metabolic rate and preferred body temperatures in some Australian pythons

Pythons have standard metabolic rates and preferred body temperatures that are lower than those of most other reptiles. This study investigated metabolic rates and preferred body temperatures of seven taxa of Australian pythons. We found that Australian pythons have particularly low metabolic rates when compared with other boid snakes, and that the metabolic rates of the pythons did not change either seasonally or on a daily cycle. Preferred body temperatures do vary seasonally in some species but not in others. Across all species and seasons, the preferred body temperature range was only 4.9˚C. The thermal sensitivity (Q10) of oxygen consumption by pythons conformed to the established range of between 2 and 3. Allometric equations for the pooled python data at each of the experimental temperatures gave an equation exponent of 0.72–0.76, which is similar to previously reported values. By having low preferred body temperatures and low metabolic rates, pythons appear to be able to conserve energy while still maintaining a vigilant ‘sit and wait’ predatory existence. These physiological attributes would allow pythons to maximise the time they can spend ‘sitting and waiting’ in the pursuit of prey.

THERMOREGULATION BY THE SPOTTED TREE MONITOR, VARANUS SCALARIS, IN THE SEASONAL TROPICS OF AUSTRALIA

Abstract 1. 1. Varanus scalaris was studied in the wet and dry seasons by measuring body temperatures (Tb) in the field and in a laboratory thermal gradient, and by claculating operative temperatures (Te) in conjunction with microclimatic data. 2. 2. In the field, predawn Tbs in the wet season were higher than in the dry season, and this is attributed to environmental conditions. Wet season Tbs were also higher during the day, but this is attributed to the behaviour of the lizards because Tes indicated that the thermal environment was not limiting. The mean midday Tb was 38.9°C in the wet season and 35.6°C in the dry. 3. 3. In the laboratory, the lizards selected a mean Tb of 38.1°C in the wet season and 35.2°C in the dry season. Thus, in both the field and the laboratory, the lizards maintained Tbs approximately 3°C lower in the dry season.

Metabolic response to feeding and fasting in the water python (Liasis fuscus)

Compared with other reptiles, pythons have a relatively low standard metabolic rate (SMR) when postabsorptive, but metabolism increases substantially after feeding. This study examined the effects of feeding and fasting on adult and hatchling water pythons (Liasis fuscus). We compared ratios of peak digestive metabolic rate (PDMR) after feeding with the metabolic rate of both post-absorptive (SMR) and fasted water pythons. If metabolic rate of a fasting snake is taken as ‘SMR’, then the ratio PDMR/SMR becomes increasingly exaggerated as fasting continues. After 56 days of fasting in adults, or after 45 days in hatchlings, the metabolic rate of water pythons was significantly lower than that of post-absorptive animals. Peak digestive metabolic rate of post-absorptive adult water pythons was only 6.3–12.0 times SMR, but the ratio was twice that if fasted (metabolically depressed) animals were used to determine the ‘SMR’ denominator. Thus, this ratio should be used with caution. Peak digestive metabolic rate after feeding increased with increasing meal size for meals less than 20% of body mass, but PDMR did not increase for meals between 20% and 39% of body mass for adult water pythons. Similarly, the PDMR did not increase significantly between 25% and 50% meal sizes for hatchlings. The digestive physiology of water pythons is apparently better suited to frequent meals of relatively small prey compared with the digestive physiology of some other pythons. D ri die G. B K. i Z10

Energetics and water flux of the marbled velvet gecko (Oedura marmorata) in tropical and temperate habitats

Abstract The gecko Oedura marmorata was studied in two different climatic zones: the arid zone of central Australia and in the wet-dry tropics of northern Australia. Doubly labelled water was used to measure field metabolic rate (FMR) and water flux rates of animals in the field during the temperate seasons of spring, summer and winter, and during the tropical wet and dry seasons. FMRs were highest in the tropical wet season and lowest in the temperate winter. The geckos in central Australia expended less energy than predicted for a similarly sized iguanid lizard, but geckos from the tropics expended about the same amount of energy as predicted for an iguanid. Water flux rates of geckos from the arid zone were extremely low in all seasons compared to other reptiles, and although water flux was higher in tropical geckos, the rates were low compared to other tropical reptiles. The standard metabolic rates (SMRs) of geckos were similar between the two regions and among the seasons. Geckos selected higher body temperatures (Tbs) in a laboratory thermal gradient in the summer (33.5°C) and wet (33.8°C) seasons compared to the winter (31.7°C) and dry (31.4°C) seasons. The mean Tbs selected in the laboratory thermal gradient by geckos from the two regions were not different at a given time of year. The energy expended during each season was partitioned into components of resting metabolism, Tb and activity. Most of the energy expended by geckos from central Australia could be attributed to the effects of temperature on resting lizards in all three seasons, but the energy expended by tropical geckos includes a substantial component due to activity during both seasons. This study revealed variability in patterns of ecological energetics between populations of closely related geckos, differences which cannot be entirely attributed to seasonal or temperature effects.

Seasonal effects on intestinal enzyme activity in the Australian agamid lizard, Lophognathus temporalis

The tropical agamid lizard, Lophognathus temporalis, has higher metabolic and feeding rates during the wet season compared to the dry season. Also, lizards from urban sites tend to be larger than those from natural sites, partly due to site differences in food availability. Therefore, we hypothesized that activity of membrane-bound intestinal enzymes and masses of organs related to digestion would differ both seasonally and between urban and natural sites. To test this, we measured activities of aminopeptidase-N (APN), maltase, and sucrase, as well as organ masses. APN activity (micromol min(-1) g(-1)) was highest in the middle portion of the intestine (section 2), followed by the proximal portion (section 1) and then the distal portion (section 3). Maltase activity was highest in section 1 and decreased distally. We detected some sucrase activity in section 1 but none in sections 2 or 3. We found similar enzyme activities within each section irrespective of site or season. However, total enzyme activities were higher during the wet season compared to the dry season for both urban and bush L. temporalis. Total wet season enzyme activity in urban and bush L. temporalis was greatest for APN (25.4; 15.8 micromol min(-1); respectively), then maltase (3.9; 3.6 micromol min(-1); respectively) and then sucrase (0.3; 0.2 micromol min(-1); respectively). The higher total enzyme activities was the result of an increase in intestinal mass during the wet season.

Habitat Differences in Body Size and Shape of the Australian Agamid Lizard Lophognathus temporalis

Abstract Body size and morphology are important traits that can strongly influence the life history of an organism. One important factor affecting these traits is habitat. Urbanization has resulted in the significant modification of many habitats, and thus, it may be a factor affecting the body size and morphology of species living in the urban environment. In this study, we compared body size and morphology in urban and bush populations of the lizard Lophognathus temporalis in and around Darwin, Australia. We made monthly measurements of invertebrate abundance and soil moisture during one year to compare seasonal variation in resource availability between habitats. We also collected all matter excreted by L. temporalis during their first four days in captivity as an index of food consumption in the field. We found that male L. temporalis were larger than females and that urban L. temporalis were larger than bush L. temporalis. Males had longer front limbs, hind limbs, and tails than did females; and in urban populations, they also had larger heads. Urban L. temporalis had longer front and hind limbs than did bush L. temporalis, although head size and tail length were similar for both groups. Resource availability was seasonally more stable in urban habitats than in bush habitats, and urban L. temporalis consumed more than bush L. temporalis all year round. We conclude that differences in resource abundance between habitats may be an important factor contributing to the morphological differences between urban and bush-dwelling L. temporalis.