Keith A. Christian

Thermoregulation of Monitor Lizards in Australia: An Evaluation of Methods in Thermal Biology

The aims of this paper are to compare the thermal ecology of four species of varanid lizards that occupy a range of habitats and climatic regions, and to assess the efficacy of methods for evaluating the extent to which ectothermic animals exploit their thermal environments. Hertz et al. (1993) have proposed several indices of thermoregulation, and these are evaluated with respect to our data from varanid lizards. The thermoregulatory characteristics of three tropical monitor lizards (Varanus panoptes, V. gouldii, and the semiaquatic V. mertensi), and the temperate—zone V. rosenbergi were studied throughout the year. Radiotelemetry was used to measure the body temperatures (Tbs) of free—ranging animals, and microclimatic data were collected to determine the range of possible Tbs that an animal could achieve. Operative temperatures (Tbs) were estimated by biophysical models for each set of animal characteristics and microclimatic conditions. The Tbs selected by animals in a laboratory thermal gradient were used to determine the set—point range of Tbs that the animals voluntarily select. Plots that superimpose Tbs, Tbs, and the set—point range across the day are extremely useful for describing the thermoregulatory characteristics of ectotherms. These plots can be used to determine the extent to which the animals exploit their thermal environment: we define an index of thermal exploitation (Ex) as the time in which Tbs are within the set—point range, divided by the time available for the animal to have its Tbs within the set—point range. Only V. mertensi was active throughout the year. In general, during seasons of inactivity, the Tbs of inactive species fell outside the set—point range, but during periods of activity all species selected Tbs within their set—point range. The temperate—zone species (V. rosenbergi) thermoregulates very carefully during periods when environmental conditions allow the animals to attain the set—point range, and V. gouldii also thermoregulates carefully in the wet season. V. mertensi selects Tbs that are significantly lower than the other species both in the field and in the laboratory, and thermoregulatory indices of this species were intermediate relative to the other species. The amount of time spent in locomotion each day was not correlated with the indices of thermoregulation: the most active species, V. panoptes, was, with respect to several indices, the least careful thermoregulator. The type of question that is being addressed, with respect to the interactions between an animals thermal environment and its thermoregulatory behavior, determines the appropriateness of the various indices of thermoregulation. The Ex index describes the thermoregulatory characteristics of ecotherms in a heterogeneous thermal environment, and in such an environment a large amount of information can easily be interpreted graphically. This index is less useful in a thermally homogeneous environment.

Ecological relations among space, time, and thermal niche axes

While the physical environment is often viewed as that with which an organism must contend, and the biotic environment as including resources that may be exploited, interactions occurring between the physical and biotic portions of animal niches suggest that animals also exploit their physical environments. The space and time required for physiological optimal interactions with the environment represent units with which the thermal environment can be quantified as an ecological resource. Characteristics of the thermal environment that an animal experiences within its home range have been incorporated into measures to quantify the quality of the home range with respect to temperature relations, to evaluate the importance of thermal transients to an animal, and to assess temporal aspects of habitat exploitation. Since the thermal environment is physiologically important to animals, and since there can be competition for space and time, this approach provides a bridge between physiological and population/evolutionary ecology.

Seasonal Shifts in Body Temperature and Use of Microhabitats by Galapagos Land Iguanas (Conolophus Pallidus)

Seasonal differences in the body temperatures (T,,) of free-ranging Galapagos land iguanas (Conolophlns pallidus) were detected by temperature sensitive telemetry transmitters. Midday 7,/s of iguanas average 4.4?C lower in the Garua (cool) season than in the Hot season. Measured T,,s and those predicted from biophysical models permitted the following conclusions: (1) lower T,,s during the Garua season represent an active shift in thermoregulation by the iguanas rather than a passive result of a cooler season; (2) the average midday T., selected by the iguanas in either season is the T., that allows maintenance of a constant T,, for the longest possible portion of the day; (3) by exploiting the warmer microclimate created by a cliff face, the iguanas are able to maintain a constant T., for a full hour longer than they could elsewhere in their home range. Census data demonstrated that the iguanas exploited the warmer microclimate created by the cliff extensively during the Garua season, and the cliff face was visited by the iguanas relatively infre- quently during the Hot season. Thus, the exploitation of the microclimate created by the cliff results in seasonal differences in the pattern of space utilization within the home ranges of the iguanas. Within the Garua season the iguanas moved away from the cliff more often on sunny days than during cloudy days. It is concluded that the physical environment is an important determinant of patterns of space utilization both within and between seasons.

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.

THE ADAPTIVE SIGNIFICANCE OF REPTILIAN VIVIPARITY IN THE TROPICS: TESTING THE MATERNAL MANIPULATION HYPOTHESIS

Abstract Phylogenetic transitions from oviparity to viviparity in reptiles generally have occurred in cold climates, apparently driven by selective advantages accruing from maternal regulation of incubation temperature. But why, then, are viviparous reptiles so successful in tropical climates? Viviparity might enhance fitness in the tropics via the same pathway as in the temperate zone, if pregnant female reptiles in the tropics maintain more stable temperatures than are available in nests (Shines maternal manipulation hypothesis). Alternatively, viviparity might succeed in the tropics for entirely different reasons than apply in the temperate zone. Our data support the maternal manipulation hypothesis. In a laboratory thermal gradient, pregnant death adders (Acanthophis praelongus) from tropical Australia maintained less variable body temperatures (but similar mean temperatures) than did nonpregnant females. Females kept at a diel range of 25–31°C (as selected by pregnant females) gave birth earlier and produced larger offspring (greater body length and head size) than did females kept at 23–33°C (as selected by nonpregnant snakes). Larger body size enhanced offspring recapture rates (presumably reflecting survival rates) in the field. Thus, even in the tropics, reproducing female reptiles manipulate the thermal regimes experienced by their developing embryos in ways that enhance the fitness of their offspring. This similarity across climatic zones suggests that a single general hypothesis—maternal manipulation of thermal conditions for embryogenesis—may explain the selective advantage of viviparity in tropical as well as cold-climate reptiles.

Comparative Analysis of Cutaneous Evaporative Water Loss in Frogs Demonstrates Correlation with Ecological Habits

Most frog species show little resistance to evaporative water loss (EWL), but some arboreal species are known to have very high resistances. We measured EWL and cutaneous resistance to evaporation (Rc) in 25 species of frogs from northern Australia, including 17 species in the family Hylidae, six species in the Myobatrachidae, and one each in the Bufonidae and the Microhylidae. These species display a variety of ecological habits, including aquatic, terrestrial, and arboreal specialisations, with the complete range of habits displayed within just the one hylid genus, Litoria. The 25 species measured in this study have resistances that range from Rc = 0 to 63.1. These include low values indistinguishable from a free water surface to high values typical of “waterproof” anuran species. There was a strong correlation between ecological habit and Rc, even taking phylogenetic relationships into account; arboreal species had the highest resistance, aquatic species tended to have little or no resistance, and terrestrial species tended to have resistance between those of arboreal and aquatic frogs. For one species, Litoria rubella, we found no significant changes in EWL along a 1,500‐km aridity gradient. This study represents the strongest evidence to date of a link between ecological habits and cutaneous resistance to water loss among species of frogs.

Integrated pest management in mango orchards in the Northern Territory Australia, using the weaver ant, Oecophylla smaragdina , (Hymenoptera: Formicidae) as a key element

Mango is the most important commercial fruit crop in the Northern Territory, Australia. Growers currently rely on insecticides to control insect pests of mango, resulting in environmental and social problems. To reduce dependency on insecticides, a suitable IPM programme is needed. Previous work showed that weaver ants can control the major mango insect pests, but they protect soft scales, damage fruits by their formic acid and annoy operatives during fruit harvest. Further research addressing these constraints showed that certain chemicals can reduce soft scale numbers without seriously affecting weaver ant populations, the isolation of ant colonies reduces fruit damage by formic acid, and water spray reduces the ant activity during harvest. A field experiment with two treatments, weaver ants plus soft chemicals (WPS) and weaver ants plus chemical insecticides (CI), was conducted at Howard Springs in the Northern Territory. In two out of the three years, the yield of first class fruit was 20% higher in WPS than in CI. This was mostly explained by lower insect pest damage, lower incidence of mango scab disease and lower infestation of lenticels in WPS. Trees in WPS produced significantly more fruits than in CI in 2002. Overall, trees in WPS resulted in a profit of A

Seasonal activity and energetics of two species of varanid lizards in tropical Australia

14.50/tree per year, but trees in CI produced only A

Diet and habitat use of frillneck lizards in a seasonal tropical environment

8.38/tree per year. During harvest, farmers experienced only minor problems with ant disturbance. An IPM model using weaver ants as a key element is discussed with respect to ‘organic’ production.

Not just small, wet, and cold: effects of body size and skin resistance on thermoregulation and arboreality of frogs

The field metabolic rates (FMR) and rates of water flux were measured in two species of varanid lizards over five periods of the year in tropical Australia. The energetics of these species were further investigated by directly measuring activity (locomotion) and body temperatures of free-ranging animals by radiotelemetry, and by measuring standard metabolic rate (over a range of body temperatures) and activity metabolism in the laboratory. Seasonal differences in the activity and energetics were found in these goannas despite similar, high daytime temperatures throughout the year in tropical Australia. Periods of inactivity were associated with the dry times of the year, but the onset of this period of inactivity differed with respect to habitat even within the same species. Varanus gouldii, which inhabit woodlands only, were inactive during the dry and late dry seasons. V. panoptes that live in the woodland had a similar seasonal pattern of activity, but V. panoptes living near the floodplain of the South Alligator River had their highest levels of activity during the dry season when they walked long distances to forage at the receding edge of the floodplain. However, during the late dry season, after the floodplain had dried completely, they too became inactive. For V. gouldii, the rates of energy expenditure were 196 kJ kg−1 day−1 for active animals and 66 kJ kg−1 day−1 for inactive animals. The rates of water influx for these groups were respectively 50.7 and 19.5 ml kg−1 day−1. For V. panoptes, the rates of energy expenditure were 143 kJ kg−1 day−1 for active animals and 56 kJ kg−1 day−1 for inactive animals. The rates of water influx for these two groups were respectively 41.4 and 21.0 ml kg−1 day−1. We divided the daily energy expenditure into the proportion of energy that lizards used when “in burrows”, “out of burrows but inactive”, and “in locomotion” for the two species during the different seasons. The time spent in locomotion by V. panoptes during the dry season is extremely high for a reptile (mean of 3.5 h/day spent walking), and these results provide an ecological correlate to the high aerobic capacity found in laboratory measurements of some species of varanids.