Aspects of the thermal ecology of the group-living lizard, Cordylus cataphractus : a spatial and temporal analysis

[摘要] ENGLISH ABSTRACT: Thermal ecology is a central theme in reptilian biology because of the thermodynamic ratedependence of virtually all biological processes in these ectothermic animals.Thermoregulation includes active processes (with associated energetic costs related toaltered behaviour and physiology) functioning to maintain body temperatures within apreferred temperature range, so that the majority of physiological functions occurs optimally,despite natural variation in the animal's thermal habitat. The recent development ofquantitative thermal indices now allows researchers to describe the thermal habitat andthermoregulatory functioning of an ectotherm within its environment from a cost-benefitperspective. The use of such quantitative biophysical approaches to reptile thermal ecologystudies is however limited in the African context. Cordylus cataphractus is one of the beststudied cordylids, and exhibits various characteristics atypical for the family, such aspermanent group-living, seasonally lowered surface activity, a low resting metabolic rate andlarge fat bodies. These characteristics are generally thought to be associated with grouplivingin a semi-arid habitat, yet, the possible links to thermal ecology remains unexplored.The objectives of the current study was: firstly, to characterize the preferredtemperature range (Tp) of C. cataphractus through the use of ecologically realistic laboratorythermal gradients; secondly, to explore seasonal and geographical variation in thermalpreference, by comparing Tp among individuals captured from a coastal and inlandpopulation and during different seasons (autumn and spring); thirdly, to describe the thermalhabitat of a C. cataphractus population during summer, autumn, winter and spring and tothen relate these findings to the seasonal activity patterns reported in literature for thespecies; fourthly, to describe the seasonal patterns of thermoregulation (during summer,autumn, winter and spring) in a C. cataphractus population through quantitativethermoregulatory indices; fifthly, to assess geographic variation in the thermal habitat and IVassociated patterns of thermoregulation in C. cataphractus among a coastal population(western range limit) and an inland population (eastern range limit). The thermal habitat of C.cataphractus was described by measuring operative environmental temperatures (Te) withhollow copper lizard models placed around rocks according to the natural surface movementpatterns of the species. Variation in thermal habitat quality was subsequently calculated (de= |Te – Tp|) and averaged. Field body temperatures (Tb) of lizards were measured withdorsally attached miniature temperature loggers. Thermoregulatory indices were calculatedfrom Te, Tb and Tp, describing: thermoregulatory accuracy, the effectiveness ofthermoregulation and thermal exploitation for each population (coastal and inland) for therespective sampling periods.The preferred body temperature range of C. cataphractus is the lowest recordedamong cordylids to date (mean Tp = 29.8oC) and was conserved among different populationsand within these populations among seasons, despite the fact that environmentaltemperatures are known to vary geographically and seasonally.Thermal habitat quality varied significantly at micro spatial scale around rocks in thecoastal population. Since C. cataphractus males are territorial, competition for thermalhabitat quality around rocks may therefore occur. Such effects will be a function of the timeof year since the variability in thermal habitat quality among rock aspects (around rocks)varied seasonally.Thermal habitat quality of crevices varied among seasons and was typically higher inthe open, outside rock crevices, during the cooler winter and spring periods, whereas insummer and autumn the crevice environments were more favourable. Thermal habitatquality was high in crevices during autumn, suggesting that the observed repressed surfaceactivity of C. cataphractus described for the time is not necessarily, as previously thought,only due to food constraints. Moreover, in contrast to earlier reports, the current results (Tbversus Te) indicate that individuals emerged from crevices in summer.The geographical assessment indicated that lizards from the coastal population, withgenerally larger groups, thermoregulated more successfully than those from the inland population. The higher thermoregulatory success in the coastal population occurred in spiteof the fact that thermal habitat quality was significantly lower at the coastal locality. Thehigher thermoregulatory success in the coastal population was likely due to reducedpredation risk associated with increased group-size. The seasonal trends inthermoregulation at the coastal and inland population corresponded to the patterns predictedby the cost-benefit model of thermoregulation, accuracy of thermoregulation and theeffectiveness of thermal exploitation being higher during the thermally more favourableautumn.

[发布机构] Stellenbosch University