Variation in Condition Indexes Due to Climatic and Seasonal Factors in an Australian Desert Lizard, Amphibolurus-Nuchalis

Abstract

The precise nature of the allometric relationships between body size and body mass, water content and solid matter was derived from a sample of over 700 lizards of the agamid species Amphibolurus nuchalis (= Ctenophorus inermis), collected during 1969-82 at Shark Bay in Western Australia. Three condition indices relating each variable to body size, as expressed by snout-vent length, were derived from these allometric relationships for male, female and juvenile lizards. The formation of condition indices was compared to analysis of covariance as a method of analysis of such data. Variation in these indices with weather and season over the period of the study were investigated by means of regression models. Variation between days within trips was relatively small, suggesting a time scale for physiological change of greater than 3-4 days. Ascribing cause to specific weather and seasonal factors was difficult, due to the high colinearity of the explanatory variables. The influence of temperature and rainfall on the condition indices was evident in the model, however, and accords well with physiological data for this species. The extent to which the condition indices could be predicted by a combination of weather and seasonal variables was encouraging, and suggests that the method may have general utility in the study of changes in body composition and condition of free-ranging animals.