Geographical variation in body size and sexual size dimorphism in an Australian lizard, Boulenger's Skink (Morethia boulengeri)

Abstract

Ecogeographical rules help explain spatial and temporal patterns in intraspecific body size. However, many of these rules, when applied to ectothermic organisms such as reptiles, are controversial and require further investigation. To explore factors that influence body size in reptiles, we performed a heuristic study to examine body size variation in an Australian lizard, Boulenger's Skink Morethia boulengeri from agricultural landscapes in southern New South Wales, south-eastern Australia. We collected tissue and morphological data on 337 adult lizards across a broad elevation and climate gradient. We used a model-selection procedure to determine if environmental or ecological variables best explained body size variation. We explored the relationship between morphology and phylogenetic structure before modeling candidate variables from four broad domains: (1) geography (latitude, longitude and elevation), (2) climate (temperature and rainfall), (3) habitat (vegetation type, number of logs and ground cover attributes), and (4) management (land use and grazing history). Broad phylogenetic structure was evident, but on a scale larger than our study area. Lizards were sexually dimorphic, whereby females had longer snout-vent length than males, providing support for the fecundity selection hypothesis. Body size variation in M. boulengeri was correlated with temperature and rainfall, a pattern consistent with larger individuals occupying cooler and more productive parts of the landscape. Climate change forecasts, which predict warmer temperature and increased aridity, may result in reduced lizard biomass and decoupling of trophic interactions with potential implications for community organization and ecosystem function.

Figure 1. The Riverina and South-west Slopes study region in southern New South Wales, south-eastern Australia.

Figure 2. Maximum-likelihood phylogenetic tree.

The tree represents 30 unique Morethia boulengeri haplotypes from 471 bp of mitochondrial DNA ND2 gene sequence, using M. adelaidensis (a closely-related species) as the outgroup. Sequences are coded according to major branches of the tree and their sampling locations are shown on the associated map. Sample names refer to the Australian Biological Tissue Collection numbers and unlabelled samples are those collected in this study (indicated by rectangle on map). Bootstrap values are shown on the figure and branches with less than 50% bootstrap support were collapsed.

Figure 3. Sexual differences in body size.

Regression analysis showing mean values and 95% confidence intervals for: a) snout-vent length (SVL mm), and b) body mass (g) between male, gravid female and non-gravid female Morethia boulengeri in southern New South Wales.

Figure 4. Regression analysis of Morethia boulengeri and candidate variables.

The relationships show a positive association with snout-vent length (SVL) and: (a) temperature, (b) rainfall (c) percent bare ground cover, and (d) density of mature trees >50 cm diameter.