A study with dragons: testing body size as a predictor of distributional range in monitor lizards

Abstract

Many important aspects of an organism’s life are closely related to its body size. As a consequence, size can be considered a driving factor on evolutionary history of animals, and some important ecological patterns have been proposed based on this measure. Recently, the Metabolic Theory of Ecology (MTE) suggests that higher metabolic rates are associated with small body sizes and this would determine species distribution patterns. Following the MTE, smaller animals should occupy smaller areas. A remarkably wide body size range can be seen on varanoid lizards. They also occupy a large variety of habitats and seem to have a very early origin, being good models for evolutionary ecology studies. We used the most recent Varanidae phylogeny to test if distribution areas reflect body mass variation. Because there is an ongoing debate about the taxonomic status of some species, we treated these as groups. We used the phylogenetic eigenvector regression method (PVR) both to detect and remove phylogenetic inertia in body and range size. The hypothesis was then tested by a Spearman correlation test using the PVR residuals. The PVR analysis indicated phylogenetic inertia on both variables (area: R²=0.68, p<0.001; body size: R²=0.44, p=0.005). However, the hypothesis test indicated that body size does not correlate with distributional area (ρ=-0.07, p=0.68). This result may be explained by the fact that monitor lizards are illegally traded as pets or source of valuable materials, possibly leading to invasions. Furthermore, the gigantism evolution in this group is reported to be independent of geographic isolation, but associated with habitat use. Therefore, body size might be limited by substrate availability. Moreover, differences in thermoregulatory skills among species may facilitate dispersion. The current findings add to a growing literature on MTE and general ecological patterns, elucidating the poor explanation provided by body size to variance in distribution range.