Ecological Segregation, Color Matching, and Speciation in Lizards of the Amphibolurus Decresii Species Complex (Lacertilia: Agamidae)

Abstract

The Amphibolurus decresii complex comprises three species, A. decresii, A. fionni, and A. vadnappa, having restricted distribution in South Australia and extreme western New South Wales, Australia. Adults of all three species are similar in appearance, apart from color and pattern differences, particularly in males, and are restricted to rocky outcrops. Amphibolurus decresii and A. vadnappa are partially sympatric, but are ecologically segregated according to the color of rocks on which they perch. Amphibolurus decresii is found on rocks which are predominantly pinkish yellow, whereas A. vadnappa is found on rocks which are dark reddish brown. Each species appears camouflaged on its respective mineral type which it prefers in choice tests. Amphibolurus fionni is allopatric from the other species and exhibits variable coloration in its "substrate races." It has a distinctive color patter of its own, but resembles one or the other species in areas where rocky habitats are correspondingly similar. Studies of reflectivity of lizard skin demonstrate that the dorsal coloration on the back, tail, and legs is relatively labile. Maximum blanching occurs at high body temperatures. Lability is correlated with latitude, southern populations being darker and possessing less lability than northern populations. The dorsal coloration is thought to be important in thermoregulation. Reflectance from dorsolateral patterns of lizards shows little lability (<3% at all visible wavelengths) and matches the reflectance of respective mineral backgrounds. These patterns also resemble the colors and shapes of lichens that grow upon the rocks. Both dorsal and dorsolateral coloration are believed to have camouflage functions, but the latter appears more important because (1) it matches the substrate more closely, and (2) it does not change appreciably with body temperature. Evidence suggests that species comprising the A. decresii complex evolved from a common ancestor in South Australia. With respect to A. decresii and A. vadnappa, a model of ecological speciation is proposed involving divergent color adaptation and possibly sympatric or parapatric populations. Color divergence and substrate matching seen in populations of A. fionni conceivably reflect evolutionary processes similar to those that promoted speciation of A. decresii and A. vadnappa.