Relative Growth in Two Sympatric Species of Sceloporus

Abstract

Osteological measurements on a growth series of two sympatric species of lizards, Sceloporus undulatus (maximum snout to sacrum length 59.1 mm) and S. olivaceous (maximum length 98.5 mm) were subjected to bivariate and multivariate morphometric analysis. Major identified sources of variability in the -sample are size, sex and taxonomy, the same that a paleontologist has to deal with. The two species are very similar osteologically but differ in size. Mixed bivariate plots show a separation between the two species for only one of 26 characters, but that character, as predicted, was strongly allometric. The single bivariate plot provided a more sensitive taxonomic separation than did a multivariate clustering technique, principal coordinates analysis. The latter did suggest, however, the existence of sexual dimorphism, previously unreported, which was confirmed by canonical analysis. Though the two species differ in maximum size by a factor of 1.5 to 1.7, the larger species is a geometrically scaled-up replica of the smaller one for only three of 28 relationships measured; many of the previously reported structural differences between species relate to the comparison between adults of the smaller species and juveniles of the larger. The effectiveness of jaw adduction of both species increases significantly through life, and positive allometry of limb length correlates with increased size of home range during ontogeny. INTRODUCTION This report is the second in a series of studiesi (Dodson, 1975a, b and c) designed to provide the paleontologist with a basis for understanding the kinds of variability potentially represented in a sample of fossils that spans a significant size range. The ultimate goal of the larger project is to test whether application of principles of ontogenetic allometry can demonstrate that 12 species of lambeosaurine hadrosaurs (duck-billed dinosaurs) of variable size and shape from a single formation may actually constitute a single ontogenetic series. The approach that is followed in all of the studies is explicitly morphometric, with a large number of cranial and po-stcranial variables being measured. In a study of a growth series of Alligator (Dodson, 1975a), ontogenetic changes of shape were described by means of allometric coefficients, and many of the changes were found, by bivariate and multivariate methods, to have expression in the function and ecology of the animal as it grows from hatchling to large size. In the present study data on two sympatric species of lizard, Sceloporus olivaceous and S. undulatus, are examined. The data are of considerable interest, for the two species are morphologically very similar, a growth series of each is included, and each specimen is 1 Present address: Laboratories of Anatomy, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pa. 19174.