Cranial Morphometry of New Guinea Crocodiles (Crocodylus novaeguineae): Ontogenetic Variation in Relative Growth of the Skull and an Assessment of Its Utility as a Predictor of the Sex and Size of Individuals

Abstract

The cranial morphometry of New Guinea crocodiles (Crocodylus novaeguineae) was investigated to quantify ontogenetic variation in relative growth of the skull to provide possible demographic insights into commercially harvested populations. Growth attributes were, in general, linear, and univariate statistics provided estimates of size from untransformed values. Curvilinear size (total length) to age transformations were attempted through applications of three growth models: a reparameterized Richards curve, the von Bertalannfy model, and a Brody curve. Satisfactory performance was attained only through the Brody model, with reliable age estimates restricted to smaller (<2 m) individuals. The utility of relative growth ratios to correctly predict the sex of known gender animals was assessed by both parametric (multivariate discriminant analysis-DISCRIM) and nonparametric (binary tree classification analysis-CART) methods. Crossvalidation data sets were created to reduce model biases and to evaluate the correctness of classification rates. CART models consistently showed greater congruence of apparent error rates with true error rates than did DISCRIM models, while the 'best fit' models of each method closely paralleled one another in providing conservative overall ability to correctly discriminate gender from measured attributes. Skull growth occurs in three distinctive stages that mimic a saltatory ontogenetic response and are hypothesized to be related to functional foraging responses.