|Geographic variation and sexual dimorphism in the skull of the dusky dolphin, Lagenorhynchus obscurus (Gray, 1828)|
Van Waerebeek, K. (1993). Geographic variation and sexual dimorphism in the skull of the dusky dolphin, Lagenorhynchus obscurus (Gray, 1828). Fish. Bull. 91: 754-774
In: Fishery Bulletin. US Government Printing Office: Washington, D.C.. ISSN 0090-0656, more
Variation in skulls of 415 dusky dolphins, Lagenorhynchus obscures, was studied based on 37 standard cranial measurements and meristic variables and 28 nonmetrical characters (NMC) by using both bivariate and multivariate analyses. Geographic variation was analysed in mature skulls from central Peru (N=189), Chile (N=22), New Zealand (N=47) and southwestern Africa (N=40). Advanced fusion in the frontal-supraoccipital suture is the most reliable (95% efficiency) cranial criterion of sexual maturity; distal fusion in premaxillary and maxillary cannot be used. Sexual dimorphism, verified with t-tests and analysis of covariance, was encountered in only 6 of 37 measurements and in none of the NMC characters. Although skull size is the same, males have a wider rostrum and a longer temporal fossa than females. Analyses of covariance and t-tests indicated highly significant differences between geographic groups. A discriminant analysis successfully classified 96.5%, 91.7%, and 90.9% of skulls from respectively the Southeast Pacific (Peru and Chile pooled), New Zealand, and SW Africa. Such differentiation is considered of subspecific level. All methods indicated higher morphological similarity within the Peru/Chile and SW Africa/New Zealand grouped pairs than between the pairs. Most strikingly, skulls from New Zealand and SW Africa are on average 3.1cm (8.5%) shorter than these from Peru and Chile. concordant with differences in body length. New Zealand specimens differ from SW African animals by smaller tooth size and a greater number of teeth. Nine cranial variables revealed significant variation between dusky dolphins from central Peru and northern Chile, but the degree of heterogeneity should be re-evaluated with a larger sample from Chile. NMC characters confirmed the trends mentioned but resolving power was limited; their use is not recommended as the principal method for subspecific discrimination. The Peruvian dusky dolphin is thought to represent the most ancestral (plesiomorph) form from which the others were derived through dispersion via the eastflowing Westwind Drift.