Morphological variation and modularity in the mandible of three Mediterranean dolphin species

Abstract

Geometric morphometric studies of dolphins have traditionally concentrated on the skull, while the mandible and its modularity have been little explored. We investigated the mandible variability and modularity in three strictly related Mediterranean dolphins: Stenella coeruleoalba, Delphinus delphis and Tursiops truncatus. The aims were to describe the interspecific differences in the size and shape of the mandible as a whole, and of its structural modules, and to detect the influence of adaptive pressures on trait variation. Data were collected on 96 specimens from the Mediterranean Sea. Eight and 10 two-dimensional landmarks were recorded respectively in the lateral and medial sides of the mandible. After General Procrustes Analysis (GPA) transformation, measurement error, sexual dimorphism and ontogenetic allometry were first investigated to allow further pooling of samples. Univariate analysis of variance (ANOVA) of centroid size was used to evaluate size differences among the species; multivariate ordination, classification and clustering methods were used to investigate interspecific variation of shape variables. Different subsets of landmarks representing distinct mandibular subunits were tested for modular integration through the RV coefficient; two-block partial least squares statistics was used to explore the patterns of covariation between modules. Size and shape differences in the whole mandible of the three species evidenced a clear morphological divergence of Tursiops truncatus and a close similarity of Stenella coeruleoalba and Delphinus delphis. The analysis of modularity identified the corpus and the ramus, with its internal foramen, as distinct modules. The corpus and ramus patterns of variation could discriminate between T. truncatus vs. the other two species. The mandibular foramen was the only trait able to discriminate each species, and the corresponding shape differences were related to selective pressures toward the differentiation of communication patterns.