Intraspecific variation of the bottlenose dolphins Tursiops truncatus (Montagu, 1821): sexual dimorphism, ontogenetic and static allometry investigated through Geometric morphometrics of the skull and the mandible. Implications for conservation and management of the European stocks

The geographic and non geographic variation of the bottlenose dolphin Tursiops truncatus (Montagu, 1821) were investigated through two- and three dimensional GM of the skull and of the mandibles. A total 422 specimens (151 males, 141 females and 130 unsexed; 65 sub-adults and 357 adults) were analyzed by 2D GM photographing various projections, and recording 20, 20, 12, 10, 8, 8 two-dimensional landmarks respectively on the dorsal, ventral, lateral, and occipital projections of the skull, and on the left and the right lateral and medial projections on the mandible, and by 3D Geometric Morphometrics digitalizing 38 three-dimensional landmarks using a 3D scanner Microscribe. Landmarks configurations of each projection were superimposed through a Generalized Procrustes Analysis; size and shape differences were evaluated by ANOVA on centroid size and by MANOVA on Procrustes coordinates. Non geographic variations included the analysis of sexual dimorphism, ontogenetic allometry, morphological integration and asymmetry; geographic variation included the analysis of populations distribution and static allometry. No differences in skull size were found between males and females, while significant sexual dimorphic traits were evidenced in the shape of the occipital bones of the North Sea dolphins, the female having a more marked incision of the cavity hosting the left tympanic bulla. Linear allometric trajectories characterized the growth of the dorsal, ventral and occipital traits, polynomial curves the lateral views of the skull, while step clines best fitted the growth of the mandibles. Presence/absence of sexual dimorphism among populations were interpreted in the light of intra and interspecific competition related to multispecific associations. Differential allometric growth patterns were discussed in relation to feeding shift and the establishment of asymmetry in the braincase during growth. The morphological integration was investigated analyzing the three modules identified in the skull and two in the mandible; the hypotheses about the boundaries among the modules identified was evaluated by partitioning the configuration in different ways and comparing the significance of RV coefficients and analyzed by 2B-PLS calculating a cross-covariance matrix for the two blocks of data. The analysis of the asymmetry was performed by Procrustes ANOVA testing the main variation's effects among individuals, age-classes and populations. The geographic variation was investigated subdividing the whole sample was in six putative genetic populations (East Mediterranean, West Mediterranean, Central Atlantic, North Atlantic, North, and Baltic) to analyze morphological differences. Were found six morphological populations: the Adriatic, the Levantine and the W Mediterranean populations in the Mediterranean Sea; the central Atlantic population along Portuguese, Spanish and French coasts and the northern Atlantic one in the Scottish waters; the North/Baltic population in the eastern North and in the Baltic Sea, showing different patterns of intraspecific allometric trajectories investigated by multivariate regression of shape coordinates on centroid size. The results obtained became relevant for the first morphological analysis of bottlenose dolphins from North and Baltic Sea and for the recognition of isolated populations in the specific habitat as the first step for the protections of the bottlenose dolphin.