Ontogenetic development and sexual dimorphism of franciscana dolphin skull: A 3D geometric morphometric approach

Abstract

The aim of this work was to study the postnatal ontogenetic development of Pontoporia blainvillei skull, identifying major changes on shape, and relating them to relevant factors in the life history of the species. We analyzed a complete ontogenetic series (73♂, 83♀) with three-dimensional geometric morphometric techniques. Immature dolphins showed a very well-developed braincase and a poorly developed rostrum, and the principal postnatal changes affected the rostrum and the temporal fossa, both structures implied functionally to the feeding apparatus, thus suggesting a specialized mode for catch fast prey in P. blainvillei. Osseous elements associated with sound production were already well developed on immature dolphins, suggesting the importance of this apparatus since the beginning of postnatal life. Sexual dimorphism was detected on both shape and size variables. Females were bigger than males, in accordance with previous studies. Shape differences between sexes were found on the posterior part of premaxillaries and external bony nares (P < 0.01), suggesting that this sexual dimorphism is related to differences on vocalization capabilities.

Skull ontogeny and modularity in two species of Lagenorhynchus: Morphological and ecological implications

“…The development of the external nares and the premaxillary sac fossa in early stages of ontogeny of our sample is also in agreement with trends found in other species. Rauschmann et al () noted that the nasal area of perinatal Stenella attenuata is similar to that of adults in terms of structure, and del Castillo et al (, in press) observed a development of premaxillae and external nares in P. blainvillei and C. commersonii . In addition, Frainer et al () found that the sound generating structures and auditory sense of P. blainvillei might already be functional at birth.…”

“…Other ontogenetic trends observed in both species were expected since they are related to specific adaptations of odontocetes. For instance, the posterior displacement of the external nares and infraorbital foramina, and the strong development of the rostrum in an anteroposterior direction are associated with the process of telescoping (Miller, ), and have been observed in postnatal ontogeny of several species, such as Cephalorhynchus commersonii, L. albirostris, Phocoena phocoena and Pontoporia blainvillei (Galatius, ; del Castillo et al, , in press). Even though telescoping is known to develop during prenatal growth, Rauschmann et al () noted that this process is not fully accomplished in perinatal dolphins, so it is expected to find telescoping‐related ontogenetic trends during postnatal growth.…”

“…A 3D digitizer Microscribe MX6DOF System (GoMeasured3D, Amherst, VA) was used to record 55 landmarks on each skull (Fig. , Supporting Information, Table S2), which were chosen following Sydney et al () with some modifications (del Castillo et al, , in press). We chose landmarks that represented the complete skull and allowed us to see important features of the development in each cranial module (see below).…”

“…We used log‐transformed CS as a measure of size because preliminary analyses showed that it resulted in better linear relationships than untransformed CS (e.g., del Castillo et al, , in press; see below). To study how shape changed in relation to size that is common to both species, we performed a pooled within‐group regression of the Procrustes coordinates against logCS.…”

“…Another goal was to test which skull region differ the most in the growth pattern between species, because specific parts of the skull in cetaceans are subject to different functional and evolutionary constraints (e.g., Guidarelli et al, ; Frandsen and Galatius, ). Based on previous studies in cetacean cranial development (e.g., Perrin, ; del Castillo et al, , in press), we hypothesize that the rostrum will be the region that differ the most, both in size and shape during growth. To test this hypothesis, we worked with the concept of modules (Olson and Miller, ) and the possibility of testing them individually.…”