Abstract
The short-nosed Risso’s dolphin (Grampus griseus) is the fifth largest member of Delphinidae, but is one of the least studied dolphin species. We studied the postnatal ontogeny and sexual dimorphism of the skull of Risso’s dolphin by applying linear bivariate and multivariate analyses of allometry and discussing our findings in a comparative and functional framework. Comparing a representative of the Globicephalinae subfamily (a typically short-nosed group) with previously studied cetaceans belonging to different families is important for evaluating variations in the growth of splanchnocranium and neurocranium in cetaceans. Results of bivariate and multivariate approaches were mostly similar. The trophic apparatus showed positive allometry or isometry, whereas negative allometry was observed in neurochranial components and in width of external nares. We detected sexual dimorphism in the growth pattern of cranial variables related to trophic functions, suggesting slight differences in the mechanics of the trophic apparatus. Our results indicate a very low degree of sexual dimorphism in skull growth compared with other dolphins with most dimorphic growth rates favoring females; this result can be related to the earlier acquisition of an optimal performance in females, which is important for reproductive requirements. The growth of the splanchnocranium, and especially of those characters associated with the trophic function, exhibits a pattern of higher growth rate than neurochranial components, regardless of the short face of G. griseus.
Acknowledgments
We thank the Japan Society for the Promotion of Science (JSPS) and the Consejo Nacional de Investigaciones Cientificas y Técnicas of Argentina (CONICET) for supporting museum visits of DAF. Yuko Tajima, Nozomi Kurihara, Naoki Kohno and personnel of the National Museum of Nature and Science assisted us during our work in Tsukuba. We are also grateful to the Japan International Cooperation Agency (JICA). This project was also partially supported by Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), project PICT 2012-1583 to DF. We thank to two anonymous reviewers who improved the quality of this manuscript.
Appendix 1
Specimens examined in this work. All specimens are deposited in the mammal collection of the National Museum of Nature and Science, Japan (NSMT).
Males: NMNS 29493, 29498, 29481, 29507, 29530, 29523, 29547, 29529, 29529, 29531, 29550, 29540, 29528, 29476, 29475, 29489, 27479, 24665, 29513, 29574, 10507, 30728, 21400. Females: 29487, 29494, 29495, 29483, 29484, 29482, 29492, 29491, 29509, 29511, 29485, 29485, 29508, 29536, 29537, 29539, 29525, 29527, 24842, 24843, 29477, 29478, 29480, 80202.
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