Morphology of the Guiana dolphin (<i>Sotalia guianensis</i>) off southeastern Brazil: growth and geographic variation

Abstract

The objective of this study was to analyze the morphology of Guiana dolphin (Sotalia guianensis) to evaluate the existence of geographical variation along southeastern Brazil. Body length and 39 cranial variables were measured of specimens stranded or accidentally captured to consider ontogenetic and geographic variations. The areas studied were Espírito Santo (ES; 18°30’S-20°40’S), northern Rio de Janeiro (NRJ; 21°35’S-22°25’S), southern Rio de Janeiro (SRJ; 23°00’S-23°07’S) and São Paulo (SP; 23°30’S-25°30’S). Body length at age zero predicted by a non-linear Gompertz model for the Guiana dolphin was 148.3cm for area ES, 108.97cm for area NRJ, 98.4cm for area SRJ and 90.9cm for area SP. Asymptotic values were reached at about six years of age for total body length and cranial variations. These results indicate that Guiana dolphins reach adult size and sexual maturity simultaneously at six to seven years of age, when specimens cease to grow. The growth pattern for body and skull size indicated that there is variation between geographic areas. Guiana dolphins found in São Paulo are smaller than those analyzed in northern Rio de Janeiro and Espírito Santo, which implies a different growth rate. A canonical discriminant analysis of the cranial metric characters indicated significant differences between the four geographic areas. Differences between areas NRJ, SRJ and SP were responsible for 54% (axis 1) and 34% (axis 2) of the variation, respectively. The third axis depicted a difference between the area ES and the others. A partial overlap between geographic areas was observed in the projection of the species on the canonical axes, suggesting parapatry. Geographic variation recorded in this study is likely to be related to environmental adaptations. One of the areas that could play a role in the distribution of Guiana dolphin in the surveyed area is the central coast of Rio de Janeiro, which is characterized by the absence of river discharges, a narrowing of the continental shelf and upwelling influence that might be limiting the species occurrence in this area.