O efeito do isolamento geográfico no tamanho de Rhinella ornata (Spix 1824) (Anura: Bufonidae) em ilhas continentais da Costa Verde do Rio de Janeiro

Abstract

Darwin and Wallace in the mid-nineteenth century were the first to observe examples of natural selection acting on island dwellers. Nevertheless, only a century later a pattern for organisms on island was described as an island rule, which states that in these communities, species with small individuals tend to gigantism while those with large individuals tend to dwarfism. Selective pressures, such as limited resources and increased intraspecific competition, modulate the size of organisms in these environments. Several groups of vertebrates have been investigated under this aproach; however, only two papers have addressed amphibians. Therefore, this work represents the third record of body size variation for insular populations of amphibian (Rhinella ornata), the first record for the Southern Hemisphere. The islands investigated were once continuous with the mainland. They became isolated as a result of sea level fluctuations that took place in the Pleistocene and Holocene. This study investigated morphometric variation in populations of Rhinella ornata (Bufonidae) occurring on three islands in the region of the Costa Verde of Rio de Janeiro (Ilha de Itacuruçá, Ilha da Marambaia and Ilha Grande) and compared them to five continental areas (Duque de Caxias, Itaguaí, Rio de Janeiro, Seropédica and Tanguá). Thus, we measured 18 variables and weight of 177 individuals. Based on this sample, we uncovered a clear tendency towards smaller body size (dwarfism) in two of the three island populations analyzed. The overall pattern seems to be determined by geographical factors, indicating the expression of the island rule inversely in populations of anurans in tropical environments (island dwarfism) compared to those of temperate areas (insular gigantism). These results are evaluated against physical characteristics of the islands, reproductive biology of the species and the history of the islands’ formation.