Development of Distinct Morphotypes in Captive Seychelles–Aldabra Giant Tortoises

Abstract

Growth patterns of captive-bred Seychelles–Aldabra giant tortoises (Aldabrachelys/Dipsochelys) were studied. This enabled the comparison of the development of 3 distinct morphotypes variously ascribed by different authors to 3 distinct species or a single, variable species (A./D. dussumieri/gigantea, arnoldi, and hololissa). Geometric morphometric analyses identified differences in growth pattern between the 3 morphotypes of 234 juvenile tortoises reared under identical conditions. In plastral characters, all 3 morphotypes could be distinguished from hatching. Initially, hatchlings were very similar in dorsal view but by 30-cm straight carapace length were distinguishable by relative warp analysis. The arnoldi morphotype is the most distinctive, with constriction in the center of the carapace resulting in the beginning of development of saddle-backed morphology from 30 cm. All 3 morphotypes show 2 distinct growth patterns, one from hatching to 20–30 cm and a different pattern above 30 cm. As these morphotypes were reared under identical, largely constant conditions, this change in growth patterns appears to be a result of ontogeny rather than environment. Differences in development of the morphotypes cannot be explained by environmental factors alone and may be the result of differences in gene expression or of small differences in genes associated with skeletal development.