Comparative growth patterns in Afrotropical giant tortoises (Reptilia Testudinidae)

Geochelone pardalis(Bell 1828) shows wide variation of body size in Africa, with tortoises in the north (Republic of Somaliland) being of comparable size to insular giant tortoises. Growth data on G. pardalis, the continental giant species G. sulcata(Miller 1779) and the Aldabran giant tortoise G. gigantea(Schweigger 1812) were re-analysed using Ford-Walford plots, and their growth patterns compared. All species and populations fitted a two-phase growth curve, with initial asymptotic growth followed by slow indeterminate linear growth. Asymptotic growth was non-Bertalanffy in all groups, with most individuals fitting Gompertz or logistic-by-mass models best. Juvenile growth (for ages 3–8) was also approximately linear in all groups. Variation between populations and species was in the rate of juvenile growth, asymptotic size reached, rate of subsequent linear growth, and survival rate of older individuals. Geochelone pardalisfrom Somaliland had similar juvenile growth rates and asymptotic sizes to other populations, and their large mean size was due to high survival and continued linear growth after the asymptote. Geochelone sulcatahad similar juvenile growth rates to G. pardalis, but reached larger asymptotic sizes; indeterminate linear growth was similar to that of G. pardalis. Geochelone giganteahad much higher juvenile growth rates than the continental species, reflecting a longer activity season. Asymptotic size and the rate of indeterminate linear growth were lower in a high-density (Grande Terre) than low-density (Ile Malabar) population of G. gigantea.The growth rate of juveniles was not affected by population density, so that the Grande Terre population had higher growth rates of juveniles relative to larger tortoises and was the only group to approach Bertalanffy-type growth, as predicted.