Comparative growth patterns in Afrotropical giant tortoises (Reptilia Testudinidae)

A. HAILEY, M.R.K. LAMBERT

Abstract


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 pardalis
from 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 sulcata had similar juvenile
growth rates to G. pardalis, but reached larger asymptotic sizes; indeterminate
linear growth was similar to that of G. pardalis. Geochelone gigantea had 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.
KEY WORDS: Africa, asymptotic size, Geochelone, growth curve, growth rate, tortoise.

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