Growth, age at maturity and sexual dimorphism in the geometric tortoise,Psammobates geometricus

Abstract

BAARD, E. H. W. 1995. Growth, age at maturity and sexual dimorphism in the geometric tortoise, Psammobates geometricus. J. Herpetol. Assoc. Afr. 44: 10–15. Capture-recapture data from a natural population of the geometric tortoise, Psammobates geometricus, were used to determine size-based annual growth rates. Juveniles and sub-adults (<90 mm CL) grow at 10.45±8.3 mm CL/yr. At 80 to 90 mm CL, males can for the first time be sexed with confidence using developing secondary sexual characteristics. Adult males grow at 1.70±1.9 mm CL/yr, while adult females grow at 1.56±1.2 mm CL/yr. Using Fabens' (1965) growth interval method and hatchling size (28.13±1.01 mm CL), a preliminary species growth model was constructed by fitting the general form of the Von Bertalanffy growth equation to the data. Independently modelled male and female asymptotic sizes agree closely with the mean sizes of the 20 largest males and females respectively. The model suggests that geometric tortoise adults may reach ages of approximately 30 years. Development of secondary sexual characteristics in males predicts their age at maturity as approximately 5.9 years. Female age at maturity remains inconclusive. Sexual size dimorphism in geometric tortoises is pronounced; females are significantly larger and heavier than males. Consistently larger female body size in P. geometricus underscores the hypothesis that natural selection for larger size in females may result in higher fecundity.