Body size development of captive and free-ranging African spurred tortoises (Geochelone sulcata): High plasticity in reptilian growth rates

Abstract

In captivity, tortoises often grow faster than their conspecifics in the wild. Here, we document growth (measured as body mass change) in three individual Geochelone sulcata over an exceptionally long period of nearly 18 years and use literature growth data (measured as carapace length change) on free-ranging animals for comparison. Body lengths almost reached a plateau in the animals due to the long observation period. After transformation of body lengths to body masses for data from wild animals, logistic growth curves by mass were successfully fitted to all data. The resulting functions yielded a 1.4-2.6 times higher intrinsic growth rate in the captive than in the wild individuals. The logistic growth model estimated the inflexion point of the growth curve at 6-9 years for the captive animals. This coincided with age at sexual maturity, because estimates corresponded well with observations of first egg-laying of a female and the masturbation of a male. The inflexion point of the growth curve for free-ranging individuals was estimated at 15 years. Raising tortoises on intensive feeding regimes in captivity may considerably shorten generation times during the breeding stage of restocking programs; but the literature suggests that slow-growing animals are more likely to thrive after release into the wild. Investigations on the health of offspring from fast-growing parents are lacking.