Feeding ecology and seasonal movements of giant tortoises on Aldabra atoll

Abstract

We investigated seasonal changes in diet and distribution of giant tortoises (Geochelone gigantea (Schweigger)) on Aldabra atoll in the Indian Ocean. Animals were counted and their activity and feeding behaviour recorded on transects where vegetation composition and primary production had been studied (Gibson and Phillipson in press a, b). There were striking seasonal shifts in tortoise distribution, and male, female, and juvenile tortoises were found in different proportions in different vegetation types. Tortoises are selective grazers, feeding on a wide range of foodstuffs of which the most important (61% of feeding observations) was tortoise turf. Diet varied seasonally, with shrub leaves (mostly litter) overtaking tortoise turf in importance in the late dry season; diet broadened as the dry season progressed. Male and female diets were not significantly different but juveniles fed on herbs and mosaic rock vegetation more often than adults. Seasonal shifts in distribution are due to movements in response to changes in food availability, measured by the foods' cover abundances and production phenologies. Tortoises concentrate on preferred foods when available, but become less selective as production falls. Some differences in size and sex class distribution between habitats can also be explained by food availability. In the late dry season density peaks on the coastal Sporobolus virginicus (L.) Kunth sward. A detailed study showed that, while at least 20% of the population uses the sward each year, visit times are short and turnover of tortoises high, as would be expected on a non-preferred food. The giant tortoise interacts with its food supply similarly to other large herbivores, except that the low maintenance needs of this large poikilotherm allow it to develop unusually high population densities.