Orientation, movement and thermoregulation in the giant tortoises (Testudo (Geochelone) gigantea) of Aldabra Atoll, Seychelles

Abstract

The giant tortoises of Aldabra engage in feeding activity in the early morning and late afternoon, with variable periods of inactivity in the middle of the day. On clear days a high proportion of tortoises orient their long axes away from the sun. Sun orientation responses of 676 tortoises were measured in coastal mixed scrub and coastal grassland habitats in July 1978. These orientations showed that a significant number of tortoises faced away from the sun. The length and area of the shadow cast in front of the carapace was calculated for eight size classes of tortoise, assuming that the animal's long axis was in parallel with the sun rays. If a tortoise seeks shade when the shadow length has been reduced to less than 50% of the carapace height, we may predict that this would occur between 09h00 and 10h00. The shadow area at 09h00 and 10h00 was reduced respectively to 8.7 and 3.8% of that produced at 07h00. The shadow cast in front of the carapace allows a tortoise to feed in the shade, through which means it may keep its head cool and increase the head-body temperature gradient. Additionally the absence of predators on Aldabra has led to the evolution of a large opening at the anterior end of the carapace which exposes a greatly expanded area of neck and limb, facilitating heat efflux. Orientation away from the sun will therefore increase the period that a tortoise may feed in the open as well as producing an efficient means of decreasing the rate of increase of the whole body temperature. Since Aldabra lies 90 South of the Equator, changes in the sun's angle of azimuth will result in a north-westerly movement drift of sun orienting tortoises from November to January and an increasing south-easterly movement from February to October. Although the gross effect of sun orientation on tortoise movement will depend upon the habitat and its degree of shade cover, it is suggested that these are at least, in part responsible for the observed movement patterns of tortoises on Aldabra.