Orientation of giant tortoises, Geochelone gigantea (Schweigger) while grazing on Aldabra Atoll

Abstract

The Aldabran giant tortoise is one of the few testudines that feeds in dense herds, and in so doing the animals frequently exhibit a conspicuous orientation away from the sun. This orientation is most marked when ambient temperature is high, and it breaks down when the tortoises are shielded from direct sunlight. It was previously hypothesized that negative heliotaxis is related to thermoregulation-to reduce heat inflow to the exposed anterior appendages and instead expose the protected posterior of the shell. There has been no test of the thermoregulatory function, and pilot experiments indicate that orientation away form the sun occurs to reduce direct glare into the tortoise's eyes. The glare of the sun seems to interfere with accurately oriented biting movements and reduce feeding efficiency. Speed and intensity of reaction to solar glare is inversely related to the animal's body size. The possibility of a thermoregulatory component to the away-from-the-sun orientation cannot be discounted, but it must be investigated directly.