Giant tortoises ( Geochelone gigantea ) on Aldabra respond to light tactile stimulation of their soft posterior regions by adopting an immobile erect stance, previously thought to be a threat posture. It is argued that this is probably a cooperative posture assisting any potential symbiont to clean ectoparasites from the areas of soft skin thus exposed. Most Aldabran tortoises over 25 cm long respond in this way and the response appears to be unaffected by activity, sex, time of day and ectoparasite abundance. The visual stimulus of a white-throated rail Dryolimnas cuvieri within 1 m may also elicit this response, and rails were observed removing ectoparasites from tortoises which had adopted the cooperative posture. The distributions of the tortoise and the rail overlap only slightly and the mutualism is thought to be ecologically unimportant to both the ectoparasites and the rail. Comparisons are made with the mutualism involving Galapagos giant tortoises, Geochelone elephantopus , and the ground finches Geospiza fuliginosa and G.fortis ,and the mockingbird, Nesomimus parvulus .