Tortoises live so long because they grow at a steady rate and hibernate. Tortoises, like other reptiles, are cold blooded, or ectothermic, so they need to ‘warm up’ by absorbing heat from their surroundings to keep active. In winter, when food is scarce and the temperature falls, tortoises in the wild stop eating, their breathing and heartbeat both slow and they go into a deep sleep. Studies show that tortoises grow rapidly during their early years and continue to grow at a steady rate that decreases with age. There is evidence of extreme age, although some reports of tortoises living over 150 years have proved unreliable, as has counting the growth rings of shell plates.
Answered by Sandra Chapman, Palaeontology Department, National History Museum London.