Reptiles
Tortoise
Testudinidae · also called land turtle
A tortoise is a turtle that gave up the water for good. It traded webbed feet for stumpy, weight-bearing legs, grew a high domed shell that a predator struggles to bite, and slowed its whole life down — which is part of why several tortoises alive today were hatched before any person now living was born.
Overview
Tortoises are the family Testudinidae, around 50 species, the fully land-living branch of the turtles. They have a high domed shell, hard scaly skin, and short columnar legs with claws, and they are almost entirely herbivorous, grazing tough, fibrous plants that ferment slowly in a large hindgut. They live on every continent except Antarctica and Australia, mostly in dry country.
The longest-lived land animals
Large tortoises regularly pass 100 years, and the island giants go far beyond: Jonathan, a Seychelles giant tortoise, is estimated at over 190 and is the oldest known living land animal; Adwaita, an Aldabra giant, may have reached 150 to 250. The giant tortoises of the Galapagos and of the western Indian Ocean are not close relatives — each set reached remote islands by floating and grew huge independently, a textbook case of island gigantism.
Living in a dome
The domed shell is hard to bite but easy to tip over, so a tortoise levers itself upright with its neck and legs; one stuck on its back in the sun can die of heat. Nest temperature sets the sex of the young, warmer nests producing females.
A slow life, badly exposed
Tortoises mature at 10 to 20 years or more and lay small clutches, so a population cannot replace lost adults quickly.
Status
Tortoises as a family are Not Evaluated, but the trend is decreasing and the group is among the most threatened of all vertebrates — over half of species are threatened by pet-trade and food collection, habitat loss, roads, fire, and introduced predators. Several Galapagos races are extinct, and the ploughshare tortoise of Madagascar is down to a few hundred.
Tortoise: common questions
What is the difference between a tortoise and a turtle?
All tortoises are turtles, members of the order Testudines, but not all turtles are tortoises. Tortoise refers specifically to the family Testudinidae, the fully land-living turtles, which have high domed shells, hard scaly skin, and stumpy weight-bearing legs with claws rather than webbed feet or flippers. The word turtle is used for the whole order in North American English and, in British English, tends to mean the aquatic species. So a tortoise is a land turtle; a sea turtle and a pond turtle are turtles but not tortoises. A terrapin is an informal term for certain small freshwater turtles and is also not a tortoise.
How long do tortoises really live?
Longer than almost any other backboned animal. Small species such as the Russian and Hermann's tortoises commonly live 40 to 80 years, well beyond the lifespan of their keepers. The large island tortoises are the record holders. Jonathan, a Seychelles giant tortoise on Saint Helena, is estimated to be over 190 and is the oldest known living land animal, Harriet, a Galapagos tortoise, reached about 175, and Adwaita, an Aldabra giant, may have lived 150 to 250 years. Ages beyond 100 are documented for several species. This extreme longevity is linked to slow metabolism, slow growth, efficient cell maintenance, and low rates of age-related disease.
What do tortoises eat, and do they drink water?
Tortoises are almost entirely herbivorous, grazing and browsing on grasses, broad-leaved weeds, leaves, flowers, fallen fruit, and, for desert species, succulents and cactus. The diet is high in fibre and low in nutrients, so digestion is slow, relying on a large hindgut full of fermenting bacteria, and a meal may take one to two weeks to pass through. Tortoises do drink when water is available, taking long draughts and sometimes sitting in a puddle to absorb water through the cloaca, but desert species can go for weeks or months on the moisture in their food, storing water in the bladder as a reserve that they can reabsorb in a drought.
Why are so many tortoises endangered?
Their biology and human demand work against them. Tortoises are slow to mature, breed slowly, and are easy to catch, since they cannot run or hide effectively, so they have been collected in huge numbers for the pet trade, for meat, and for shells. Their dry-country habitats are heavily converted to farming and development, and roads through those habitats kill adults faster than the population can replace them. On islands, introduced rats, pigs, dogs, and goats eat eggs and hatchlings and strip the vegetation. Because a female may take twenty years to reach breeding age, losing adults is close to irreversible on any useful timescale, which is why more than half of all tortoise species are now threatened.