Aldabra's giant tortoises graze so hard that the grass evolved to hide
On a remote Seychelles atoll, some 100,000 giant tortoises crop the vegetation so relentlessly that a distinct habitat called tortoise turf has formed. More than 20 grasses and herbs there grow dwarfed, setting seed close to the ground where the tortoises' jaws cannot quite reach.
The Aldabra giant tortoise ranks among the world's biggest, comparable to the Galápagos species. It has a tall brown or tan shell, thick scaly legs and an unusually long neck for reaching branches up to a metre high. Females are smaller than males; average animals measure about 91 centimetres along the shell and weigh 159 kilograms. Shell shape tracks diet: ground feeders have domed shells that dip over the neck, while browsers have flatter tops with a raised front.
Giant tortoises once lived on many western Indian Ocean islands, but European sailors hunted them hard, and by 1840 apparently only the Aldabra population survived. Aldabra's atoll, shielded from human disturbance, remains the stronghold, with smaller groups on Frégate Island, in a Seychelles marine park, on Changuu near Zanzibar and in parks on Mauritius and Rodrigues. Genetic work suggests the species split from an extinct Madagascar relative about 4.5 million years ago. Conservation began early: in the late 19th century Albert Günther of the British Museum, backed by Charles Darwin and others, worked with Mauritius to set up a preserve.
As the largest animal around, the tortoise acts rather like an elephant, knocking down trees and opening paths other creatures use. It eats mostly grasses, leaves, stems and fruit, occasionally invertebrates or carrion, and gets most of its water from food because fresh water is scarce. Herds graze on open grassland in the morning, then shelter in shade, caves, wallows or pools through the midday heat.
They can live remarkably long. Some are thought to exceed 200 years, though the tortoises tend to outlive whoever is keeping count. Esmeralda, at 182, is second only to Jonathan, a Seychelles giant tortoise, among verified elders. Mating runs from February to May, and females lay 9 to 25 eggs in a nest 30 centimetres deep; after about eight months the young hatch between October and December, fully independent.
Source: Aldabra giant tortoise
