A Reptilian Kingdom
The Aldabra Atoll supports the world's largest population of giant tortoises (Aldabrachelys gigantea), with a fluctuating population of around 100,000 individuals. These reptiles are the last survivors of a dozen giant tortoise species that once inhabited the islands of the Indian Ocean before being hunted to extinction by sailors. The tortoises are ecosystem engineers; their intense grazing on a variety of grasses and herbs creates a unique, species-rich habitat known as "tortoise turf". Males can weigh up to 250 kilograms and measure 1.2 meters in length, with lifespans that often exceed a century.
The atoll is also an important habitat for the Aldabra rail (Dryolimnas cuvieri aldabranus), the last flightless bird in the Indian Ocean. Fossil records show that the bird's ancestors colonized the atoll over 130,000 years ago and evolved to be flightless. When a major sea-level rise completely submerged the atoll, that population went extinct. After the atoll re-emerged thousands of years later, the same rail species recolonized it and, in a rare case of iterative evolution, evolved to be flightless for a second time.