Aldabra tortoises, commonly cited as Geochelone gigantic, feed in the early morning and again in the late afternoon.
During the heat of the day, they stop eating and rest in shade to avoid overheating.
The Aldabra Village -
Giant Aldabra Tortoise
Today Aldabra reminds us of an ancient era when reptiles ruled the world. Millions of years ago giant tortoises roamed virtually every continent on earth. But by the end of the last ice age, some 10,000 years ago, competition and predation from mammals had driven all the giant tortoises to extinction, except for a few populations on remote oceanic islands of the Pacific and Indian Oceans. Scientists believe that the ancestors of these populations floated from mainland on ocean currents. Even today it is not uncommon at Aldabra to encounter a hapless tortoise floating at sea - sometimes inside the lagoon, sometimes at sea well beyond the perimeter of the fringing reef.
When humans first settled Seychelles in 1770, giant tortoises inhabited most of the islands of the group. But they were intensely exploited for food, and by the middle of the 19th century, all known wild tortoise populations in the Indian Ocean were extinct except for those on Aldabra. Even the Aldabra populations reached dangerous low levels. Captain Wharton of the Royal Navy, visiting in 1878, reported that "the reptiles are now very scarce", and that his men were only able to find a single animal "after much trouble and search."
In 1892, the atoll's lessee, claimed that there were more than 1,000 animals but other scientist thought this was a considerable over-estimate. Reports in the early 20th Century suggest the animals were very scarce and absent from where they are now common.
The Aldabra giant tortoise is mainly herbivorous although it will opportunistically scavenge off the caresses of dead animals, Including those of other tortoises. On Grande Terre much of the staple diet is 'tortoise turf'. a mixture of genetically dwarfed plants that has evolved under constant pressure from tortoise grazing. This complex of some 22 species of grasses, sedges, and herbs only survives where tortoises are present. On other islands where tortoises no longer exist, most of the associated plants also died out, replaced by more vigorous vegetation.
Fortunately, protective measures were implemented for giant tortoises in the late 1800s. They responded well to the protection. A census conducted in 1973-1974, showed that the population had risen to approximately 129,000 animals.
A1997 survey indicated that the population subsequently declined to 100,000, probably in response to environmental damage brought by a combination of tortoise over-population, habitat destruction by feral goats, and drought.
Nevertheless, Aldabra today has the largest population of giant tortoises in the world, many times the size of the only surviving natural population in the Galapos.
Tales of tortoises longevity are often exaggerated but there are recent authenticated cases of captive giant tortoises in the granitic Seychelles living for more than 150 years. In captivity and with no shortage of food, tortoises can grow to extraordinary sizes and weigh more than 300 kilos. Unfortunately, in the harsh environmental conditions at Aldabra tortoises cannot survive for such long periods or grow so large. Dehydration, over-heating, and starvation during extended droughts are constant threats. Tortoises grow slowly at Aldabra and reach maturity after some 16 to 30 years, depending on food availability. Some tortoises make annual migrations from land to coastal feeding grounds during the wet season. But, with rare exceptions, an Aldabra tortoise is likely to spend most of its relatively long life within a distance of only a few kilometers from its birthplace.
After heavy rain, the trudge through the mud begins. At moments like this, the resilience that tortoises must have to survive in this landscape in apparent.
During the dry season giant tortoises can survive without water for considerable periods. As soon as it rains, small depressions in the stony surface fill with water, and tortoises emerge from the undergrowth to drink thirstily.
To enable them to exploit even the shallowest water filled crevices they have evolved the ability to drink through their nostrils.
The dry Season, in the latter half of the year, can be merciless. Not only is food scarce, but the foliage wilts and provides little shade. The 'cold-blooded' tortoises is unable to regulate its own body temperature. Instead it must seek shelter from the ravages of the midday sun to avoid overheating. During dry season when food is scarce, tortoises crowd together under few available leafy tress waiting patiently until the first rain arrives. bringing cooler temperatures and a more abundant food supply. At Aldabra, the tropical rain can transform the landscape within a few hours.
Growth rings in the carapace plates can provide an indication of the age of a tortoise when its relatively young. During the rainy season the keratin in the shell grows faster than during the dry season. It is not entirely accurate method, however, because after ten or fifteen years the lines begin to merge, fade and abrade away. In the 1970s, several thousand of these creatures were tagged with titanium disk glued to their shell to keep track of their movements and growth rates.
Aldabra giant tortoises vary greatly in size and shape according to their environment. In the densely populated flat plain limestone of Grande Terre, Competition among tortoises has produced animals that are generally smaller, mature later and lay fewer eggs per clutch than do those on Picard or Malabar. Their lives are in constant jeopardy. In addition the danger posed by dehydration and over heating, they face a rocky terrain that is pitted with deep holes into which they can easily fall and become trapped.
Reproduction in tortoises begin with noisy affair of mating which takes place during the rainy season of January too May.
The male has a concave plastron, or lower shell, to enable him to mount the female and not fall off. The flat plastron of the female probably enhances her capacity to carry eggs. Eggs are laid from July to October in concealed nests that are difficult to find. Where tortoise population density is high, clutch size tend to be low. On Picard. the vegetation is richer and the population less dense (because of past exploitation) so that a clutch averages nineteen eggs, compared to five eggs on Grand Terre.
Clutch size also increase during years with higher rainfall. The hatchling emerge after 98-148 days.
Thew young tortoise are inconspicuous for the first six years or so of life, until they are big enough to fear no predator. The young have plenty of predators.