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An Aldabran Giant Tortoise.
Photo: Chris Johnson

December 11, 2002
Land of the Lumbering Giants - The Giant Tortoises of Aldabra
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Log Transcript

This is Genevieve Johnson speaking to you from Aldabra atoll. This is my friend 'Biskwi'. Giant tortoises are probably the islands most famous inhabitants, 'Biskwi' is one of 110,000 giant tortoises that live here.

With legs like an elephant, a long retractable neck and an enormous scaled shell, the lumbering giant tortoise is the undisputed 'King' of Aldabra.

But how did they reach such a remote place? The evidence indicates that all organisms living on atolls such as this one were carried to them by the wind, the sea, vegetation mats washed down by rivers, or in the claws or crops of other animals. The tortoises and other reptiles are no exception and may have floated across the ocean to populate Aldabra.

All giant tortoises are at home in the water, but, unlike most others of their kind, the giants of Aldabra have a flat nose that enables them to drink through their nostrils when the only water available is held by shallow pools in the coral limestone. I this way, they can make the most of the scarce water in this arid terrain.

'Biskwi' is one of the largest of the estimated 110,000 giant tortoises on Aldabra. He weighs well over 100 kilograms.
Photo: Chris Johnson

Chelonia, the reptilian group comprising turtles and giant tortoises evolved about 180 million years ago. Giant tortoises originally dominated many environments but were later brought to extinction by the large carnivores that evolved on continental land masses and spread to the whole world except for islands that were far offshore

However, on Aldabra, this ancient species of herbivorous reptiles, which for all intents and purposes live their lives entirely in slow motion, still dominates, growing to such enormous proportions takes decades.

Genevieve holding a young giant tortoise-

"This is a young giant tortoise, probably between five and ten years of age. You can actually tell their age by the growth rings on the top of the carapace. This on is big enough where he is safe from predators like rats that have been introduced onto the islands. So he may well live for the next hundred years or so, maybe longer."

The primary food of giant tortoises is grass but they can also use their coarse, serrated beaks and blunt, rough tongues to feed on tree leaves, flowers, and fruits.

They also retain food in their digestive systems for an especially long time which ensures that they extract all available nutrients from it. Their metabolic rate is ten times lower than that of a mammal of the same size.

The absence of predators has allowed giant tortoises to develop a relatively large opening in the carapace - an adaptation that assists them in regulating their body temperature.
Photo: Chris Johnson

Although hatchlings can fall prey to robber crabs and birds, adult tortoises have little to fear and sleep peacefully, letting all of their appendages hang out. The absence of predators has allowed them to develop a relatively large opening in the carapace - an adaptation that assists them in regulating their body temperature.

At dusk and dawn they move slowly outwards from shelter and orient themselves so their backs face the rising sun. A crescent shadow in front of the carapace protects the tortoise's head, neck and forelimbs from overheating.

Their numbers are largely determined by the availability of water and food and by access to nesting sites and shade from the harsh tropical sun. In places where turf is available (for example on Picard Island and the area around the research station) these great, hulking herbivores grow very large and some, like 'Biskwi', weigh well over 100 kilograms. However, on Grande Terre island, which is marginal habitat for tortoises (owing to the difficulty of finding food, the greater harshness of the terrain and the stronger competition for shelter) an animal of 'Biskwi's' age may weigh only 20 kilograms

Although a tortoise requires the heat of the sun to maintain a high enough body temparature if it is to be active enough to feed and move around, by midday the sun is so hot here that unless they can find adequate shade they run the risk of baking alive in their shell. So although they are out, moving about and feeding early, by about 9 am, the race for shelter is on. Unfortunately, some tortoises don't make it, and their scattered carapaces are evidence of the price they pay.

Terrence Mahoun - Head Ranger, Aldabra Research Station

Grass is one of the majors element in the diet of giant tortoises.
Photo: Chris Johnson

"The last census that was carried out in 1997 if I am right, roughly about 100,000. I would say this figure has declined a bit due to the harsh conditions and environment."

Genevieve Johnson-

Remarkably, there are between thirty and fifty thousand kilograms of biomass (or six to seven hundred tortoises on average per square kilometre on Aldabra). To put this into context, this is a greater biomass than has ever been recorded on the mammal-dominated African savannahs.

Dr. Peter Teglberg Madsen - Chief Scientist R/V Odyssey

"Even though the Aldabra atoll is one of the most dry and arid places on earth, we find a biomass of 30 tons per square kilometre primary made up of these giant tortoises. It is important to stress that even though we have such a large biomass, that this system is extremely fragile and is highly susceptible to human impacts.

Genevieve Johnson-

Giant tortoise populations in the Indian Ocean were first discovered in the sixteenth century. Subsequently, seafarers and whalers, ruthlessly exploited these defenceless reptiles as a source of fresh meat and thereby exterminated almost all populations apart from those in Aldabra and the Galapagos Islands.

Today, the giant tortoises of Aldabra enjoy complete legal protection and are free to roam the island as they have for millenia. Unfortunately, the effects of humans are still evident. Poaching of young animals is a constant problem. Feral animals such as cats and rats threaten the survival of hatchlings, and goats compete with the tortoises for food, although eradication of these competitors have begun.

During the day, giant tortoises must seek refuge from the blazing sun, or slowly bake in their shells. Scattered and bleached carapaces are found throughout the atoll, a constant reminder of the harsh enviroment.
Photo: Chris Johnson

Guy Esparon - Warden, Aldabra Research Station

"You know we have a goat eradication program. I think that goats too its tool on the tortoise program. But I think by eradicating the goats it would give the lower vegatation a better chance to grow. But it breaks your heart to see them. Because a [giant] tortoise of 60 years only weighs ten kilograms at the most [on Grande Terre ] if they get to live that way. They get stuck in the mud. It is a very harsh environment for them."

Genevieve Johnson-

To spend time among these ancient animals that are tame and entirely trusting is a privilege. The giant tortoises serve as a reminder that the island and its ancient treasures should be left to themselves with their survival ensured, they deserve no less.

Written by Genevieve Johnson

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