Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"

Something different this time! I used oil pastels for this drawing. It is an Aldabra Giant Tortoise, Aldabrachelys gigantea (and for once the scientific name means exactly the same as the common name). It is one of the largest remaining tortoise species, rivalled only by the giant tortoises of the Galápagos Islands. Its carapace reaches 1.2m/4' in length, and it reaches a weight of 250 kg/550 lbs. This giant tortoise only occurs naturally on the Aldabra Atoll in the Indian Ocean, some distance off the African coast. It also occurs on the Seychelles Islands, to which Aldabra belongs politically, though being geographically separated from them by a good stretch of ocean. Some have also been introduced to the small island of Changuu, near Zanzibar just off the coast of Africa, and to the Mascarene Islands Mauritius and Rodrigues. The main populations on Aldabra currently numbers about 100 000 animals; it is the only large land vertebrate to occur on the island.

Giant Tortoises and Turtles of the Earth

Tortoises and turtles are an ancient group, dating back at least to the Triassic Period (about 250-200 million years ago) with some possible precursors going back to the Permian. It's still not clear exactly how they're related to other reptiles; there are signs that they may actually be closer to crocodiles than to lizards and snakes – in which case they'd also be closer to birds. Many enormous kinds existed in the past. The largest ones ever to exist, were marine and freshwater turtles: Archelon ischyros was a marine turtle which reached an overall length of 4 m/13' and measured about an equal distance across its flippers. Archelon had a soft shell with minimal internal bony support, and lived in the Cretaceous, about 80 million years ago, while non-avian dinosaurs still ruled on land. The more recent freshwater turtle, Stupendemys geographicus, by contrast had a massive shell, as wide as or wider than it was long – a measurement of over 3 m. Stupendemys lived about 5-6 million years ago in South America, alongside an amazing diversity of other freshwater turtles and crocodilians.

These ultra-huge aquatic turtles have died out, but we still have some large ones in the form of the oceanic Leatherback Turtle, Dermochelys coriacea. Not quite as vast as Archelon, they reach a length of around 3 m and at least 650 kg in weight.

The largest land-living tortoises were close in size to these immense marine species. The biggest one we know of at present, is the species Megalochelys atlas. This one occurred over a large range in Asia, centred on India and Pakistan, and might even have reached southeastern Europe. It was truly immense, reaching a weight of one to perhaps two tons – the size of a rhinoceros! It lived for a very long time also, from the Miocene Period, about twenty million years ago, to the Pleistocene, which started about two and a half million years ago. It might have co-existed with early humans, who might even have been responsible for its extinction.

Africa also had its own giant land tortoises, and Australia too – in the form of the bizarre, horned Meiolania-species, which also occurred on many of the islands of the Pacific. The meiolaniids reached about 2.5 m in length, and had long, club-tipped tails much like those of the giant armadillo-relatives the glyptodonts of South America – which are mammals and thus only very distantly related – and also resembling the armoured dinosaurs, the ankylosaurs, which flourished in North America and Asia in the Cretaceous.

Continental giant tortoises all went extinct in prehistoric times – though very likely as a result of human activity. Their immense carapaces could protect them against everything but human hunters. However, many kinds of giant tortoises still persisted on isolated islands. Those on the Seychelles, Aldabra and the Galápagos are still with us – though some local species or subspecies did go extinct on some of the islands – but the equally impressive tortoises that lived on the Mascarene Islands are now extinct. As I said, the Aldabra giant tortoise has been introduced on those islands in lieu of the local species which went extinct. The indigenous Mascarene tortoises were of a different genus, and different in appearance. Some had amazingly long necks, enabling them to browse on high shrubs or low trees. The Aldabra giant tortoise also has a long neck, but not nearly as long as those of these extinct species.

The Mascarene giant tortoises were completely wiped out by human sailors. They were said to once have been so numerous that you could walk from one end of the island to the other entirely on the shells of the tortoises! They had no defenses against the humans, who stacked them onto their ships and kept them alive as sources of meat and oil on long journeys, and raided the populations until there were none left. They were even fed to pigs. Also, pigs, rats and monkeys introduced to the islands fed on their eggs and hatchlings, wiping them all out by the Nineteenth Century.

Luckily, the Aldabra giant tortoises escaped this fate, Aldabra not being on the main route from Africa to the East Indies. So today this is the only remnant of an entire group of Indian-ocean giants. The main race on Aldabra differs slightly from those on the Seychelles, which are very rare now. The tortoises come in two main forms: the dome-shelled, which feeds mainly on food close to the ground, and the saddle-shelled, where the front of the shell is raised to allow the neck to stretch upward, so that it can browse on bushes and trees.

Tortoise Turf

Being the only large herbivores on the island, these tortoises actually shape the landscape and vegetation, as elephants do on the African continent. They are catholic in their tastes, eating pretty much any and every kind of plant to be found (and even some non-plant food like carrion), and existing in all habitats: grassland, scrub, forest and mangroves. They don't need drinking water, being able to extract all the moisture they need from their food. With their thick feet they stomp out well-travelled paths through the bush. They dig holes, using their blunt nails, to shelter in, and also nests to lay their eggs in. They enjoy wallowing in swamps on hot days. In open areas they sometimes occur together in large groups.

The greatest effect all these turtles have on the landscape, is the creation and maintenance of a very specific kind of vegetation, called Tortoise Turf. This consist of a very low, dense growth of grasses, sedges, herbs and small shrubs from a variety of plant families including the Pea Family, the Euphorbia Family and several others. Though not all being closely related, they all tend to grow low and to produce their flowers and seeds even lower, around their bases, hopefully away from the ever-chomping tortoise jaws. These patches of tortoise-turf on Aldabra are thus similar to the open grassy plains of the African continent, which are similarly maintained by the actions of millions of large grazers and elephants. On Aldabra, only the tortoises use these turf plains … the only other land vertebrates are birds, which tend to prefer the regions with trees and bushes.

Where the vegetation grows higher, the saddle-shelled types of tortoise reach up to browse using their long necks. They will even sometimes reach higher still by rearing up on their hind legs – which is perilous, since they may topple backwards, in which case these huge and heavy tortoises might very well not succeed in righting themselves again and may perish from drought, hunger, heat and exhaustion. They also risk toppling when mating, with the male having to poise himself precariously on top of the female.

Ultimate Old-Timers

Provided they don't perish in any stupid feeding or copulating accidents, Aldabra giant tortoises can live for very long; they're perhaps the longest-lived of any land vertebrate. They start out tiny. The female lays what is a rather small clutch, by tortoise standards: 9 to 25 eggs, and about half of them not fertile, to boot. They incubate for about eight months. The tiny tortoises emerge and are able to walk and find their own food – there's no parental care at all. But Aldabra has no large land predators. As soon as they've grown too big for a gull or other seabird to snatch, the new tortoises are safe. They grow slowly throughout their very long lives. Precisely how long, is not clear yet … meticulous documentation and record-keeping is indeed a younger thing than many of the adult tortoises themselves. It's been claimed that a tortoise named Adwaita had been brought from the Seychelles by British sailors in the Eighteenth Century, transferred to a zoo in Calcutta in the late Nineteenth Century, and died in 2006, which would have made him over 250 years old. The oldest surviving giant tortoise is called Jonathan, of one of the Seychelles races, at the time of writing over 180 years old.

Today, these tortoises are very well-conserved on Aldabra itself, and the populations on the Seychelles are being bred and monitored. The species has, as I've said, also been introduced to the Mascarene Islands, to ecologically replace the local species which have been driven extinct (and which, like on Aldabra, played vital roles in maintaining the structure of the vegetation, even being necessary for the propagation and seed-distribution of several plant species). Having them on more than one island is very much a case of not having all one's eggs in one basket.

In addition, these giant tortoises are also found in a number of zoos. With their slow lifestyles and low metabolisms, they do not need much, and several babies per brood can survive if protected and fed. In the optimal environment of a zoo, a female can even lay two clutches per year. As such, this is a rather easy species to protect, and thus unlikely to go extinct anytime soon. Still, we mustn't rest on our laurels. These are the last of a wondrous giant tortoise fauna that used to inhabit the isolated islands of the Indian Ocean, and we should do all we can to ensure that they continue their tranquil existence.

Colours of Wildlife Archive

Willem

12.06.17 Front Page

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