Aldabra Tortoises In The Wild

 

 

Do They Swim?

   Aldabras can swim in a basic form, for brief periods, but it is much more floating than doing the front crawl. They much prefer to sit in a mud wallow rather than deep water though.  Even though they are buoyant, because of their very thin bones in their shells and their large lungs being located at the top of their Carapace, healthy Aldabrans are dense and heavy, sinking down in the water. Luckily they can use their long necks like a periscope to keep their head above the water, to both see where they are going and to breathe. It is not a natural excersise for them, they are not turtles so they do not have the muscle tone for long swims, they just have to hope that the sea remains calm and they get washed up to shore. It is thought that they are able to speed up and change direction, if the current is not too strong.

 

   They don’t ‘swim’ in the traditional sense, it is more that they ‘walk’ through the water, using the same locomotion as they use on land, its a little like a Hippopotamus, (only they tend to walk on the river bed holding their breath). Aldabras will be much more controlled by the current and luck, as to where they end up, in their final destination, as they float and drift, which is likely how they got to Aldabra in the first place, probably from one of the other Islands in the Seychelles. Their trip would not have been quick because it would be governed by the currents speed and it is around 1000km from Mahe to Aldabra.

 

   At times there are limited food supplies on the sland and if they are foraging on the beach, or go into the Mangrove area to cool down, at low tide they can get caught out. If  they don’t make it back to dry land before the tide comes in, they can be swept out to sea. Most of the time they are still within the lagoon and they simply let themselves be carried back to shore like flotsam.

 

   In salt water, and an unhealthy animal (one which isn't carrying as much body fat) may just manage to float and paddle, over a long distance, this was how a gravid female  got to the Aldabra Atoll in the first place, to begin the population, when they were washed out to sea from neighbouring islands. They are not like turtles though and would not go deep diving for the fun of it.

 

   They first arrived on Aldabra by oceanic drifting, as a giant tortoise. Unlike the Galapagos Giant Tortoises, from South America, who were a smaller species who grew and evolved to fill the top position available on their islands.

 

   The Atoll has been completely submerged at least four times since the Aldabra tortoises settled on it, so at least five colonisations may have occurred, within the last hundred and fifty thousand years or so.

   

Important to note :

 

Some species of tortoises can float, but not all of them do, and they sink quickley, so please do not put any tortoise in deep water as you could drown it! Also, don’t force your pet to stay in water longer than it is comfortable with, it will cause them stress.

 

WARNING:

   Please also be aware that in the wild some species of tortoises use the same nesting beaches as turtles and they hatch at similar times. As they can look very similar people, with good intentions, often take the baby hatchling to the water to ensure that they make it there safely, without realising that IT IS A TORTOISE, THAT LIVES ON LAND, AND NOT A TURTLE, THAT LIVES IN THE SEA!

 

   The result is that the tortoise had been quietly making its way off the beach and onto land, after battling its way out of the egg, when some well meaning human picked it up and threw it into the water and suceeded in drowning it rather than saving its life.

 

   IF YOU ARE NOT 100% SURE THAT IT IS A TURTLE please leave it be, the animal will sort itself out. If you want something useful to do- scare the preditors away, that will benefit both species.

 

 

Do They Have Any Predators?

   Over the years man, particularly old fashioned sailors and the crews of Whaling ships, were the main predator, which made the Aldabra tortoise the only variety left of the 18 species of tortoise that flourished in the islands of the Indian Ocean.

 

   The whalers in particular,  took them for their oil and the sailing ships wanted them as a source of fresh meat, which they could keep alive on-board ship, without having to feed them. They could then slaughter as needed, on long voyages, keeping the crew healthier. Their abundance of shell also proved useful for storage purposes or if it was cut up it was used for something to carve, like ivory was. It also made a trading item when they arrived somewhere with a vibrant market. Apparently, in 1842, just, two ships were reported to have taken 1200 tortoises from Aldabra, they were harvested for meat.

 

   Previous human residents of the Atoll also introduced species such as rats, cats, dogs and pigs to the island, which predated and trampled the young tortoises and eggs and limited the increase in population.

 

   Introduced species of goats, who ate the tortoises limited food supply were eradicated from Aldabra in 2012. All cats have also been removed from all islands except one and research of a feasibility to eradicate rats has also been done.

 

   The tortoise's shell is like heavily designed armour plate that protects vulnerable body inside, and the leathery skin is covered with scales to help protect them from a coral atoll abraded by jagged limestone, which has eroded in places causing fissures they fall down and die. Younger tortoises, up to about 4 years old, which are less than 8 inches (20cm) long, are liable to be prayed on by large bird species such as the Ibis and the Aldabra Rail. Land crabs and feral cats are other dangers.

 

   Their greatest danger is accidentally falling down crags, pits created in the eroding rocky sandstone terrain of the Atoll because once they are down there they cannot get back out and will starve to death. Dehydration, overheating and starvation during droughts are their main natural causes of death.

 

How Did Aldabra Tortoises Manage To Survive When So Many Other Species Were Poached To Extinction?

   It was a very close thing, in 1892 it was reported that there were just 1000 tortoises left on Aldabra. One of the reasons they survived is that Aldabra Atoll was a long way off the direct shipping lane from the Cape of Good Hope to India and the Orient. Also the heavy surf that surrounds the Island made it very difficult for ships to anchor safely, so many ships were put off from even trying and then foliage was so dense that it was difficult to see the tortoises when the did land. All this meant that the plunderers preferred to find somewhere easier to scavenge, leaving the tortoises to their own little world.

 

   Worldwide, at least 36 species of large and giant tortoises have gone extinct in the last 2.6 million- 11,700 years,  many because humans played a significant part in their demise, leaving 32 living species. However, most of these tortoise extinctions occurred on islands and some were due to submergence of their islands prior to human contact, so man cannot be blamed for wiping them all out.

 

How Would They Spend Their Day In The Wild?

   The natural temperature on Aldabra is over 30 degrees Celsius, so like most tortoises, these giants are Crepuscular, meaning they are more active in the morning and early evening, when it is cooler. They are usually active for less than 4 hours in early morning and late evenings.

   

   The largest concentrations are mostly found amongst the scrubby, dense, low lying vegetation, in the grasslands called 'Plantins' where a mixture of herbs and grasses known as ‘Tortoise Turf' grows, in the thin soil. During the dry season there is an even dispersal of animals throughout the habitats but when the first drops or rain, heralding the rainy season, start they instinctively migrate onto open grass land and coastal scrub beaches between the dunes.

  

   The tortoises go off to feed solitarily but gather back together in large herds at the swamps, spending the hotter parts of day in shade or lounging in shallow water holes, lagoons and mud wallows, to protect themselves against mosquitoes and the heat. 

 

  Aldabra tortoises can sleep 18 out of 24 hours but it varies on the individual and environment. There is a large tidal lagoon on Aldabra, which is shallow and 2/3rds dry during low tide and around a waterhole. Like this, it is possible to see 50 or more animals together at a single time. They can gather together so closely that it sometimes looks as if the area is paved with their shells.  On Aldabra, the biomass (total weight of organism in a given area) of Aldabra tortoises is calculated to be between 3.5 - 58 tonnes per square kilometre.

 

   Giant tortoises can travel 3 or 4 miles in a day on rugged terrain areas, covered in thick coastal scrub, typically where there is a mixture of trees, shrubs and dunes. They will noticeably alter habitat during searches for food and have been known to knock over small trees in search of nutritious leaves creating pathways for other animals to travel along in the process. They can also feed on some of the rotting wood, particularly if there are bugs and lava there, as it is a good source of protein for them.

 

   With rare exceptions, Aldabra tortoises, in the wild, will actually spend all of their long lives within just a few killometers of their birth.     

 

Aldabra tortoises in Mauritius

 

What Is Their Natural Habitat?

   Aldabra Atoll is 285 miles from Madagascar and 425 miles East from mainland Africa, in the Indian Ocean and is the most south westerly part of Seychelles. It is an Ecosystem dominated by reptiles which is something that is not seen anywhere else since the time of the dinosaurs and is a place where nature has been left to take its course. Varied habitat from large mangrove swamps bordering the lagoon to coastal dunes to grass land and scrub forest.

 

   The Atoll of Aldabra consists of 13 limestone islands, which are the result of ancient volcanic or seismic activity. It was formed about 125, 000 years ago when volcanic material was deposited on the sea bed and continued to grow until it broke the ocean surface. Coral grew around the island at sea level and when the old volcano subsided beneath the sea, a coral atoll was formed.

 

   The lagoon connects with the ocean in 4 places, so Aldabra Island is actually composed of 4 Islands- North or Middle Island also known as Malabar. Grand Terre, which is the South Island and is the largest, Polymnie and a small settlement island called Ille Picard or West Island. Tortoises are most plentiful on South Island but a few live on North and West. The lagoon is teeming with Black Tipped sharks and Manta Rays.

 

   Aldabra is only 8 metres (26ft) at its highest elevation above sea level, and so is at great risk from rising water caused by climate change. The island is the worlds 2^nd largest coral atoll and is 34km (21miles) long by 13km (8.1miles) wide. The coastline is approx 50 miles long with only about 1% of it being beach, the rest is nearly impossible to cross on foot. Thus because it is quite difficult to even get to the atoll, it has remained largely uninhabited and the giant tortoises have been free to colonise it and become the largest occupants.

 

   The tortoises are a valuable agent of pollination as they disperse seeds during excretion, after they pass through its digestive tract. Their intense grazing actions have put pressure on natural flowers to alter the way they pollinate so the tortoises can't take flower heads off too soon, making them directly responsible for plant evolution taking place. Reportedly there are 307 different species of plant and animals on Aldabra including one of the largest populations of nesting Green Sea Turtles in Western Indian Ocean and these lay their eggs on the small beaches.

 

   The copious amount of poo they produce is important for nutrient recycling, on this sometimes barren island, as well as seed dispersal. They can eat large amounts of fruit and swallow relatively large fruits and seeds. Aldabra's in the wild can consume 26 million lbs or 11.8 million kg of plant matter each year. Tortoises have a variable gut passage time depending on the species.  The average range from a few days to 3 weeks, allowing for potential dispersal distances of hundreds of metres. Some seeds can take up to 3 months to pass through an Aldabra gut.

 

   The unfortunate side effect of them producing all of this poo is that Aldabra permeates a strong aroma (it stinks!), as there is no dutiful human who goes around, with a shovel, and cleans up after them. So, with a hundred thousand plus animals living on Aldabra, there is alot of faeces scattered around.

 

   Being located nearer the equator the weather is opposite to the UK. There the Average temperatures range between 22-31 degrees Celsius and it is cooler in August and warmer in December with a Monsoon season November to March. So they are well able to cope with the UK rain!

 

   After heavy rain on Aldabra the barren earth can quickley turn to thick gloopy mud, which is both difficult and tiring for them to trudge through.

 

 

 

What does the habitat on Aldabra consist of?

There are 8 different natural habitats on the Aldabra Atoll and a lot of it is is inhospitable to both tortoises and humans, which is another reason the Island was never populated, for long periods, or in large numbers, by humans, in the past.

 

 

1) Champignon covers 4.8% of the Atoll. This is a highly eroded and severely fractured limestone area, made up of ridges, chasms and other impassible formations, caused by water erosion, of the soluble rocks, over time. There is not much vegetation growing and the razor sharp edges, on the rocks, make it practically impenetrable and very hazardous to limb injury for the tortoises.

 

2) Platin 12.9%. This is a small, relatively flat, and unvegetated area but this is where the puddles of fresh water accumulate after the rains so it is a vital area for the tortoises. As Aldabra is a largely limestone Island, the water seeps away through the rocks, leaving a shortage of fresh water, for the tortoises during the non rainy season, which is how they have adapted, over thousands of years, to go without water for many months of the year.

 

3) Pemphis Scrub 23%. Again this contains highly eroded limestone with razor sharp edges, making it treacherous and inaccessible to humans. However, it is densely populated by a heat and salt resistant plant, so it a valuable food resource area for the animals but the many vertical crevices pose a threat to the tortoises, if they fall in they can’t get back out and will die. Unfortunately, that is a risk they have to take in order to feed to survive. Life on Aldabra is harsh.

 

4) Dense Scrub 27.2%. These are shrubs that form a closed canopy and provide vital shade areas for the tortoises to hide away during the excessive heat of the midday sun. Overheating is one of the main causes of death for the tortoises on Aldabra.

 

5) Open Scrubland 16.3%, this includes the area of Aldabra exclusive tortoise turf. This is the primary area tortoises come to feed.

 

6) Tortoise turf only covers 2.9% of the Atoll. It is surrounded in the undergrowth scrubland vegetation, which helps to protect it from the elements and this is the preferred habitat of the tortoises. Some of the plants have even directly adapted to the way that the tortoises feed. They produce their flowers low down so that the tortoises do not immediately bite the heads off before the plant has had chance to reproduce. Some of the plants growing on the turf are exclusive to Aldabra.

 

Tortoise turf consists of:

Bacopa monnieri

Boerhavia elegans

Bulbostylis basalis

Cassia aldabrensis

Cyperus dubius

Cyperus ligularis

Cyperus obtusiflorus

Dactyloctenium pilosum

Eragrostis decumbens

Euphorbia prostrata

Euphorbia stoddartii

Evolvulus alsinoides

Fimbristylis cymosa

Fimbristylis ferruginea

Hypoestes aldabrensis

Lagrezia madagascariensis

Lepturus repens

Mollugo spergula

Panicum aldabrense

Phyllanthus maderaspatensis

Pleurostelma cernuum

Plumbaga aphylla

Pycreus pumilus

Ruellia monanthos

Sclerodactylon macrostachyum

Sida parvifolia

Solanum nigrum

Sporobolus testudinum

Sporobolus virginicus

Tephrosia pumila

 

7) There is an extensive mangrove line 11.1%, which grows in and on the edge of the crystal clear lagoon, where the tortoises spend a lot of their time wallowing, to cool down. The mangroves consist of 8 different tree species, but is predominantly Red Mangrove. Sandy patches between the roots, on the banks, give the tortoises opportunities to escape the noon heat of the day, during the dry season, and rest on land rather than in the water.

 

8) Sand 1.8% is shaped highly inland, with dunes up to 20m high. These outer areas of the Atoll provide important nesting sites for the Green Turtle.

 

 

 

 

Have They Changed Much As A Species?

   Tortoises have existed for many millennia; there was a form of giant tortoise around at the time of the dinosaurs. The Aldabra Giant Tortoise arrived on its native Atoll as a giant tortoise, unlike some other species of larger tortoises who have got bigger, over the years, to fill a niche on the evolution scale.

 

   Fossil records indicate giant tortoises once occurred on every continent, and many islands, in the world, with the exception of Australia and Antarctica. The earliest tortoise fossils came from the late Palaeocene era (66-56million years ago) and were found in Mongolia.

 

  The basic turtle/ tortoise body shape arose in the late Triassic period, which is 237 - 201 million years ago and it has not really changed since. If it works for them they obviously do not feel the need to alter their design, The shell is basically an extension of the rib cage, which are outside rather than inside the body.

 

   This slow resistance to evolution is probably something that will work against them, with the rapid hastening of climate change, they will not be able to adapt quickly enough to compensate.

 

   It is thought that the shell divergence tha some Indian Ocean Giant tortoises and Galapagos  Giant Tortoises have developed is down to ecology not genetics. Barriers preventing movement between islands means each Island species evolved slightly differently out of necessity. The number of Scutes on their shell has remained constant since their early evolution.

 

   Turtles evolved to go into the sea, from being land animals. We know this because they must come to the surface for air, and ashore to lay their eggs. Present day tortoises descend from turtles that swam in the sea and not the animals that remained on the land. For an unknown reason some of them decided to retreat from the water and become 'toe walkers', on the land, once more.

 

Are They Endangered?

   Unlike the Galapagos Giant Tortoise from South America, which are an endangered species, the Aldabra Giant Tortoise is classed as Vulnerable on the Red List Of Threatened Species of (I.U.C.N) the International Union for Conservation of Nature. The Aldabra Tortoise, Aldabrachelys Gigantea, comes under the protection of Appendix II in CITES.

 

   The Aldabra Giant Tortoise was one of the first species to be protected to ensure its survival. Charles Darwin and other notable Conservationists of the late 1800's along with the then Governor of Mauritius set aside a breeding population on Mauritius as well as protecting the Aldabra Atoll and introducing them to the nearby Island of Reunion.

 

   There has been a tiny permanent settlement on the Aldabra Island since 1874 however; a ban on killing the tortoises was introduced in 1891. By 1900 giant tortoise species in the Indian Ocean were hanging on by a thread, thanks to man taking them as easy prey. At one time there used to be 4 subspecies of Aldabra tortoises- The Aldabra Tortoise, The Seychelles Giant Tortoise, The Arnold's Tortoise, The Hololisaa but only The Aldabra thrived beyond 19^th century.

 

   After WW2 restrictions were imposed on how many people could stay on the island, in the settlement. Most of these people are locals, connected to the small research station, or are invited scientists there to study the tortoises and the Atolls other various wildlife and fauna.

 

   In the 1960's plans for development of the Atoll into an RAF base were proposed. However this produced outrage from the scientific community, and the plans were abandoned in 1967 after an international protest by scientists, which became known as the Aldabra affair.

 

   The Atoll received full protection when it became a designated UNESCO World Heritage site on 19th November 1982; it is now managed and protected by Seychelles Islands Foundation (SIF). As a World Heritage site it ensures that the largest population of Aldabra's are now safe from any outside human influence. Eco tourism is controlled and tourists are not allowed to stay on the island and are encouraged to visit the Conservation Parks on Mauritius and Rodriguez Islands instead.

 

   In the 1970's several thousand tortoises on Aldabra were tagged with titanium disks, glued to their shell, so researchers could monitor movements and growth rates.

 

   Captive bred Aldabra are being used, increasingly, to rewild islands in the Seychelles region, to replace extinct giant tortoises and to resurrect degraded island ecosystems. However, these tortoises are all coming from the same captive groups so they are all genetically related. Between 1978 and 1982 a large scale translocation of 250 tortoises from Aldabra to Curieuse Island took place.

 

   A nature Reserve was created on Rodriguez Island in 2007 and stocked with captive bred tortoises, there are almost a thousand Aldabras living there now.

 

 

N.B This is Samson eating stickey weed, and not a wild Aldabra!

How Many Sub Species Of Aldabra Tortoises Are There?

 

There are four sub species of Aldabra:-

 

1) The Daudin’s Giant Tortoise has been extinct since 1850.

2) The Aldabra Giant Tortoise is alive and well on Aldabra Atoll, and being used to repopulate other local islands.

3) The Arnold’s Giant tortoise / Seychelles Saddleback Tortoise is found on Mahe Island.

4) The Seychelles Giant Tortoise / Hololissa Tortoise is found on the Islands of Silhouette, Round, Praslin, Mahe, Fregate, Cousine and Cerf.