Aldabra Tortoises In The Wild

What Is The Scientific Name For Aldabra Giant Tortoises?

   Tortoises is the common name for some shelled reptiles, they are part of the order of Chelonia, specifically Testudindae.

 

   The Taxonomy of an Aldabra Giant Tortoise is

Kingdom Animalia

Phylum Chordata

Class Reptillia

Order Testudines

Family Testudinidae

Genus Aldabrachelys

Species Aldabrachelys gigantea

Type Reptiles

 

   Aldabra Giant Tortoises have at least 4 different scientific names but the most common Species names are Aldabrachelys Gigantea Gigantea and Geochelone gigantea and Dipsochelys dussumiieri. Geochelone means large species.

   According to the decision of the International Commission on Zoological Nomenclature (ICZN) in 2013 winch aimed to standardise the naming of the Aldabra tortoise, they decided that it would be known as Aldabrachelys Gigantea gigantea.

 

What Is The Aldabra Island Group?

   The Aldabra Island Group consists of four neighbouring Islands Aldabra, Assumption, Astove and Cosmoledo. Aldabra Atoll has an overall outline of 365km and is roughly 34km long and 14km wide. The land itself is around 155km2 but in actual fact only 30km2, that is just under 20%, offers usable habitat to the tortoises.

   If you were able to actually walk around the shoreline of Aldabra, which is not possible due to the inhospitable landscape in a lot of places, it would almost be the equivalent of walking from Cambridge to Newcastle Upon Tyne, in the UK.

 

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 of tortoises 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.

   Aldabra become active and start foraging at dawn but they can foresee when a day will be hot and cease activity around 10am. They will then congregate in larger groups, in shaded locations and under shrubs, anywhere that offers the shelter. Once the greatest heat is over at around 3 or 4 pm, they will emerge and be active again, foraging until sunset.

   The tortoises go off to feed solitary 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. 

   In the wild, when the temperatures are very high, Aldabra tortoises can sleep 18 out of 24 hours, in the shade. As it is too hot to waste energy trying to move around, it is better for them to wait until it cools to a more acceptable temperature but it varies on the individual and environment. 

   However, here in the UK, Aldabra tortoises are more active, in summer, with the lower, more tolerable daily temperatures. On a nice summer day our Aldabras can be up at 7am and not go back to bed until 9pm, spending most of their time eating, being active and wandering around. If it is a very hot day they will be looking to hide in the shade from around 10am till 2pm before coming back out to forage, just like their wild brethren. On days like this, some of them are very reluctant to go to bed, they are happy to just chill out and enjoy a balmy summer evening, especially if they are in the wallow. A bit like someone who enjoys relaxing, with a glass of wine, in their hot tub. 

   If your UK tortoise is consistently sleeping for most of the day and reluctant to move around, you need to check that it has correct the husbandry. You may need to improve your heating, cooling or lighting facilities. If you do not see any improvement after doing this, it would be advisable to book an appointment with an exotics vet so that they can do a blood test and check to see if there is an underlying health issue for the tortoise. Your Aldabra should be active for most of the day, in normal temperatures. They can slow down over winter, and just want to bask under heat lamps, which is fine, and normal, because its too cold for them to be outdoors.

   For 30% of daylight hours, in the wild, they will be inactive and resting in the shade. The largest cave on Aldabra is 5 meters deep and can shelter up to 85 tortoises. But space and shelter is at a premium, and not finding shade during the hottest parts of the day can be life threatening for them. Tortoises who seek to cool down in the water, of the mangrove beaches, can risk being washed off, and into both the inner lagoon and even open ocean, as the tide goes out.

   There is a large tidal lagoon on Aldabra, which is shallow and 2/3rds dry during low tide. 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 to 58 tonnes per square kilometre, depending on the sizes of the animals in that area.

  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 use, 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 kilometres of their birth.    

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, of varying sizes, which are the result of ancient volcanic or seismic activity, they range in height from 2 to 4, to 8 metres at the highest point, above sea level. It was formed about 125, 000 years ago when volcanic material was first deposited on the sea bed and continued to grow until it broke the ocean surface. Coral grew around the island at just below 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 main 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 narrow islands each average around 2km wide and they are situated around a shallow lagoon, which is teeming with Black Tipped sharks and Manta Rays.

  Aldabra is only 8 metres (26ft) at its highest elevation above sea level, but most of it is much less than that, so is at great risk from rising water caused by climate change. The island is the worlds 2nd 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 not 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 the tortoises 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 tortoises, in the wild, can collectively 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 a lot of faeces scattered around. However, this is what breaks down to form the basis of the soil that the plants grow in. Without the tortoises presence over the millennia there would not be enough nutrients on the soil for an abundance of plants to survive.

  Being located nearer the equator the weather is opposite to the UK. There the Average temperatures range between 22 to 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 quickly turn to thick gloopy mud, which is both difficult and tiring for them to trudge through . However, if they rest too long in the heat, and the mud dries out around them, they can get stuck and die from heat exhaustion, being unable to break free.

   Along the coastline of Grand Terre, and over many millennia, a hundred thousand generations of Aldabra Giant Tortoises have transformed the habitat into, short cropped, grassy lawns and created their own highly productive grass, to feed on. The result is that plants that have evolved in certain ways as a direct reset of the feeding habits of the tortoises.

 

Aldabra tortoises in Mauritius

 

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. These are also the mini islets that you will often see photographs of, amongst the clear azure sea, in holiday brochures or on websites.

2) Platin 12.9%. This is a small, relatively flat, and non vegetated 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 not 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. The Pemphis plants can grow to 3 or 4 metres high and is so dense and impenetrable that you need a machete to get through it.

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, which covers and area about 60 feet wide. 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. This is a well used and vital area, and around 30 Aldabra tortoises per acre will graze on the Tortoise Turf, at any one time.

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. The tallest dune is on South Eastern Grand Terre and is just 60 feet above sea level. This is the Aldabra Atolls highest point.

How Much Of Each Terrain And Life Is There On Aldabra?

• 178 species of indigenous flowering plants. 8% are naturally exclusive to Aldabra
• 50% limestone terrain which is covered by salt tolerant scrub thicket.
• 19% mixed scrub, offers the tortoises more sheltered areas.
• On the flat limestone there is mixed growth of low trees. This limestone plain is where many generations, of tortoises, over hundreds of thousands, or potentially millions, of years, have come to drink after the sparse rains. It is one of the few places that rainwater collects and does not immediately seep away. Over those many thousands of years, millions of their feet and shells have worn smooth edges around the puddles as they repeatedly came to drink the vital water.
• 17% of the mangrove swamp, covering 17% of islands surface, grows around the edge of the lagoon.
• Sea grass meadows provide grazing for the hungry mouths.
• 100 species of bird help fertilise the Tortoise Turf with their guano.
• Insects form 38% of the estimated 1000 species, on the Atoll.
• 1997 it was estimated there were 100,000 tortoises maintaining a self sustaining population, now it is more like 150,000, of varying sizes and ages. Whilst females are still laying there have been noticeably reduced clutch numbers or less frequent nesting occurrences compensate for the increase in population numbers.
• 94,000 of the tortoises live on largest island of Grand Terre. In several places they are confined to a certain area due to impassable natural land obstructions.
• Sections of the landscape of Aldabra are covered with deep fissures of jagged limestone, with very sparse vegetation and are impenetrable to both human and tortoises. 4000 years ago sea levels were several metres higher and the thrashing of the water over the submerged Atoll eroded limestone limestone to sharp spikes and pits, over much of the islands

 

 

What Is The Highest Point On Aldabra?

   The highest point on the Island is Dune Jean Puis, which is a sand dune measuring around 8 meters, or 26 feet, tall.

 

How Big Is The Lagoon And Do They Use It A Lot?

   Even though the large lagoon, in the middle of Aldabra is filled with sea water, so they are unable to drink from it. The blue lagoon is a vital area that the tortoises to use to cool down in and it is so large that the whole of City of Portsmouth (40.3 square km) would fit in there nearly 5 times over! The lagoon is linked to Indian Ocean by 2 major, and 1 smaller channel, as well as several small reef passages.

   There is a tidal range of more than 3 meters and it produces strong currents through the channels to the open ocean, from the lagoon. The main channel drains about 60% of the water in the lagoon whenever the tide goes out, leaving it at around 2 to 3 metres deep at low tide.

   The surface area of the massive lagoon covers 196 square kilometres, and has several small islands, visibly shaped like mushrooms, with eroded spires of dagger shaped limestone, poking from the water. Up to 70 Aldabra giant tortoises live on these smaller lagoon islands called Champignns.

How Many Tortoises Are On Aldabra And Where Do They Live?

   It is difficult to get an accurate number for exactly how many tortoises live on the islands of the Aldabra Atoll because many places are inaccessible to the scientists, to search for them. Also the smaller tortoises are very difficult to spot in the dense undergrowth, so there could be a dozen new hatchlings hiding under foliage just a couple of metres away, that they cannot see and know nothing about. It could take five, ten or more, years for one of those animals to be spotted for the first time by a scientist, or researcher.

   There are actually more tortoises on Aldabra than would fit in the whole of Old Trafford Football Stadium and Murrayfield Rugby Stadium combined. And they outnumber the human residents of the Seychelles Archipelago, by around 60,000.

   It is thought that the self sustaining population numbers on the Atoll have been consistent for around the last 15 years, now that most of the introduced predators have been eliminated. However, those tortoises are distributed unequally across the different areas.

   With an estimated 150,466 tortoises it is not possible to GPS tag all of them and get accurate signals to find them again each time. However, in the 1970s several thousand tortoises on Aldabra were tagged with titanium disks, screwed to their shell, so researchers could monitor movements and growth rates. For nearly a 100 years scientists have been constantly studying this unique group of animals, and a growing number of tortoises have been permanently identified with these metal plate identifications.

   97.7% of Aldabra Giant Tortoises live on Grand Terre, which is about 146912 tortoises . This is the largest island and has low vegetation to graze and 10% tree cover, to shelter beneath. The tortoises that live there are much smaller than some of the other islands of Aldabra Atoll. On Grand Terre full grown adults will be 30 to 50kg with a blacker shell.

   Around 60% of those, occupy an area of just 33.6km sq, which covers 38% in the East coast of the island, with a tortoise population of 18 per hectare. Whilst 24 % occupy the Southern coast, of Grand Terre with a more spacious population of 13 tortoises per hectare. Dense thicket, jagged limestone and sea water make natural barriers in the terrain to separate the population dispersal, especially in the East of South Island, so there is limited interaction between the different groups.

   Malabar Island is smaller and has a total area of 29.56 sq km available but is only about 4% occupied. There are, around, just 2250 living in an area of 3.2 sq km, giving them a hectare for every 12 tortoises.

   A further 1235 live on Picard and these are known to be the larger animals with lengths of around 1m and weights of up to 120kg. They are also more brown coloured in the shell, rather than the traditional grey. Tortoises in captivity can get much heavier, the heaviest known is 363kg but you would never get those weights in Aldabra Tortoises in the wild, they would not naturally be able to find that level of food on the island, to produce such bulk.

   Up to 70 tortoises live on the smaller lagoon islets, these uplifted, mushroom shaped, coral formations are known as Champignons. They are covered by very dense scrub, which is unsuitable for most of the tortoises to live on but a few hardy ones have made them their home ranges.

   At the moment, no tortoises are thought to live on Polymnie.

   There is some gene flow between the island populations, it is limited by the terrain but they are not totally isolated groups, they float or drift, usually within the lagoon, from one island to another. The risk of over population, in certain areas could be a real problem for them, in the future.

   Unfortunately, there is some evidence to suggest that the tortoises on Aldabra are not reproducing to the same extent, possibly as natures way of preventing overcrowding because they will have a limited food supply or maybe for other reasons. Due to the fact they all live on an Island about the size of the Isle of Wight, there is also a greater chance of their inbreeding developing into issues as the years go on.

 

Why Do Tortoises Only Live On Certain Parts Of The Aldabra Atoll?

   The terrain governs a lot about where the tortoises can access, and successfully survive, on Aldabra, and there are places there that are inhospitable to them. So, the tortoises are unevenly distributed across the 155 sq km of coral limestone Atoll, 80.7% of tortoises occupy just 37% of the area available. The tortoises do not roam areas with deep limestone fissure, or they risk falling in and dying because there is no way for them to get back out.

   Density of Aldabra Tortoise numbers in an area depends on several factors:

• Feeding access. If all of the food is too high up, and out of reach, the tortoise will not spend their time in that area, as there is no benefit for them, they have to make the most of eating whilst it is cool enough for them to do so.

 

• Quantity of food. The tortoises wont remain in the area, if they have grazed it flat within an hour, they will move on to somewhere new. On the South West coast of Grand Terre, Malabar and Picard Island the feeding tortoise are found mainly on a relatively narrow coastal strip of grassland and mixed vegetation.

 

• Quality of food. The tortoises will be attracted to areas like the Tortoise Turf where there is a better quality of food available to them. If they can take in more nutrients in one sitting then they will have to feed less, or at least not starve for as long.

 

• Shade. They need to be under cover during the hottest parts of the day or there is a risk of dehydration killing them. The shade provides an essential limit to the evaporative loss of water, from the body. So, the ones on Malabar drink less because there is less water available, but to compensate there is more shade so they loose less to evaporation. The tortoises will obtain most of their fluid in take from the browse and vegetation that they they eat. They will calculate how far they can go to eat before they need to retreat back into shade. This can cause inhospitable height planting to grow, where they do not feed, making it impossible for them to move around certain areas.

 

• Habitat. They need an area that they can move around in safely, without being hindered by dense thickets or limestone fissures. They do not want to take their life in their hands every time they step out for something to eat.

 

• Density of other tortoises. More tortoises around means they have a greater chance of finding a mate and successfully passing on their genes to the next generation, which is something that all life is driven to do. However, too many other tortoises in the locality means there will be grater competition for the food and shade available, so it is a fine balance.

 

• Seasonal requirements. Females will move to certain areas in the breeding season to lay their eggs. This is a very important consideration because not all terrain is suitable to dig a nest in and as it takes a lot of effort for them to dig their nests, they want to increase the chances that they will have a successful outcome as much as possible. So, they carefully choose where they want to lay, sometimes going to the effort of digging test holes first.

 

• Water. Tortoises will also be drawn to certain areas during the rainy season, because water collects at certain points, like in the flat limestone areas. Aldabra tortoises have to spend many months without fresh water to drink so, when the rains do arrive the need to make the most of it and rehydrate, before it evaporates. On Grand Terre, in the East and South East, there is a large lens of fresh ground water that is raised close to the surface by high spring tides, which allow the tortoises a vital chance to drink after months of access to very little water. This water source is probably one of the reasons why there is significantly more tortoises per sq km on Grand Terre, than any other island in the Atoll. This water is probably also a reason why the vital Tortoise Turf grows there successfully.

 

Is There Physical Difference Between The Tortoises That Live On The Aldabra Atoll?

   60% of the Aldabra population live at the Eastern end of Grand Terre, and they only grow seasonally, as a result they are generally much smaller than the 2000 or so who live on Malabar, who grow continuously.

   Adults on Grand Terre are much smaller than those on Picard, and have a blacker shell. Fully grown, an animal on Grand Terre will be between 40 to 60kg, whereas, Picards brown shelled residents can be up to 120kg.

   This is thought to be primarily because of the limited shade factors on Grand Terre, which impede the length of time they can spend feeding.

   The higher up the former volcano sides the tortoises live, and feed, on Aldabra, the more domed and compact their shell shape is because they are a grazing feeder, meaning their food is on the floor, so their head stays down. Whereas, the ones who feed more on browse need to be able to lift their head higher and stretch further to reach food on bushes, and low tree branches. They have they have a lower, longer shell which is more open at the front so they can lift their head up, without being blocked by the overhang of their Carapace. The lower shell also alters their centre of gravity so that they are less inclined to be pulled backwards, to roll over, if they raise more than one foot from the floor. Some Aldabra can actually stand up on just their back legs, allowing them to reach to up to 4ft of the ground for food.

   Just like some of the Galapagos Giants, found on Espaniola, Pinzon, Pinta, Santa Cruz and Isabela Islands, in South America. The Arnolds Giant tortoise, a sub species of the Aldabra Giant Tortoise, which is now only found on Mahe Island, the capital of the Seychelles, has developed the distinctive, high fronted, Saddleback shaped shell to help with easier access to high reach feeding, giving it the alternate name of the Seychelles Saddleback Tortoise. The shaping is not as extreme as the Galapagos Saddleback Tortoise though.

   The Aldabra subspecies of the Seychelles Giant Tortoise, or Hololissa Tortoise, is found on the Islands of Silhouette, Round, Praslin, Mahe, Fregate, Cousine and Cerf. These tortoises also have slight shell differences from the main Aldabra group, including more flared leg scutes and different outlines of Plastron scutes, however, they are subtle and difficult to identify with certainty.

Are There Any Other Species Of Tortoise On Aldabra?

   No, the environment on Aldabra is harsh and there is not enough food, water and living space for another tortoise breed to thrive. It is dominated by the Aldabra Giant Tortoise species, who pick all of the accessible graze down to bare minimum in order to live there themselves. Smaller breeds would be in direct competition with young Aldabra tortoises and they would not survive the competition. They would also not be able to cope with the lack of drinking water, their smaller bodies would dehydrate too quickly, and they could not store fluids like the giants do. Even the smaller Aldabra tortoises have physical internal differences to a normal tortoise, to allow it to survive longer without water.

   Also, a smaller breed would not make the voyage to get to the island in the first place, they would not survive the potentially weeks, or months, long oceanic drift, of potentially several hundred kilometres, from the nearest land mass, that have tortoises living on them already. Neither could they survive in the water for that long, without being eaten by a predator, or drowning. And it is highly unlikely that they would be able to cling to flotsam, in rough weather, for so many days, not to mention they would be exposed to the overwhelming power of full sunshine and have nothing to eat.

   Aldabra tortoises were giants when they arrived, not ordinary size tortoises that grew fat when they landed. It is this fact that allowed them enough stored fat and water to endure the long voyage and survive and even thrive when they landed. Such a journey is rare and very few Aldabra tortoises have made it.

 

 

 

 

How Did Aldabra Tortoises Manage To Survive When So Many Other Giant Tortoise 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, and the area had a reputation for pirates. Also, the heavy surf and coral reef that surrounds the Island, just under the sea level, made it very difficult for ships to anchor safely, a valuable ship could be wrecked on the coral reef. So, many ships were put off from even trying, and then foliage was so dense, and inhospitable and perilous to traverse, that it was difficult to see the tortoises when the sailors did land. There was no dependable freshwater and beyond the heavy tortoises there was not much else to eat. All this meant that it was unattractive to the early sailors, and plunderers preferred to find somewhere easier to scavenge, leaving the tortoises on Aldabra to their own little world.

   Worldwide, at least 36 species of large and giant tortoises have gone extinct in the last 2.6 million to 11,700 years, many because humans played a significant part in their demise. 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.

   In the 18th century, on neighbouring Rodriguez island, in just 18 months 30,000 tortoises were take from that island and eggs were also take for food or trampled by domestic animals such as cattle and pigs, who where introduced by settlers. By 1800 not a single tortoise remained alive on the island.

   Not all of these animals were destined for Western tables and interest. In 1737 the Governor of Mauritius organised a regular service of frigates, to transport tortoises to his island. A typical cargo would compromise of 5000 land tortoises and 50 turtles. Unfortunately the transport was so brutal that only a small percentage survived the 8 day voyage.

   Natives of Aldabra continued to kill them for food until 1874 when the Governor of Mauritius was partitioned to take steps to preserve the remaining tortoises from extinction. In response paddocks were established on several islands of the Seychelles and as many adults as could be located were taken there. Killing the tortoises, on Aldabra, was banned in 1891. Today some of their descendants live in the breeding farms, or conservation parks, that tourists can visit.

   After so many years of poaching tortoises the sailors were struggling to find them on the islands and when they did it was more difficult to get them onto the boats, they were having to carry them further over treacherous terrain, and they are not easy to carry at the best of times. They decided that they preferred goat, as they were easier to move and could be tied up on an island for a few hours to feed, whilst the men searched for other food and water for the stores, before being taken back to the ship. Unfortunately, some of these goats escaped their tether and their descendants created problems with competition feeding with the tortoises, for generations to come, until they were finally eradicated from Aldabra in 2012, after twenty years of trying. The sailors new preference for goat was what saved the final few numbers of tortoises on Aldabra, or else they too would have been taken to extinction like so many of the others.

 

Did The Old Fashioned Sailors Just Take The Tortoises For Their Meat?

   No, although fresh food for the crew was a primary attraction, there were a lot more uses for the tortoises and turtle species that they took, so there was not a lot of waste. The tortoises were so useful that seafarers, of the age of exploration, which was 1500s to 1800s, plundered all of them to almost extinction.

   Firstly, fresh tortoise meat was delicious and nutritious to eat, and a welcome alternative to salted beef and pork, which was the sailors usual diet in those days. Tortoise steaks were served to the Captain, senior staff and special visitors. The meat from along the Plastron, and other shell parts, was scraped free, when cooked and, along with other scraps, made into soup for the crew. This stew would be served with Sippets, which is bread or toast.

   The eggs harvested, both from nests, ashore, and the female, tortoise and turtles, they killed, and brought aboard, were a good source of protein for sailors and again were more often used for the menu of the officers table, in various forms.

   The fat was used for butter, and cooking with.

   There is a book The Art And Mystery Of Cooking, which has survived from 1665, which details several methods they used to cook them. You can find some more details of this in the How To Cook A Tortoise article in the General Tortoises Information section of the web site. (might not have been uploaded yet).

   Tortoises could be tied down, in the storage area, or on deck, and be kept alive on board the ship for a year or more without food or water. Then they were slaughtered, when required, and the crew had a supply of fresh meat.

   The sailors even drank the tortoises blood, and the stored water from the tortoises over large bladder, so that when they got desperate, in times of drought, whilst far from land, they had drinkable fluid available.

   Shells could be carved or made into decorative objects, both for functional reasons but also to keep the men occupied, as well as making items to trade and sell for money. Shell would be valuable and could be divvied up for payment of the crew.

   Oil was more of a primary target for the sailors on the whaling ships, just like seals and whales on other trips, to the colder areas of the planet. Oil was made by removing the tortoises from their shell and boiling down the body parts, including the genitalia, to make tortoise oil. Oils had many uses on board the ship, such as cooking, used as lamp fuel, to even caulking the ship to keep it water tight, and oiling mechanical machinery.

   Any not used for ships purposes could be sold for money. For use in traditional beauty products, lubricants, and traditional medicine purposes in some countries. One barrel of tortoise oil would have taken 400 to 500 tortoises to make it, so it was expensive to buy. We are not sure if that is ordinary sized tortoises but suspect it could be the giants.

   Finally, the impressive Giant Aldabra Tortoise could be given as a gift, or a bribe, and is how a number of them ended up in zoos so far from home. Our own late Queen Elizabeth II was given two by the Seychelles Government and its People, in 1972, as an honoured gift and these tortoises went to live with the Zoological Society London, at their Regents Park site. We do not know what happened to them but London Zoo no longer keeps Aldabra Giant Tortoises, they only have the Galapagos Giant Tortoise, which are a fairly recent addition.

 

How Many Sub Species Of Aldabra Tortoises Are There?

   There are four sub species classifications of Aldabra Giant Tortoise, based on the morphology (shell shape) of the tortoises:

1) Aldabrachelys gigantea daudinii. The Daudin Giant Tortoise , has been extinct since 1850.

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

3) Aldabrachelys gigantea arnoldi. The Arnolds Giant tortoise is also known as the Seychelles Saddleback Tortoise is found on Mahe Island, the capital of the Seychelles also in some zoos in America.

4) Aldabrachelys gigantea hololissa. The Seychelles Domed Giant Tortoise or Hololissa Tortoise is found on the Islands of Silhouette, Round, Praslin, Mahe, Fregate, Cousine and Cerf. This sub species is also identified as some of the animals in the worlds zoo population, as well as suspected to be the species of some of the giant tortoises in the UK and, potentially, in the worldwide pet trade.

   However, genetic studies show very low genetic intraspecific differrentiation and do not support the distinction of subspecies of Aldabrachelys Gigantea. DNA tests indicated that there is only one species of Aldabra Giant tortoise, which is Aldabrachelys gigantea gigantea.

Are Aldabra Giant Tortoises 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, since 1996, because of their restricted range. The Aldabra Tortoise, Aldabrachelys Gigantica, 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 1800s, 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 Giant Tortoise, The Seychelles Giant Tortoise, (Hololissa) The Arnolds Tortoise (Seychelles Saddleback), and The Daudin Giant Tortoise which is extinct. Only The Aldabra has thrived beyond 19th century.

  After WW2 restrictions were imposed on how many people could stay on Aldabra 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 1960s 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 supposed to ensure that the largest population of Aldabras 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 1970s 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. 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.

   Today's worldwide Aldabra population, is estimated to be at under 200,000 animals. However, because the majority of those animals live on one specific place, namely the Aldabra Atoll, and the surrounding Seychelles locality, they could mostly be wiped out if one powerful natural disaster hit the area. This why they are classed as being a vulnerable species. However, these tortoise numbers are 10 times as many tortoises as can be found in the Galapagos Islands, and why the Galapagos Giant Tortoise is an endangered species, despite scientists hard work to save it.

  The Aldabra Atoll is only a few metres above sea level, in most places, and with the climate crisis happening causing sea levels to rise, it is just a matter of time before something like a Tsunami strikes and decimates the wild population of the region. It has happened before, in Aldabras past, and it will happen again, it is just a question of when?

  The unfortunate consequences of such devastation on Aldabra could easily mean that the whole population of the region both wild and captive bred are eradicated. In that case it would be down to animals in collections around the rest of the world to help repopulate. Considering that there has only been breeding achieved in America and very limited recent success in Europe and Asia, it would not be enough to re populate even the smallest of the islands, on Aldabra.

 

Do They Have Any Predators?

  Over the years man, particularly from old fashioned sailors, looking for food, 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 once 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 Norway Rats, cats, dogs and pigs to the island, some were unintentional , but they killed and trampled the young tortoises, and eggs, as well as limited the increase in population.

  An introduced species of goats, who ate the tortoises limited food supply, were eradicated from Aldabra in 2012. At one stage there were thought to be over 600 cats on the Atoll All cats have now been removed from all islands except one and research of a feasibility to eradicate rats has also been done.

  The tortoises shell is like heavily designed armour plate, that protects the 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 could fall down and die in. 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. Large land crabs are another danger for the younger tortoises. These Coconut Crabs and the Norway Rats are the ones that will swarm a dead body to decimate any flesh and keep the island free from disease from rotting corpses.

  The greatest natural danger to the tortoises 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.

   Young Aldabra tortoises and turtles are met with predation by Ibis when they are newly hatched. As the eggs of a clutch tend to hatch around the same time the bird has plenty of choice of victims. It can swallow a hatchling down in 2 or 3 quick movements but will occasionally spit a half dead one back out if it has gorged too much. Once they are in the water, that surround the island, the endangered baby Hawksbill and Green Sea turtles are also at risk from the sharks that live off shore, Aldabra in not a hospitable place to try to survive.

 

As They Are An Island Species, Do They Swim?

  Aldabras are confident in calm water and 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. Their very thin bones, streamlined shells, and their large lungs being located at the top of their Carapace, all act as buoyancy aids. However, 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 exercise for them, but they do seem to enjoy the weightlessness, it must make a nice change, to take the weight off their feet, from hauling their heavy weight around on land.

They are not turtles, even though there is a genetic link from around 50 million years ago when tortoises first emerged from the water to live permanently on land again. So, they do not have the muscle tone for long swims and they would not go deep diving for the fun of it.

  They do not swim in the traditional sense, it is more that they walk through the water, using the same locomotion as they use on land, not doing the breast stroke like a turtle would.   Aldabras will be much more controlled by current and luck, as to their final destination. As they float and drift, their tails and necks help by acting as rudders as they bob along. With no land bridge, between Aldabra and the mainland, or any local island, this long and slow, Oceanic Drift was how a lucky gravid female successfully made it to the Aldabra Atoll in the first place, to begin the population, thousands of years ago. It was likely that she was washed out to sea, from a neighbouring landmass, several hundred kilometres away, probably from one of the other Islands in the Seychelles. The trip would not have been quick because it would be governed by the currents speed, and it is around 1000km from the main Seychelles Island of Mahe to Aldabra.

Female Aldabra tortoises can store viable sperm inside themselves for up to 4 years, allowing them to fertilize eggs, when the conditions are right, even if there is not a male present. This is how one lone female could have begun the Aldabra population.

  At times there are limited food supplies on the island 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. However, some have floated slowly, all of the way, to mainland Africa, after months adrift.

  They first arrived on Aldabra as a giant tortoise. Unlike Galapagos Giant Tortoises, who were a smaller species that arrived on floating debris, unable to survive long distance in the water, and then grew big and evolved to fill the top position available on Galapagos.

  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 quickly, so please do not put any tortoise in deep water, as you could drown it! Also, do not 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 finally battling its way out of the egg, after months of growing, when some well meaning human picked it up and threw it into the water and succeeded 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 predators away, that will benefit both tortoise and turtle species.

 

What Happens When An Aldabra Tortoise Is Washed Off The Island Into The Sea?

   Many times it is not good news for a tortoise to be washed into the water. If they are lucky and are still within the lagoon, then the incoming tide can return them to shore like a piece of flotsam, and no harm done. If the tide is outgoing then they risk being swept out into the vast Indian Ocean. Being at the mercy of the tide and currents, they might be able to turn themselves around but they are not strong enough swimmers to get to safely to shore and they are hundreds of kilometres from another landmass. They cannot fight against the currents and could be dashed, being severely injured or killed, against the outlying rocks or coral reef, as it was dragged, by the tide, from the safety of land.

If the tortoise does make it past the outer reaches of the Atoll, then they face other challenges:

• The intense heat from the sun, with no shade to hide under, will cause them to dehydrate.
• No food available to eat.
• No fresh water available to drink. They cannot drink the salt water that is surrounding them.
• Threat of attack from sharks, could result in severe injury or death.
• Threat of collision with boats, could result in severe injury or death.
• Threat of being poached for food by a, ruthless, passing sailor.
• Exhaustion from being buffeted by the waves, and having to keep their head above water, could result in them drowning.

   It is not often that tortoises are seen out in the open ocean, and there is very few recorded sightings, at sea, so it is more than likely that they perish.

   A long Oceanic Drift like this will be how Aldabra tortoises first, and subsequently, colonised the Atoll over many thousands of years, when they came from the mainland or neighbouring islands. Tortoise of this size would have difficulty clinging to a vegetation type raft for long periods, they are not designed to hug, and hold on for dear life.

   However, in December 2004 an Aldabra tortoise successfully made the return journey, probably from Aldabra, back to the mainland. A female washed up on the shoreline in Kimbiji, 35km south of Daresgalaam in Tanzania, on the East coast of Africa, she was 740km from Aldabra and was Emaciated and had extensive growths of Goose Barnacles on its shell and legs but she was alive and walked ashore, when she was found early in the morning.

   There was no way to know of her exact point of origin, there were several possibilities, she could have come from Changuu Island in the Zanzibar Channel or Curieuse or Fregate Island. It was later thought, that looking at her shell shape it was likely that she had come from Aldabra itself, probably Grand Terre, the Southern Island. It was likely that she was washed away in a stormy monsoon and floated on the South Equatorial Current, with its Westerly monsoon winds, until she was beached in Kimbiji.

   The Westerly current of 1 to 3 knots could have meant a drifting time of 6 to 17 days but possibly 3 weeks. However, the amount of barnacle growth supported the theory that the journey lasted much longer, probably 6 to 7 weeks, or even months, in the sea water.

   When she was first found the female was 77cm long and 74cm wide, over the top and weighed just 25kg. In the following 3 months she gained just 2kg as she began to slowly recover from her big adventure. However, by 2006 she had settle in to her new home and grown to 85.5cm and 83cm wide, over the top, and then weighed 42.5 kg which was an impressive 60% increase in her bodyweight.

   On a personal note, we were surprised how small the tortoise was. Yes, she was large compared to a Hermann tortoise but then you realise that the measurements are for over the Carapace of the shell and not underneath the Plastron. So, for an Aldabra tortoise she was still a juvenile size. For those of you who have seen our tortoises at one of the shows in the UK, and know some of the individuals, she was around Zippys size, who is currently around the 38kg mark and 2ft long. We would have definitely expected it to have been a bigger, sub adult, tortoise to have been able to successfully make such a long trip. It was a very impressive feat, given that she probably lost about a third of her body weight whilst bobbing about in the Indian Ocean, desperately hoping for somewhere to land.

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

What Are The Main Threats To Tortoise In The Wild?

1. Illegal trade taking animals directly from the wild.

2. Habitat loss both from man and natural disasters.

3. Invasive species out competing them for food.

4. Climate change.

5. Plastic Pollution

6. Being eaten by humans

 

   More than half of tortoises in the world are classed as critically endangered or vulnerable, including the Aldabra Giant Tortoise. This will make them of greater interest to unscrupulous collectors and so poaching is an ever present threat.

   Incredibly, numerous real Tortoise shells, from various breeds, are still used in products for sale today, and you can legally buy them on sites such as Amazon, as not every species of tortoise has the same level of legal protection.

   With more humans coming to live in the immediate area, and more visitors potentially being allowed onto Aldabra Island, to raise money for its upkeep, the delicate ecosystem is under greater threat than ever before from an unintentional bio hazard getting ashore. Although biosecuity is taken very seriously, each person and their belongings are thoroughly searched before being allowed ashore and decontaminated if necessary. All soil and dust must be removed from anything to be taken ashore. However, it is still possible to accidentally introduce invasive species, such as the mealy bug, which managed to seriously damage the native vegetation, before controls could be put in place to stop it. Since that incident, improved controls to limit the spread of each plant have been introduced and are in place.

   It is a fact that climate change is causing the current sea level to rise, this effect is poised to drown substantial parts of the Aldabra Atoll, within the lifespan of the tortoises living on Aldabra today. In the next 100 years the projected overall decline of Aldabra Giant Tortoises in the area, from climate change, is a loss of 40-60% of the tortoise population.

   For an idyllic island, the beaches on Aldabra are becoming swamped in items of unsightly plastic rubbish contamination, and it is becoming a very serious threat to their survival. These are over 513 tons of potentially dangerous and harmful items, that have been dumped at sea, both accidentally and intentionally, and then washed ashore by the currant, with most of it coming from Asia. It is greatly interfering with the green turtles nesting sites on Aldabra beaches, they have to literally climb over piles of rubbish to find somewhere to dig and then battle their way back to the sea again, potentially threatening their future generations. Both the turtles, the Aldabra tortoises as well as other animals residents, are at risk from eating some, out of curiosity, or becoming entangled in it and being injured. You can find out more about this problem by searching UNESCO Plastic Odyssey.

 

Is Illegal Wildlife Trade A Serious Problem?

   Yes, globally there is 23 billion dollars a year in illegal wildlife trade, and that it just the stuff that the authorities know about, there will be many more trades that get through undetected.

   Wildlife trade is the 4th most prolific Serious Organised Crime in the world, behind:

1. Drug smuggling and sale.

2. Firearm smuggling and sale.

3. People Smuggling.

 

   Wildlife trade is usually connected to these other aspects of criminality.

   The Aldabra Giant Tortoises that you buy in the UK are captive bred youngsters, so there is no negative impact on the wild population. They are not a traditionally poached animal, smaller breeds of tortoises are easier to move. Plus, to find Aldabra youngsters on the island will be much more difficult and take longer, leading to a greater chance of discovery and interception by the authorities. However, it has been a threat in the past and could be a threat, once more, in the future, if more people move into in the area to live, with the development on neighbouring Assumption Island. People will be closer and the temptation and access will be easier.

 

What Is CITES?

   CITES is the Convention of International Trade in Endangered Species of wild fauna and flora. It is a worldwide institution and there are very few countries that have not signed up to join and agreed to adhere to these standardised regulations, to improve the protection offered to these vulnerable species.

   CITES started in 1975 to protect wild tortoise populations, from being plundered to extinction, by the pet trade. Later European lawmakers added sections a, b and c to elevate the protection. No tortoise species on the CITES list is allowed to be taken from the wild. All of these animals need an appropriate A10 certificate to be traded.

   Unfortunately, not all tortoise species are on the CITES list, Russian Horsfield tortoises are one of these species and the fact that they do not currently required an A10 to be bought, and sold, is one of the reasons these animals are now becoming so popular within the pet trade.

 

How Many Tortoises Die On Aldabra?

   It is difficult to give an accurate figure, as there are too many tortoises to physically go around and count. In previous years scientists have calculated the annual tortoise mortality rates for Malabar Island to be 2.7%, of the 2250, making the figure around 60 per year, of varying sizes.

   The much larger Grand Terre Island has approximately 44 tortoises per sq km, for animals over 60cm carapace length, and 2.9%, or 4260 will die per year, in normal conditions.

   The most common causes of death on Grand Terre is over heating, as they cannot sweat or pant. 79%, or 3365, of Aldabra Tortoises deaths, will die in this manor. Meanwhile, approximately, 937 or 22% will slowly die after falling into a rock crevasse, or a hole, and being unable to get back out.

 

Do Aldabra Tortoises Hunt?

   In the not too distant past the answer would have been a definite no. They are primarily vegetarians, but some would describe them as Opportunistic Omnivores because they would scavenge dead fish washed up on the shoreline or fallen comrades who have died for whatever reason. Food like that is too vital to waste and leave to rot, in the harsh environment they live in. The younger animals would also eat worms and things, millipedes are a very good source of protein for a growing giant tortoise. They will eat scavenged bones and small snails as calcium sources.

   Then, scientists began seeing some tortoises adapting and using Finching as a way of catching food. Finching is when the tortoise stands up tall, with all limbs extended and the tortoise patiently waits, as still as a statue. The local birds come down and start pecking parasites from them, symbiotically benefiting both the tortoise, who stays healthy and the bird, who gets extra food and respite in the shade.

   In this hunting scenario, the tortoise remains standing still and the bird, or sometimes a crab, gratefully takes shelter beneath it, where it is degrees cooler in the shade. Once beneath it the tortoise suddenly drops back to the ground, like a stone, and squishes the bird beneath it, killing it with its weight, and the tortoise gets a free, high protein, meal. This is not something that all tortoises do, it seems to be learned behaviour. but curiously it is something that Galapagos giant tortoises, half way around the other side of the world have also learned how to do. The two types of giant tortoise have learned to do it independently of each other, there is not contact between the two species but it does make you wonder if there is some programming in their genetics that has passed this skill down from their very ancient ancestors from over 5 million years ago, rather like our Neanderthal characteristics, and it is only just coming to light. At that time dinosaur type tortoises were considerably bigger than they are now, with the likes of Megalochelys and Titanochelon roaming around. So, it was highly likely that they would have subsidised their diet with meat, to energise such a huge body, and not just been grazers. Hunting could be in the giant tortoises DNA and they have jut never tapped into the skill before.

   Curiously, in 2020 Aldabras on Fregate Island, which is privately owned eco tourism island, in the Seychelles, were seen actually hunting chicks, of Lesser Noddy and Fairy Tern, which had fallen from their nests. This is physically going after them and, targeting them, and not just getting tired of standing up and accidentally squashing one when you sit back down.

   In 2013, approximately 3000 Aldabra were estimated to live on Fregate Island, with a bias towards the males. Fregate is a 219 hectare area of mostly mixed woodland, which is popular with the tortoises. The 2020 incident was seen at Anse Parc, Video footage of this can be seen on the internet, after it was recorded for the first time. The flightless chick was sitting on a log, and it was killed by the tortoises open mouth and not by the Finching, falling down and squashing the prey method, as had previously been seen, with a few other Aldabra both in the wild, and also on Fregate. The Tortoise was witnessed actively following the retreating chick, as it flapped its wings in panic, before closing its jaws over its victim head and killing it. Taking less than 2 minutes to kill it and then swallow it whole.

   It is thought that observation of other tortoises hunting allows it to become a learned and adapted behaviour by other Aldabra tortoises in the area. This tortoise population is a mixed group of wild introduced, and relocated from captivity animals, as well as wild bred on Fregate Island tortoises, so most are unrelated. It Is unknown which category the hunting tortoises came from.

   These are flightless chicks and thus unable to fly out of danger. A ground nesting species would have parents nearby to defend them, or be more instinctively adept at avoiding predation, by disappearing into undergrowth. But the parents of these fallen chicks do not head to the ground to feed them, they are considered lost.

   The Terns nest in Pisonia Grandis trees between June and September, and high winds blow the chicks out of their nests. There are approximately 265,000 Noddy Terns on the island, and these are the most noted predations, so, the tortoises hunting a few is not going to have major impact on the population. These fallen chicks would naturally be predation by other creatures, such as crabs so, again, bird numbers are not being endangered by rare tortoise predation.

   Other tortoises in the area have been seen making similar attempts, a small number of attacks, predominately by females but not exclusively so, were seen by scientists and observers, in 2022, most taking place early in the morning. The bird would sometimes be caught by a wing tip, when fledgling fluttering its wings, thus unable to escape. Clearly we are witnessing evolution in action.

   Gene traits , such as this hunting, can quickly become typical due to inbreeding within a small population, so it is highly likely that this behaviour will be seen more often in the future, in this, now isolated, new population of Aldabra Giant Tortoises.

 

Do Aldabra Tortoises Migrate?

   On Aldabra, some animals will remain in the same few thousand hectares for their whole life, due to natural blockades. Others are more curious and do roam and migrate to different parts of the island at various times of the year. This is likely due to seasonal changes and availability of food.

   In the wild, tortoises can move half a mile in a day and roughly 5 miles in a season. Although, the average daily distance moved is 190 metres, (about 0.12 miles), although one male has been noted to travel 0.75 miles (1207 metres) in a day.

   They have been known to walk 7 or 8 kilometres, in a direct straight line from one end of an island to the other, and then turn right around and walk a straight line back to where they started. So, the do not always go on a random meandering course, sometimes they have purpose, we might not know what that purpose was but they clearly do!

   Males usually have larger home ranges of around 14 hectares and females around 8 hectares (about 8000 square metres) but she may go elsewhere during breeding and nesting periods, as some will come down from higher ground to lay. Those that do migrate, and move around the area looking for food, water, shelter or a mate, can cover 25km in a few months.

   From personal experience, we know that our Aldabras have different favourite areas of the pen that they like to head to. Some of them are happy to remain in their own field but others are more eager to explore, if we leave the gate open and let them wander into the surrounding areas. A few of them, mainly females will pace up and down behind the gate or even try to push their way out if they know there is fresh grazing out there. Thankfully, most of the time they will head back to bed, of their own accord, in the evening though.

   The Giant tortoises on the Galapagos Islands have been studied by scientists, in relation to their migrating habits, and they will travel over rough terrain looking for food, water, and nesting sites. Every spring the female Eastern Santa Cruz Galapagos Tortoises will migrate, down from the highlands to the lower elevations, which is 10-15 km away (around 6-10 miles), to lay eggs in the soft earth. They will follow ancient winding pathways to return to these nesting sites.

 

Why Do They Migrate?

   On Aldabra, some of the tortoises that live on Grand Terre will migrate to the coast during the rainy season, to benefit from the new fresh growth of vegetation that they do not have access to in the more shaded areas.

   These migrants are often females and the boost in this food intake allows them to lay larger clutch numbers. The disadvantages are that the rainy season is the hottest time of the year and they have to venture out into the open areas, away from the shade, increasing the chance of them overheating. Competition for the limited shade can often lead to the death of smaller tortoises, who are exposed to the sun for too long and overheat.

   Once rainy season is over, and they have cleared most of the vegetation, they will return inland to where there is more shade available. Between June and December females will migrate to arid zones to nest.

   In the wild they will venture further away from the shade to reach the vegetation but then they risk another tortoise nipping in and pinching their shady spot. So when they return there is no where for them to hide. It is a fine balance between benefits and the dangers exposing themselves in this manor, on cloudless days.

   Often at these times they will Brumate, where they just shut their systems down to minimal levels and just sleep away the hottest days, until more favourable temperatures return.

   Over the year, 56% of identified tortoises do not move more than 500m, from their initial marking point, of being first seen by scientists.

 

Are Aldabra Tortoises Territorial?

   Not in our experience, they wont defend a territory, they are communal and like to live in a mixed sized herd and share close contact with others. They do not have territories, they are happy to share with males and females and are not confrontational. Aldabra tortoises lack a sense of personal space and do not claim ownership of food, as a dog does with a bone. Usually, a large male will allow a much smaller, juvenile, Aldabra Tortoise to feed alongside them, without feeling threatened.

   They are an island species, so they seem to have have an inbred tolerance of close proximity, as they were unable to get away from each other, and have limited space in the wild. However, we have noticed that some of them definitely have preferred areas that they like to go back to. Others prefer to travel around and explore, each tortoise is different.

   In contrast Galapagos Giant Tortoises are very different, even though they are also an island species. They not a herd animal, like the Aldabra, and they are strongly territorial, especially adult males. Food is theirs ,and they warn others off by sound, lunges and bites to the air ,and at the intruder. If the intruder does not leave, the next bite could cause them to loose their Nares, the front of their nose. Permission for a rival to be in their space is given by head raising, and eye contact, and much fighting. The most dominant one is the one with the highest positioned head. Both male and female Galapagos tortoises will fight.

 

Have Giant Tortoises Always Been Top Of The Food Chain?

Have There Never Been Predators On Aldabra?

   Certainly in this last colony of Aldabra there have not been any apex predators on the island. Baby tortoises would have predators such as Norway Rats, Coconut Crabs and feral cats but for larger tortoises, other than being targeted by the rats, if there was a shortage of food, they have no predators. Their greatest threat is overheating or falling into a jagged limestone ravine and being unable to get back out.

   In the distant past there used to be an Aldabra crocodile, which would target the tortoises front section as they came near the waters edge, probably to drink. Then it would strike, grabbing for their head or legs, any soft part that it could hold onto because it could not get its mouth around the shell, nor did it have the bite pressure to crush it.

   The Aldabra crocodile Aldabrachampus dilopus lived around Aldabra in the late Pleistocene era, which was about 90-125,000 years ago. It was identified in 2006 from fossil fragments but is known to be a smaller species of crocodile, possibly even a dwarf species, and would not rival modern Nile crocodiles. This would point towards this crocodile being more of a scavenger than a killer. Dead adult giant Aldabra would not be easy to move around and it would be very difficult for anything to break into an adults shell to access the meat there. This would leave them to feed only on the exposed meat of head and limbs, leaving the shell covered body mainly intact.

   Today, tortoise decomposition is mainly achieved by the large coconut crabs, rats, cats, ants and insects descending on the body to feed, and lay eggs for maggots to hatch. They deal with the dead bodies of any animals and keep potential spread of infection under control.

   In the 1700's it was reported that enormous salt water crocodiles lived in the Inner Granitic Islands such as Mahe but there were no reported sightings on Aldabra. However, larger remains have been found, in the mud of the Lagoon, on Aldabra, as well as ancient damage to tortoise bones and shells which would indicate that there was also a nomadic apex crocodylian predator, occasionally, hunting there as well. Scientists have found evidence on Grand Terre of large crocodile bones and larger giant tortoise material for animals larger than tortoises today grow but more investigation is needed. They think that it was nomadic because if it had lived there permanently then the crocodiles would have regularly come ashore to hunt and there would be much more evidence of predation available. More than likely man eradicated it before the crocodiles population had chance to thrive.

 

How Many Islands In The Seychelles Have Tortoises?

   The Seychelles consists of 115 islands, of very varying sizes and heights, which include 42 granite islands and 73 coral islands. They range from being between 480 to 1600km from the East coast of Africa. The majority of Seychelles is coral islands that are relatively low and flat, making them more vulnerable to sea level rises, and not all of them are able to support tortoises living on them.

   Giant tortoises were first notably discovered in the region in the 16th century, however by 1859 there were just 15 islands who had tortoises, from the 23 that had them in the past, with a population of around 500 adults. Most islands tortoise population was extinct by 1840, with only the coral island of Aldabra having over 100,000 tortoises.

   Between 1850 and 1990 efforts were made to reintroduce giant tortoise to 8 islands in the region, however, by this stage 15 other islands no longer had them, for various reasons.

   In 2013 there were tortoises on 9 granitic islands. Transrelocations of tortoises to uninhabited islands, over the years have been successful to varying degrees, including on Round Island. The outer southern islands are doing well in their reintroduction projects.

Eco tourism has become more popular in the area and as such some private island owners have used tortoises presence as a tourist attraction, whilst others have chosen to remain private islands.

 

There are a number of islands that you can go to to see giant tortoise. (ones in bold are popular visitor attractions where you can interact with them)

Moyenne Island

Curieuse Island

La Digue Island

Desroches island

Praslin Island

Cousine Island

North Island

Silhouette Island

Round Island

Mahe Island

Fregate Island

Cerf Island

Bird Island

Aldabra Island

Rodriguez Island

Darros Island

 

Maritius

Madagascar

 

How Are Aldabra Tortoises Actively Being Used As Eco Engineers In The Wild?

   In 2000 researches introduced Aldabras to Ille Aug Aigrettes reserve on Mauritius, where the Ebony forests had been cleared by logging. The tortoises were used to eat and spread Ebony fruits, through their poo to help naturally restore the tropical forest.

   Tortoises do not have teeth, and they do not chew their food, so any seeds the swallow are taken down whole and undamaged. Some seeds, like the Ebony trees, benefit from spending a long time in the tortoises gut. Of the seeds that do not pass through a tortoise only about 2 or 3 percent will germinate but for the seeds that the tortoise has eaten there will be an 80% success rate of germination. 10 years later they had successfully achieved their projects aim and Ebony forests were once more growing.

   Aldabras are ecological engineer and key to many seeds dispersal and they can out graze invasive species, which is why they were put onto Round Island to help restore the islands natural foliage. Their presence changed how the island has evolved. certain species of native plants require going through the tortoises gut in order to germinate. seeds are not damaged through chewing and remain intact during passage through the gut. Tortoises can carry the seeds a long way during their digestion time and this help the plants to expand their range,

   Tortoises on Aldabra eat 20 species of seeds on the island, second only to the blue pigeon which eats 26 varieties. However, unlike some of the bird species which can pass seeds through their body in as little as 5 minutes, so their grouping on the floor is very close to each other. Tortoises move much further before depositing the seed via their poo. Food spends 2 to 3 days in the foregut and 5 to7 days in the hindegut, whereas elephants throughput time can be a day or less.

   The terrestrial hermit crab feeds on the Aldabra droppings. Their dung is dismantled by the crabs , when fresh it is dense and wet and quickly entices a swarm of flies over it. However, in a week it will be half dried with its nutrients soaked back into the thin soil of the landscape or the ancient coral island. Tortoise dung is one of the richest nutrient providers and helps seeds to fertilise and grown in this harsh environment where little can live.

   Their movements across the islands alter the landscape, creating access to habitats for other species. They create open space and let in light for other plants to grow. just by walking, things brush and catch on their shell, to be blown or deposited elsewhere. they mix up and aerate the soil, with their footsteps, to entice more to grow or just by kicking up beetles and invertebrates they provide food for birds to scavenge.

   Plants in the famous Aldabra Tortoise Turf have evolved in direct correlation to how the tortoises feed from it. Flower heads are much lower to the ground so they re not picked off as easily and are given chance to be fertilised by the insects before they re eaten by the tortoises and passed through their gut into a new area.

   The tortoises are hardier, and need less attention, than using domestic animals such as goat, cattle and pigs. They will also target different and more invasive species to transform the environment back to more what it was intended to be.