Aldabra giant tortoise (Aldabchelys gigantea)
Species Description
One of two remaining species of giant tortoise on Earth, the Aldabra giant tortoise, is the last extant species of a clade of giant tortoise that inhabited islands of the Western Indian Ocean. They shared a common ancestor with the Galapagos tortoise around 20 million years ago. Origins of the species branch back approximately 17.5 million years to Madagascar from where they dispersed to islands of Seychelles, Mauritius, and Réunion. Although the clade was once widespread across the region, human hunting of tortoises confined the remaining giant tortoises to Aldabra Atoll, from where they get their name.
This true giant species, evolving from island gigantism, is distinguishable from their cousins in Galápagos due to their narrow faces and pronounced domed carapace (top shell). Adult male Aldabra giant tortoises can reach up to 1.3m/4ft and can weigh up to 350kg/770lbs in captivity. Adult females are generally smaller than males to facilitate mating, where the curved plastron (bottom shell) of a male tortoise allows him to mount the female. Aldabra giant tortoise shells are usually light to dark brown in colour, but older females develop smoother shells often with darker colouration due to buffing from mating activity. There is a distinct breeding season where mating attempts are made between November to March, with nesting occurring from February to May after which the hatchlings emerge from September to December.
Famously, Aldabra giant tortoises are known for living extraordinary long lives. The current oldest land animal on Earth is an Aldabra giant tortoises named Jonathon, who is reportedly 190 years old. In the wild, they live to over 100 years old, but aging them molecularly is difficult, and size is not an indicator of age, therefore reports and old photographs are used to guesstimate the age.
Taxonomy
The Aldabra giant tortoise has a chequered taxonomic history was described with the species given nearly 40 different names. In 1812, August Schweigger, a German botanist, named a specimen in the Naturel history Museum of Paris, Testudo gigantea, but the type specimen was misplaced until resurfacing in 2006 when it was found again by Dr Rodger Bour. In the intervening decades, other French biologists had described two species as T.elephantina and T.gigantea, the later becoming the common description for most of 20th century until Dr Bour argued it should be renamed T.elephantina. American turtle biologist, Jack Frazier, strode into the debate, and a giant battle commenced to cement the species name of these fantastic animals. Frazier, promoted gigantea, whilst Bour fond the original type Schweigger specimen, and suggested a new name; ‘dussmieri’. The quarrel continued until 2013 when taxonomists at the International Commission of Zoological Nomenclature convened to put the matter to bed. Finally, the Aldabra giant tortoise had its name; Aldabrachelys gigantea.
Distribution
The natural range of the Aldabra giant tortoise is Aldabra atoll, which has a population of around 100,000 tortoises. In the wild, they are restricted to the atoll, but there are introduced free-roaming herds on other islands in the Seychelles. Most notably, Fregate Island is home to the largest rewilded herd of Aldabra giant tortoises with a population of more than 3000 tortoises. There are numerous successful rewilding projects in Mauritius, where the tortoises have been introduced to replace the extinct Cylindraspis giant tortoise and helping to restore island ecosystems. Being such gentle giants, they make very popular pets and there are many hundreds of Aldabra giant tortoises in zoos and private collections across the world. During the early 20th century, the governor of Seychelles offered giant tortoises as diplomatic gifts, the most famous of which is Jonathon, the oldest living land animal in the world, living on St. Helena. They are widespread across the world due to their demand in the pet trade as one of the most lucrative reptilian pets on the market. However, due to their restricted range in the wild and the threats the wild population faces from climate change, the species is considered ‘Vulnerable’ to extinction on the IUCN Red List.
Habitat
Aldabra giant tortoises are terrestrial and occur in a wide variety of habitats, including scrub forests, mangrove swamps, and coastal dunes and beaches, each with their respective vegetation. The largest populations of tortoises are found on Aldabra atoll in areas of grassland called "platins." Due to thousands of years of heavy grazing, a habitat known as “tortoise turf”, consisting of a variety of herbs and grasses, has developed in certain areas of platin. This area is where the highest density of giant tortoise in the world can be found; approximately 80 tortoises per hectare.
Aldabra Giant Tortoises roaming open grassland habitat – Dennis Hansen
Movement and Home Range
Aldabra giant tortoises on Aldabra atoll show a variety of movement types with some tortoises being relatively sedentary, while other individuals roam widely. Sedentary or ‘resident’ tortoises remain within areas of a few hundred meters, yet tortoises with a tendency to roam exhibit movement paths like the great migrators of the African savannah as they move away from and return to the place of origin at distinct times of year. These movements are linked to seasonal changes in vegetation, where tortoises move to the inner part of the atoll during the driest periods to access water pools and pool vegetation. In keeping with their slow pace of the life, the average daily movement is 190 meters, although one particularly athletic male giant tortoise roamed 1.2km (0.75 miles) in a single day.
Home range size also varies between individuals and regions of the atoll. The sex of the tortoise can influence the home range size, with male giant tortoises having larger (~14ha) home ranges than females (~8ha). However, females may leave their home range to find preferential nesting grounds during the breeding season.
Aldabra giant tortoises show behaviour consistent with other ectotherms that reside in semi-arid climates in that their activity fluctuations with daily temperature. This pattern is characterised by a concentration of activity in the mornings, followed by a long period of rest during the middle and hottest part of the day, with a final short period of activity in the afternoon. Activity is considerably decreased if the temperature is above 32°C for a few hours during the day.
A typical start to the day for a giant tortoise is to spend their time basking in the early morning before starting to graze until the middle part of the day. Prolonged exposure to the sun increases the risk of mortality so tortoises spend long periods of time under shade trees or in pools at this time of day to avoid overheating. During the rainy season activity levels are higher as there is more water and food available compared to the dry season.
Aldabra giant tortoises resting in the shade - where they spend most of their time - Rich Baxter
Diet
Though they primarily feed on plants, Aldabra Giant Tortoises are opportunistic in their diet. Besides grazing on herbs and grasses, they have been observed to consume small invertebrates and carrion (including dead turtles and tortoises). Due to their large numbers and heavy grazing pressure on vegetation, giant tortoises influence the vegetation structure and dynamics in their habitat by creating large swathes of open grassland called “Tortoise Turf.” The turf consists of grasses and herbs, and is a favored habitat for foraging. Constant grazing by the tortoises keeps the turf short, potentially increasing the amount of new growth, which is less tough to eat and more nutritious. During the dry season, as the turf dries out, fallen leaves from shrubs and trees become the preferred food type. Juvenile tortoises feed on small fleshy herbs found in crevices and holes in limestone rock formations.
Aldabra Giant Tortoises are vital components of seed dispersal on islands. Due to their large gape size, they can swallow large fruit whole, meaning that fruit seeds are not damaged by chewing and remain intact during the passage through the gut. The average gut passage time is between 17 to 21 days after which the undamaged seeds emerge in a pile of fertile dung ready to germinate. During this time, giant tortoises can easily move several kilometers thus potentially dispersing the seeds wide and depositing them away from parent trees.
Little fresh water is available in the tortoises’ natural habitat, and they must obtain most of their water from food sources. However, they have an adaptation in the nasal cavity that allows them to ‘snort’ water up through their noses. This allows them to access very shallow pools of water collected on rock surfaces, or even on large leaves. The Aldabra Giant Tortoises can survive for long periods of time without fresh water.
Aldabra giant tortoises grazing on tortoise turf - Rich Baxter
Reproduction
Sexual maturity is determined by size rather than by age; however, Aldabra Giant Tortoises appear to be mature at around 25 years of age or when they reach approximately half their full-grown size in the wild. Mating occurs from December to May. Females lay clutches of 9 to 25 rubbery, 2-inch diameter eggs in a shallow, dry nest from April to August, with incubation lasting around 110 days. The sex of a tortoise is determined by the average substrate temperature during incubation; female hatchlings emerge from the warmer top of the nest, and male hatchlings emerge from the bottom of the nest where the temperatures are slightly cooler. Once a female has laid her eggs there is no further parental involvement and the young hatch as 8cm (3-inch) long tortoises and dig out of the nest on their own. Females can produce a second clutch within the same breeding season if in good body condition.
Mating attempts only occur when tortoises are active, during early morning or late evening. Mounting results from casual encounters, although there is evidence that females produce a musk attracting males during the breeding period. Over this period, more mating attempts occur than the rest of the year. The initiation of a mating attempt begins with a male approaching a female and climbing onto her back with his neck fully extended. Once in this position, he pushes off his back feet and thrusts forward emitting a loud bellow with each thrust performed. The female will often respond to the male’s mount by trying to walk away or by dropping her rear into the ground, preventing access for the male. Males appear to be promiscuous in their selection of prospective partners, not all of which are necessarily female. Most mating attempts are not successful.
Ecosystem Engineers
The ecosystem impacts of giant tortoises on islands are potentially on par with the likes of elephants and buffalo in continental ecosystems as drivers of ecosystem structure and function. Supporting this comparison is the fact that on Aldabra their biomass is greater than the biomass of large mammalian herbivores on the African savannah. Moreover, they can access most habitats on the atoll (only areas of deeply recessed channels and ‘fissured limestone’ prevent movement between regions). Since tortoise activity and behavior are crucial for many ecosystem processes on oceanic islands, they can be considered ecosystem engineers. Specifically, their impacts are crucial for the following reasons:
Tortoises can facilitate seed dispersal and germination; engineering impacts of a species are tied to the length of time a population has been present in the ecosystem. Aldabra has been colonized by giant tortoises two or three times in the last 400,000 years.
They affect the distribution of food supply for other animals (for example, hermit crabs feeding on tortoise dung).
As the main herbivore they are essential for the dispersal and cycling of nutrients through consumption of plant material and soil turnover.
Overall, the tortoises provide critical ecosystem functions that encompass fundamental ecological needs of the community of organisms on Aldabra. Obtaining a detailed understanding of the movement patterns and activity of the tortoises is a key step in understanding how they influence the distribution of vegetation and how they maintain ecosystem functions on islands. These abilities to enhance ecosystem functions makes the Aldabra Giant Tortoise incredibly effective in habitat or island restoration projects.
Aldabra giant tortoise browsing on the Aldabra atoll - Dennis Hansen
Conservation Concerns
Giant tortoises were common on the islands of the western Indian Ocean with no significant predators or competitors for food until the 1600s when increasing numbers of explorers and settlers visited the Seychelles, Mauritius, and Reunion, and removed or killed them in large numbers. Due to their abilities to survive without food and water for long periods of time, giant tortoises were easily caught and stored on ship as food. By 1840 the only giant tortoises surviving in the wild in the region were on Aldabra atoll. Aldabra tortoises were saved, in part, by appeals from prominent scientists of the time including Charles Darwin and Richard Owen, advocated for tortoises to be shipped from Aldabra to Mauritius where ancestors of those translocated tortoises remain in free roaming herds on Mauritian islands. Today the UNESCO World Heritage Site, Aldabra atoll is home to around 100,000 Aldabra giant tortoises. Captive breeding programs for conservation and rewilding projects make use of the tortoises to replace extinct giant tortoises are active in Mauritius and Rodrigues. Aldabra giant tortoises are incredibly popular pets around the world, and international trade in the species is controlled by CITES (Convention of International Trade in Endangered Species of Wild Fauna and Flora). The Seychelles government has regulations in place to control the trade from captive populations, but trade in wild caught tortoises is prohibited.
On Aldabra, increasing drought and higher temperatures are already serious issues, and current sea level rise is poised to drown large parts of the atoll within the lifespan of some of the tortoises living on Aldabra today. Action is urgently required to mitigate the potential loss of the wild Aldabra giant tortoise population. Work has already begun translocating tortoises to islands with higher elevations to protect the species and rewild islands missing giant tortoises.
The Indian Ocean Tortoise Alliance (IOTA) seeks to drive and support the rewilding of these empty islands with the Aldabra giant tortoise and strengthen conservation initiatives of this tortoise in the wild. Working with local and international partners, IOTA strives for a future where Aldabra giant tortoises and their role in island ecosystems is not only protected but thrives. IOTA‘s work will accelerate, support, and further develop tortoise rewilding as a spearhead of island restoration in the region.