Aldabra Giant Tortoise

Species Description

One of two remaining species of giant tortoise on Earth, the Aldabra Giant Tortoise, is the last extant species of a group of giant tortoise that inhabited islands of the Western Indian Ocean.  They shared a common ancestor with the Galapagos Tortoise around 25-30 million years ago.  Origins of the species branch back approximately 17.5 million years to Madagascar from where they dispersed to islands of the Seychelles.  Meanwhile, Cylindraspis, another more ancient group of giant tortoises, had colonized the Mascarene Islands.  Both groups of giant tortoises were once widespread across the region, but human hunting of tortoises led to the extinction of all, except for the giant tortoises on the Aldabra Atoll, from where they get their name.  

This truly giant species, a classic case of so-called island gigantism, is distinguishable from their cousins in Galápagos by their narrow faces and pronounced domed carapace (top shell).  Adult male Aldabra Giant Tortoises can reach up to 1.3m (4ft) in length and can weigh up to 350kg (770lbs) in captivity.  Adult females are generally smaller than males to facilitate mating, where the concave plastron (bottom shell) of a male tortoise allows him to more easily mount the female.  Aldabra Giant Tortoise shells are usually light to dark brown in color, but older females develop smoother shells often with darker coloration due to buffing from mating activity.  In the wild, there is a distinct breeding season with mating attempts between November to March, and nesting occurring from February to May, after which the hatchlings emerge from September to December.  

Famously, Aldabra Giant Tortoises are known for living extraordinarily long lives.  The current oldest land animal on Earth is an Aldabra Giant Tortoise named Jonathon, who is reportedly 190 years old.  In the wild, they can easily live to over 100 years old, but aging them is notoriously difficult, with size not being a good indicator of age.  Most reported old ages are based on written sources or old photographs to guesstimate the age.

Taxonomy

The Aldabra Giant Tortoise has a checkered taxonomic history that was described with the species being given nearly 40 different names.  In 1812, August Schweigger, a German botanist, named a specimen in the Natural 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 latter 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 in 2006 by proposing a specimen from Aldabra as the neotype, and fixing the species name as Aldabrachelys gigantea.  A giant battle between different camps of taxonomists and conservation biologists lasted until 2013, when the International Commission of Zoological Nomenclature convened and finally put the matter to bed by cementing Aldabrachelys gigantea as the name for the Aldabra Giant Tortoise.

Distribution

The current wild, natural of the Aldabra Giant Tortoise is restricted to the Aldabra atoll, which has a population of around 100,000 tortoises.  There are, however, 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, Rodrigues, and Madagascar, where the tortoises have been introduced to replace the extinct Cylindraspis and Aldabrachelys Giant Tortoises, to help to restore island ecosystems.  Being such gentle giants, they make very popular pets and there are likely several thousand 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 the aforementioned Jonathon, the oldest living land animal in the world, living on St. Helena.  The Aldabra Giant Tortoise is thus 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 classified as ‘Vulnerable’ to extinction on the IUCN Red List.

Habitat

Aldabra Giant Tortoises are terrestrial and occur in a wide variety of habitats in the wild, including scrub forests, mangrove swamps, and coastal dunes and beaches, each with a different vegetation.  In rewilded herds on other islands they also live in dense forests and can be found all the way to the top of granitic hills and small mountains.  On Aldabra, the largest populations of tortoises are found on large, flat areas of grassland and mixed scrub.  The open grasslands are the result of the tortoises’ heavy grazing, and form a habitat known as “tortoise turf,” consisting of a variety of herbs and grasses.  These areas are where the highest density of giant tortoises in the world can be found; approximately 80 tortoises per hectare – a vertebrate biomass higher than that of large mammals in many African national parks.

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 thousand square meters, yet other tortoises have a tendency to roam, exhibiting movement behavior like the great migrators of the African savannah as they move between areas several kilometers apart at distinct times of the 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 and vegetation in pools.  In keeping with their slow pace of the life, the average daily distance moved is 190 meters, although one particularly athletic male giant tortoise covered 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 its home range size, with male giant tortoises having larger (~14ha) home ranges than females (~8ha).  However, females may leave their core home range to find preferential nesting grounds during the breeding season.  

Aldabra Giant Tortoises show behavior consistent with other ectotherms that reside in semi-arid climates, in that their activity fluctuates with daily temperature.  They are highly active 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 on warm days whenever the temperature is above 32°C (89.6F) in the shade.  

A typical start to the day for a giant tortoise is to spend their time basking in the early morning sun, then 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 to avoid overheating.  During the rainy season, activity levels are higher as there is more water and food available compared to the dry season.

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.

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.