Radiated Tortoise
Species Description
Radiated tortoises (Astrochelys radiata), or Sokake in Malagasy, are considered to be one the most beautiful species of tortoise in the world. They are native to the dry, spiny forests of southern and southwestern Madagascar but have also been introduced to the nearby islands of Reunion, Mauritius and the Seychelles as pets. The Radiated tortoise gets its name from its unique shell pattern, with yellow lines radiating from the centre of each scute. Apart from a black patch on the tortoise’s head, the rest of its skin is yellow. They can weigh up to 16 kg and easily live over 100 years. The oldest Radiated tortoise, named Tu'i Malila, lived to the age of 188 before passing away in 1965. Males tend to be slightly larger than females, with adult carapace length ranging between 28 - 40 cm in males and 24 - 36 cm in females. The species is considered Critically Endangered on the IUCN red list, largely due to centuries of human exploitation. They were heavily exported during the 18th and 19th century, mostly to Reunion and Mauritius where their meat was considered a delicacy. In recent decades, they have suffered major habitat loss and are collected from the wild for their meat, and for souvenir and exotic pet markets.
Habitat
Radiated tortoises only occur naturally in the southern and southwestern regions of Madagascar, scattered along a narrow band 50-100 km from the coast. They can be found on the high plateaus inland as well as on the coastal dunes. In areas that have been subject to high levels of commercial harvesting, tortoises are either absent or occur at very low abundances. In more remote regions, however, they have persisted at densities of up to 2500 individuals per km2.
Diet
Radiated tortoises are primarily herbivores. They feed mostly on grasses, fruits and succulent plants - their favourite being the Opuntia cactus. They are known to ingest animal matter on occasion, including bone, shell, dried carcass and faeces. They obtain 80-90% of their nutrition through grazing and individuals regularly return to the same patch of grass to feed on. By exerting continuous grazing pressure, they supply themselves with highly nutritious new growth, which they prefer over mature grasses.
Reproduction
Radiated tortoises begin mating at a carapace length of around 31 cm in males, and slightly longer in females. Courtship is preceded by inter-male conflict, whereby rivals will ram one another with the front of their carapaces. The successful male will then initiate the courtship by smelling the female’s hind legs and cloaca. Occasionally, he will lift her up using the front edge of his carapace, preventing her from moving away. He will then mount her from the rear and begin striking the posterior part of his plastron against her carapace. These movements are often accompanied by hisses and grunts. If mating is successful, the female will excavate a hole in the ground, 15-20 cm deep. She will lay anywhere between 1-12 eggs, which will hatch after 5-8 months. Juveniles are approximately 3-4 cm in length upon hatching and tend to be off-white in colour.
Movement & Home Range
Radiated tortoises are diurnal. They are most active early morning (6.30am-10.00am) and late afternoon (3.00pm-6.30pm), though the exact times vary depending on the season. They exhibit particularly high levels of movement during the wet season, between November and April, when herbaceous plants have new growth and food is more abundant. The wet season also coincides with their breeding season, so many individuals are on the move in search of reproductive partners. They are least active during the dry season, which occurs between May and October. During these months, rainfall is low, so the tortoises aestivate to avoid dehydration. Consequently, their monthly home range may vary between a few square metres during the dry season to 11 hectares during the wet season.
Conservation Concerns
Radiated tortoises were once quite abundant in some parts of Madagascar. Often spotted along roadways, they were previously considered a cultural symbol of the south Madagascan region. They have disappeared from almost 40% of their original geographical range and today, they are one of the rarest tortoises species in the world. Not only are they heavily poached for bushmeat and the exotic pet trade, they have also suffered severe habitat loss as a result of deforestation for agriculture and charcoal production. Although they have been protected under Malagasy law since 1961 and listed on the CITES Appendix 1 since 1975, it is estimated that up to 45,000 Radiated tortoises are harvested annually resulting in a population decline of about 80%.
There are two ethnic groups of people within Madagascar, the Mahafaly and Antandroy, whose land overlap with the tortoise’s range. Yet, it is not them who exploit the tortoises as they consider it ‘taboo’ to eat or even touch them. Malagasy villagers from other areas of Madagascar will come to the south, collect the tortoises and sell them to middlemen for as little as $3. They are then shipped to Europe and Asia, where they can be sold for $1000-10,000.
Conservation Efforts
In 2018, a team of biologists, veterinarians and construction experts rescued over 10,900 Radiated tortoises from a wildlife trafficking scheme in Toliara - a town on the southwestern coast of Madagascar. Authorities had received complaints about a pungent smell coming from a two-story house. The inspection revealed thousands of juvenile Radiated tortoises occupying the floors, sinks and bathtubs of the house. They had been left without food or water, and were essentially living in their own waste. Over 500 individuals were already dead at the time of the inspection. The tortoises were taken to a rescue facility run by the NGO, Le Village Des Tortues. With the help of seven other conservation NGOS from across the world, the facility received over 500 pounds of medical and building supplies, in support of the tortoises’ care over the next several months.
Durrell Wildlife Conservation Trust has kept Radiated tortoises at Jersey Zoo in the Channel Islands, since 1976. The four males and two females they received from Madagascar have helped raise awareness about the species’ critical status in the wild. They welcomed their first Radiated tortoise hatchling in 2017. It will be cared for indoors until it is at least 10 years old and approximately the same size as its mother, at which point it will be brought to the main enclosure. Durrell also works with Radiated tortoises in Mauritius as part of an island ecosystem restoration program, where they are used to restore degraded habitats by facilitating key ecological processes such as grazing, seed dispersal and nutrient cycling.
The Turtle Conservancy runs the largest breeding program of Radiated tortoises in North America. Their sanctuary, situated in Ojai, California, has shown great promise, having hatched over 300 Radiated tortoises so far.
The Turtle Survival Alliance Madagascar program has a goal to engage communities to protect tortoises in their native habitat, conserve forests, dissuade poaching, seize and provide long-term care for illegally collected tortoises, infiltrate and dismantle tortoise poaching networks, and return tortoises to the wild where possible. Their efforts aim to protect tortoises from illegal trade and conserve their natural habitat while also empowering local communities to be stewards of their environment.
Additional Facts
During heavy rains, Radiated tortoises will stand as tall as they can and shake their bodies from side to side in an attempt to remove debris and excess water off their shells.
When attacked by a predator (or poacher), the tortoise will retract inside its shell and emit a high-pitched cry in an effort to frighten the predator away.
Their generation time is approximately 42 years.
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