Radiated Tortoise

Scientific Classification

Class Reptilia

· Modern reptiles include crocodilians, turtles, tortoises, lizards, snakes, tuataras, and birds. Although there are some exceptions, non-avian reptiles share characteristics including scales, shields, or plates, and claws on their toes (if they have toes). Young are miniature versions of their adults, but may vary in color and pattern. Non-avian reptiles are ectotherms, meaning they rely on their environment for their body temperature.1

Order Testudines

· The order Testudines contains all modern and extinct turtles, tortoises, and terrapins. Turtles are set apart from all other vertebrates by the presence of a shell. Shells are not exoskeletons, rather, they are modifications to the ribcage and vertebrae. Modern turtles do not have teeth, although some extinct ancestors did. Turtles and tortoises go back 220 million years in the fossil record.2,16

Radiated tortoise by Stephen Nelson

Family Testudinidae

· Members of testudinidae are land dwelling turtles, often referred to as tortoises. Tortoises can be found on every continent except for Australia and Antarctica. Tortoises typically have elephantine legs and a domed shell, although this is not always the case. There are about 15 genera in the Testudinidae family and over 50 species, although some are already extinct.2,3

Genus Astrochelys

· There are just two species in the genus Astrochelys: the radiated tortoise and the ploughshare tortoise. Both are endemic to Madagascar and are the two largest species of tortoise that live on the island. These species likely diverged from a single species 2.5 million years ago.1,4

Species radiata

· The word radiata refers to the patterning on the shell, where yellow markings radiate from the center of the scute (shell scale). There are no recognized subspecies of radiated tortoise. Radiated tortoises are referred to as sokake in their native country of Madasgascar.4,8

Distribution and Habitat

Distribution

· Radiated tortoises are found on the south and southwest coast of Madagascar. They are endemic to the country, which means they can only be found on that island. While 15 years ago, they could be found as far north as Tulear, their range has contracted due to poaching. Now this species can only be found as far north as Tsimanampetsotsa National Park and as far east as Ambovombe.1,2,4

Core range of Radiated tortoise in Southern Madagascar in Leuteritz et al. 2005. The range has since contracted.

Habitat

· Radiated tortoises are found in xerophytic spiny forest habitat with low irregular annual rainfall. These are narrow bands of forest, only 50-100 km wide. They can also be found on inland plateaus and sandy dunes close to the coast.1,10

· The vegetation in the spiny forest habitat varies widely due to differences in soil types, however, it is dominated by species from the Didiereaceae and Euphorbiaceae families, which are well adapted to arid climates.4,10

· This climate experiences a wet and dry season, with most of the rainfall occurring between November and April, with January and February being the wettest months of the year. The annual temperature ranges from 59 to 91 degrees.4

Ifaty Madagascar Spiny Forest by Global Travel Press

· To help regulated body temperature, tortoise prefer partially shaded habitat, with leaf litter.6

Migration

· Radiated tortoises do not migrate seasonally and stick to a relatively limited habitat range.

Population

· As of 2019, there were approximately 3 million radiated tortoises living wild in Madagascar. This represents a 75% decline since the year 2000. While it is difficult to determine exactly how many tortoises are taken from the wild every year for bushmeat and the pet trade, the population trend suggests 500,000 are poached every year.8,22,24

Physical Characteristics

Size

· Female radiated tortoises typically range from 9.5 to 14 inches in length from each end of their carapace, the top of the shell. Males tend to be larger than females, ranging from 11 to 15 inches. Radiated tortoises may weigh up to 35 lbs.1,10,22

Shell

· Like many tortoise species, radiated tortoises have a domed shell. Radiated tortoises have distinctive yellow markings that “radiate” from the center of each scute. Individual patterns vary widely across the species and the lines can merge into yellow blobs at the center of the scutes as the tortoises age.10,13,22

· Shells are composed of two main parts. The top part of the shell is called the carapace. The bottom part is called the plastron. The shell protects the soft tissues of the body and allows the tortoise to pull its legs and head inside when threatened.13

· Shells are extensions of the ribcage. Early turtle ancestors display widened ribs that eventually fused into a shell. The vertebrae are fused inside the top of the shell. Therefore, the shell is not a shelter that the tortoise can leave, but a part of the body. Shells grow with the tortoise as they age.13,22

Beautifully patterned shells by BBC Nature

· Shells are made up of 59 to 61 bones and are covered by plates called scutes, which are made of keratin. Between the bones and keratin scutes is a layer of nerves and blood vessels.16,21

· The plastron of a tortoise can often indicate the sex of the individual. Males have concave plastrons that allow them to mount the females during mating, while females have a flat plastron.8

· Shells are used as protection from predators. When a tortoise is threatened, it will pull its head and legs into its shell.16

· Radiated tortoise shells are typically smooth, with small ridges called annuli on each scute, indicating growth. In human care, scutes can sometimes appear raised, which is a deformation attributed to improper environmental conditions and diet.21,22

Anatomy

· Radiated tortoises have a three-chambered heart including left and right atria, and a single ventricle.18

· Radiated tortoises shed the outer layer of their skin, although not in one piece as in snakes. Frequency of shedding is influenced by age, environmental conditions, hormones, damage, and disease.18,21

· The length of a radiated tortoise’s tail can help identify males from females. Males have longer tails than females.21

· Radiated tortoises have elephantine feet without individual toes.2

Thermoregulation

· Radiated tortoises, like other reptiles, are ectotherms (cold-blooded). They do not have adaptations that help them retain heat such as feathers, fur, or fat reserves. The surroundings of a tortoise influence its body temperature.18

· Radiated tortoises alter their behavior and choose specific microclimates in order to keep their body temperature lower than the ambient temperature in their desert climate. During the heat of the day, radiated tortoises remain inactive or in burrows, and typically forage in the afternoon.6

Dentition and Digestion

· Tortoises do not have any teeth, but instead have a beak with a hard, sharpened edge. This beak continuously grows and is worn down through feeding. The beak can become overgrown if conditions are not right and may need to be trimmed.13,16,21

· Like other reptiles, tortoises have slower metabolisms than mammals.18

· Tortoises excrete waste through their cloaca, a single cavity through which the tortoise also lays eggs and mates.17,21

Senses

Hearing

· Tortoises do not possess external ears, but have tympanic membranes. The eardrum is located behind a scale just behind the mouth. Tortoises respond mostly to low-frequency noises.17,21

Eyesight

· Tortoises have upper and lower eyelids and a nictitating membrane, which helps lubricate the eye and clear it of debris. Tortoises possess a limited ability to retract their eyeball into the socket.18,21

· Tortoises possess both rods and cones and are able to detect color.18,21

· Tortoises have round pupils, related to the fact that they are more active during the day. The eyes are black.18,21

Tactile

· Turtles and tortoises feel pressure and pain through their shells, similar to how we feel through our fingernails.16

· Though the skin of turtles and tortoises looks thick and tough, it is sensitive to the touch. Some tortoises in human care enjoy being touched and scratched.16

Taste

· Tortoises have a greater number of taste buds than most other reptiles, indicating that they can taste their food items. They are not, however, picky eaters in their native habitat.19

Smell

· Smell is a very important sense for tortoises. It is used during mating and may help tortoises find their way back to familiar territory.14

· Tortoises possess a Jacobson’s organ. As air is pulled in through the nose, it detect scent particles.21

Behavior

Sociality

· Radiated tortoises do not have a social structure, but occur at relatively high densities across their limited range. This density was previously 2522 tortoises per square kilometer. In 2012, it was reported at a much lower 420 tortoises per kilometer.8

· Male ranges tend to be larger than female ranges, which suggests a promiscuous breeding systems in which males’ ranges overlap female ranges.9

· One study suggested that males are the dispersing sex, while females remain close to their natal grounds. Because she is familiar with the territory, female tortoises may be able to attain a larger body size which can result in larger and more plentiful eggs.11

· Radiated tortoises show some site fidelity, meaning that they prefer a particular range and may be found there year after year.22

· Radiated tortoises will sometimes congregate at rest and shelter sites.9

Activity Patterns

· Radiated tortoises are considered crepuscular, which means they are active at dawn and dusk.8

· Radiated tortoises tend to be more active during the warm, rainy season, when their food is more readily available. Adults will continue some activity through the dry season, but juveniles typically become dormant during this time.22

Locomotion and Travel

· When a radiated tortoise travels, it lifts its plastron off of the ground and uses its elephantine feet to move over sandy and rocky terrain.2

· Over the course of a month, one study found that tortoises might travel through a range measuring as large as 27 acres. They may, however, spend several days inactive in a burrow and some individuals may stay within a few meters of one location, mostly under a tree, for up to a month.9

Sleep

· Radiated tortoises rest in burrows or under shelter during the heat of the day. Several tortoises might share the same rest site.9

Interactions with Other Species

· Radiated tortoises share the same range as Northern spider tortoises and have similar patterns of activity. They are sometimes found resting together.9

· Madagascar buzzards, Indian civets, and snakes are predators of young tortoises.22

Food and Eating Habits

Food Preferences

· Radiated tortoises are mainly herbivores, feeding on low-lying vegetation, dead leaves, and fallen fruits. One study reported that 91% of their diet is composed of leaves, 5% flowers, and 4% fruits, from 109 different plant species. Other studies have suggested that their main diet is composed of grasses.1,4,9,22

· Radiated tortoises will opportunistically feed on carrion, animal bone, and shells. This may provide tortoises with calcium and other nutrients.4,22

· Radiated tortoises have shown preference for food items with higher contents of digestible energy during the dry season. This includes a preference for new growth over old growth as it provides higher protein and lower fiber.9,22

· Radiated tortoises have been known to eat invasive Opuntia (prickly pear) species.1

Methods of Collecting and Eating Food

· During the wet season, radiated tortoises forage in the early morning before 10am. They then rest in the shade during the heat of the day and resume foraging after 6pm.9

Water Intake

· While radiated tortoises get much of their water from the food they eat, they will drink from pools of water collected in rock depressions as well.1,4

· Radiated tortoises are able to drink through their nostrils, which may help them exploit water sources from shallow pools.22

Reproduction

Sexual Maturity

· Male radiated tortoises reach sexual maturity around 12 inches of length. Females may not breed until they are a few inches longer. This may take 16 to 20 years.14

· Before sexual maturity, males and females can be difficult to distinguish from each other. At maturity, males tend to have a concave plastron, versus a flat one in females. Males also have longer tails.21

Mating Activity

· Males occupy a larger range than females, which helps them find more females to mate with.11

· Males often circle, chase, and butt shells with females before mating. Receptive females will lift the back of her shell by extending her legs, allowing the male to mount her and slide his tail beneath her. Thuds and hisses can be heard during copulation.4

· Females are promiscuous and mate with multiple males. They may also have the ability to store sperm from year to year. This can result in multiple parentage for her offspring from the same clutch.11

· 82% of mature females breed in an average year.1

Gestation and Birth

· Larger female radiated tortoises are more likely to lay larger eggs in larger clutches than their smaller counterparts are. They are also more likely to lay multiple clutches of eggs. Larger eggs contain more egg yolk, which helps the baby radiated tortoise survive incubation.12

· Females dig a nest during the wet season, when the earth is softer. She often travels to the edge of her range to lay her eggs, digging a flask shaped nest approximately 6 inches deep with her back feet.2,4

· Radiated tortoises may lay as many as 12 eggs in one clutch, but average 1-5 eggs per clutch in the wild. Eggs are spherical and approximately 1.5 inches in diameter with a soft shell. They may lay up to 3 clutches a year, but this is highly dependent on resource availability and most tortoises do not lay multiple clutches in a single year in the wild.10,15

· Once the eggs are laid, they incubate for 263-342 days, a wide range that seems to be at least partially dependent upon temperature. Some eggs have reportedly incubated in 71 days.4,10,15

· Incubation temperature determine the sex of offspring, rather than genetics. Temperatures lower than 82-84 degrees produces male offspring, while females incubate at higher temperatures.17

Top: Radiated tortoise laying eggs in a nest by Garden State Tortoise.

Bottom: 1.5 month old radiated tortoises at the Indianapolis Zoo by Carla Knapp.

· Radiated tortoises have an egg-tooth that helps them break through the shell of the egg and emerge.16

Care of Young, Development, and Independence

· Radiated tortoises receive no parental care. They are independent from when they hatch out of the egg.12

· Radiated tortoise hatch out as miniature version of adults. Their shells begin soft, but hardens as the tortoise ages. Rings of keratin, called annuli, are added to the rim of each scute. While the annuli may or may not correspond with age, as the tortoise grows, by 15 or 16 years old the annuli become too thin to count. This correspond with a slowing in growth with maturity.22

· Radiated tortoises do not reach mature size until there are between 16 and 20 years old. At that time, secondary sex characteristics like plastron shape and tail length can help visually indicate sex.4

Longevity and Mortality

In the Wild

· Radiated tortoises are one of the longest-lived tortoise species with some individuals surviving past 100 years.4

· Ridges on the scutes of radiated tortoises are not good measures of age. The lines on the shells of older individuals may be worn down so that the yellow appears as blotches.22

In Human Care

· Radiated tortoises likely live longer on average in human care as they do not have natural threats like predators and are provided a consistent diet and health care. The oldest reported radiated tortoise was Tu’i Malila, who was presented by Captain Cook to the king of Tonga in 1777. She was estimated to be 189 years old when she passed away in 1966, although some sources consider this report unreliable.4,20

Conservation

Importance in the Ecosystem

· Radiated tortoises stick to a relatively small range, grazing on grasses and keeping them short. They may also play a role in reducing invasive Opuntia (Prickly Pear) species which are spread from zebu livestock.1,23

Cultural Significance

· Radiated tortoises have long been considered a symbol of southern Madagascar. The Mahafaly and Antandroy peoples in the south have traditionally held a fady (taboo) against touching the radiated tortoise. This taboo has helped protect this species in certain parts of their range.7,9

Conservation Status

· The IUCN Redlist lists the radiated tortoise as critically endangered. Some experts predict that this species will go extinct in the next twenty years.1

Threats

· Habitat Loss: The conversion of land from natural forests and grasslands has impacted the availability of viable habitat for radiated tortoises. Slash and burn agriculture, livestock grazing, and charcoal production have all reduced the spiny forest habitat. Additionally, invasive species have encroached on the spiny forest, threatening the main tortoise diet of native vegetation. The radiated tortoise has disappeared from over 65% of its historic range.1,22

· Collection for Meat: Radiated tortoises are killed for food in their native range by Vezo and Antanosy peoples. This species is now considered a delicacy at restaurants and during holidays in their native range and far north in the capital. Adult radiated tortoises are targeted for this trade as they provide more meat than juveniles do. While this trade is difficult to track, one study found almost 2,000 discarded carapaces in dumps in just one city over a four-year period. Poachers often target adult female tortoises because their meat is fattier and they may contain eggs. This not only skews the sex ratio, but also leaves the smaller females, who produce smaller eggs with lower rates of survival in the population.7,8,9,11

· Collection for the Pet Trade: Radiated tortoises are highly desired in the pet trade. Juveniles are targeted for this trade, as they are easier to conceal during travel. This species is protected and trade is illegal. Between the collection for meat and for the pet trade, the number of tortoises taken from the wild is likely up to 500,000 per year.8,22,24

Conservation Efforts

· Protected Status: Radiated tortoises are a protected species in Madagascar and are listed under Appendix I of CITES, which affords them the highest level of protection. Some spiny forest habitat lies in protected areas. Unfortunately, weak and inefficient enforcement, political instability, and extreme poverty continues to undermine conservation efforts.1,7

· Community-Based Conservation: Because much of the national enforcement for wildlife trafficking comes from the capital of Antananarivo in the north of the country, there is not effective enforcement of anti-poaching laws in the south of the country. The community around Antsakoamasy have a strong fady against harming the tortoises, as they are seen as an embodiment of their ancestors. The TSA engaged with these communities. By building a primary school to show their commitment towards the welfare of the community and the value in protecting tortoises, the Turtle Survival Alliance has established an example for Malagasy people about the rewards of conservation. 22

· Rescue, Care, and Release: The Turtle Survival Alliance provides care for over 24,000 confiscated radiated tortoises in Madagascar. After conducting a survey to determine suitable and safe sites for release, the TSA plans to begin reintroductions to the wild in 2020.

At AZA Institutions

· Radiated tortoises have one of the longest running Special Survival Plan of any reptile. They are considered a Green Level SSP and over 300 individual tortoises live in AZA institutions.

10,000 tortoises were found in a house. Photo by the Turtle Survival Alliance.

· In 2018, 10,000 radiated tortoises were found in a house in Madagascar, destined for the pet trade. AZA facilities answered the Turtle Survival Alliance’s call to action, sending dozens of animal care professionals to Madagascar to help with the triage effort. Using their experience gained at their institutions, they were able to help provide the specialized care needed in the rescue effort and helped expand the TSA’s facilities to cope with thousands of tortoises.

· The Radiated Tortoises SAFE program was established in 2017.

What Guests Can Do

· Do Your Research Before Buying a Pet. The pet trade is a major threat to many species of tortoise throughout the world. Before buying a pet turtle or tortoise, do your research about the care of the species and the origin of the individual you are purchasing. Buying a captive bred individual, or adopting a rescue tortoise, can help drive down the demand for wild caught individuals. Make sure that a tortoise is the right pet for you and that you can care for its needs throughout its life.

· Support AZA Institutions: Attending and donating to zoos and aquariums supports conservation. AZA institutions not only provide exceptional for care animals living on their grounds, but also support the protection of animals out in the wild. AZA institutions work with partners like the Turtle Survival Alliance, providing critical funding for rescue centers in Madagascar and sending experts in times of need. In 2018, after the Malagasy government confiscated 10,000 poached tortoises, 45 AZA institutions sent animal care and veterinary professionals to Madagascar to assist the TSA in caring for the animals and expanding housing to accommodate them. We are only able to accomplish this because of the support of visitors and donors.

Resources:

1. Leuteritz, T. & Rioux Paquette, S. (Madagascar Tortoise and Freshwater Turtle Red List Workshop) 2008. Astrochelys radiata. The IUCN Red List of Threatened Species 2008: e.T9014A12950491. http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T9014A12950491.en. Downloaded on 30 October 2019.

2. Pecor, K. 2003. "Testudinidae" (On-line), Animal Diversity Web. Accessed November 01, 2019 at https://animaldiversity.org/accounts/Testudinidae/

3. https://www.britannica.com/animal/tortoise

4. Radiated tortoise. The Orianne Society. Retrieved from http://www.oriannesociety.org/sites/default/files/fact-sheets/Radiated%20Tortoise%20Fact%20Sheet.pdf

5. https://www.youtube.com/watch?v=fLjXIFvafSE

6. Ronto, W., Rakotondravony, D, and J. Ganzhorn. (2013). Thermoregulation of Radiated tortoises in an altered landscape in Madagascar. http://www.tropentag.de/2013/abstracts/links/Ronto_ZTZAbXhm.pdf

7. Manjoazy, T, Razafimanahaka, JH, Ronto, W, Randrianavelona, R, Ganzhorn, JU, and RKB Jenkins. (2016). The supply of illegal tortoise meat to Toliara City, south-western Madagascar. Oryx. International Flora and Fauna. Retrieved from https://www.madagasikara-voakajy.org/71-the-supply-of-illegal-tortoise-meat-to-toliara-city-south-western-madagascar/file.

8. Rafeliarisoa, TH, Walker, RCJ, and EE Louis. (2013). Decline in the range and population density of radiated tortoises. Chelonian Research Monograph. Retrieved from http://www.chelonian.org/wp-content/uploads/file/CRM%206/15-Rafeliarisoa&al.pdf

9. Rasoma, RVJ, Raselimanana, AP, Ratovonamana, YR, and JU Ganzhorn. (2013). Habitat Use and Diet of astrochelys radiate in the Subarid Zone of southern Madagascar. Chelonian Research Foundation. http://www.sulama.de/files/Rasoma_et_al__2013.pdf

10. https://portals.iucn.org/library/sites/library/files/documents/SSC-OP-005.pdf

11. Paquette, SR, Louis, EE, and FJ Lapointe. (2010). Microsatellite analyses provide evidence of male-biased dispersal in the radiated tortoise astrochelys radiata (Chelonia: Testudiniade). Journal of Heredity. Retrieved from https://academic.oup.com/jhered/article/101/4/403/906192

12. Hammer, JM. (2013). Do bigger females produce bigger eggs? The influence of female body mass on egg mass in astrochelys radiata. Turtles on the Brink in Madagascar. Retrieved from http://www.chelonian.org/wp-content/uploads/file/CRM%206/20-Hammer.pdf

13. Arthur, Alex. (1989). Shell. Alfred A Knoff: New York, NY.

14. Egeler, J. 2000. "Astrochelys radiata" (On-line), Animal Diversity Web. Accessed November 01, 2019 at https://animaldiversity.org/accounts/Astrochelys_radiata/

15. Leuteritz, TEJ, and Ravolanaivo, R. (2005). Reproductive ecology and egg production of the radiated tortoise (Geochelone radiata) in southern Madagascar. African Zoology. Retrieved from https://www.tandfonline.com/doi/pdf/10.1080/15627020.2005.11407322?needAccess=true

16. San Diego Zoo. Turtles and Tortoises.

17. Kuchling, G, Goode, EV, and Praschag, P. (2013). Endoscopic imaging of gonads, sex ratio, and temperature-dependent sex determination in juvenile captive-bred radiated tortoises, astrochelys radiate. Turtles on the Brink in Madagascar. Retrieved on http://www.chelonian.org/wp-content/uploads/file/CRM%206/19-Kuchling&al.pdf

18. Tristan, T and Hannon, D. (2015). ARAV Master Classes. ExoticsCon 2015 Main Conference Proceedings. Retrieved from https://cdn.ymaws.com/members.arav.org/resource/resmgr/Files/Proceedings_2015/Main_Conference/ch23.pdf.

19. Heiss, E, Natchev, N, Schwaha, T, and Salaberger, D. (2011). Oropharyngeal Morphology in the Basal Tortoise Manouria emy emys with comments on form and function of the testudinid tongue. Journal of Morphology. Vol 272(10): 1217-29. Retrieved from https://www.researchgate.net/publication/51152102_Oropharyngeal_Morphology_in_the_Basal_Tortoise_Manouria_emys_emys_With_Comments_on_Form_and_Function_of_the_Testudinid_Tongue.

20. Stanford, Craig B. (2010). The Last Tortoise: A Tale of Extinction in Our Lifetime. Harvard University Press. Retrieved from https://books.google.com/books?id=bsf1mRVbH-wC&pg=PA47&lpg=PA47&dq=radiated+tortoise+diapause&source=bl&ots=Gn8tHsR-Tv&sig=ACfU3U0K7sx7I-tTMSWontMc4gSjd3P_FQ&hl=en&sa=X&ved=2ahUKEwiV0vewxMvlAhVFbKwKHRUCBNIQ6AEwF3oECAkQAQ#v=onepage&q=radiated%20tortoise&f=false

21. Know Your Tortoise. (2019). British Chelonian Group. http://www.britishcheloniagroup.org.uk/caresheets/know?LMCL=XDguJh

22. Castellano, C, Rhodin, AGJ, Ogle, M, Mittermeier, RA, Randriamahazo, H, Hudson, R, and Lewis, RE. (2013). Turtles on the Brink in Madagascar. Chelonian Research Foundation. Retrieved at https://www.researchgate.net/profile/Gerardo_Garcia3/publication/300819409_An_integrated_research_management_and_community_conservation_program_for_the_Rere_Madagascar_Big-headed_Turtle_Erymnochelys_madagascariensis/links/5782ae9108ae01f736e8f06e.pdf#page=69

23. Radiated Tortoise. Smithsonian’s National Zoo. Retrieved from https://nationalzoo.si.edu/animals/radiated-tortoise

24. Walker, Ryan. Notes on the current population status of the radiated tortoise Astrochelys radiate 2012-2019. Personal communication. Email.

Population of Radiated Tortoises in Over the Past 20 Years

Population of Radiated Tortoises in Millions2000201220191263