The Madagascar big-headed turtle (Erymnochelys madagascariensis) is a critically endangered turtle species native to western Madagascar. This highly aquatic turtle inhabits permanent slow-moving rivers, backwaters, and lakes. Facing threats such as habitat destruction and exploitation for food and traditional medicine, understanding the species' diet is crucial for effective conservation efforts.
Habitat and General Behavior
The Madagascan big-headed turtle (Erymnochelys madagascariensis) is a turtle native to the waters of permanent slow moving rivers and lakes in western Madagascar. This turtle occurs in freshwater areas and is diurnal (active in the daytime) and highly aquatic. Venturing onto land solely to lay its eggs, the Madagascar big-headed turtle prefers to bask on logs, rocks or banks surrounded by water. It is quite rare to find it on sandy shores, preferring rocks and rugged terrain.
Adult Diet: Primarily Herbivorous
As an adult, the Madagascar big-headed turtle is mainly herbivorous, feeding on plants, and the fruits, flowers and leaves of plants overhanging the water.
Threats to the Species
Despite their vulnerability to extinction, they are commonly eaten for food and they are still commonly shipped from Madagascar to Asia to help meet the demand of Asia's traditional medicine market. The main threat for this species is that they are heavily exploited for food, caught in nets, fish traps and by hooks and lines. It is also hunted for illegal export to Asia for the traditional medicinal market. This species is heavily exploited for food because it is easily caught in nets, fish traps, and by hook and line. It is also caught and exported illegally from Madagascar to Asia for the traditional medicine market. Furthermore, land development in Madagascar is a serious threat to this turtle as it destroys its natural habitats. Clearing forests for agriculture and timber opens up Madagascar’s untouched habitat and leads to terrible erosion. Their numbers have drastically decreased due to human interference. They are exploited for food and their natural habitat is degraded and fragmented.
Conservation Efforts
The Turtle Conservation Fund is working on a five year ‘Global Action Plan’ for this species that includes captive breeding and reintroduction projects, trade monitoring, and establishment of rescue centres, local conservation plans and educational programs. The Durrell Wildlife Conservation Trust is also independently involved in the conservation and protection of this species. A captive breeding program has also been started to prevent the species from becoming extinct.
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Taxonomic Classification and Conservation Status
The Madagascan big-headed turtle (Erymnochelys madagascariensis) is a turtle native to the waters of permanent slow moving rivers and lakes in western Madagascar. It is a highly evolutionary distinctive species, the only member of the ancient family Podocnemididae found outside of South America.
The Madagascan big-headed turtle is one of the most endangered turtles in the world, and is also included in the Turtle Conservation Funds (TFC) top 25 endangered. It has a hard dark brown shell enclosing all the soft parts of the body and, as its name indicates, a very large head. It has been evaluated as the most endangered turtle in the world by a 2018 review.
The Madagascar big-headed turtle is one of the most endangered turtles in the world and is included on the Turtle Conservation Fund’s (TCF) top 25 endangered turtles list. It is classified as Critically Endangered by the International Union for the Conservation of Nature (IUCN) and listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), which protects them from international trade.
Physical Characteristics
It has a hard dark brown shell enclosing all the soft parts of the body and, as its name indicates, a very large head. The Madagascar big-headed turtle gets its name from the size of its head. Most turtles are able to retract their head all the way into the shell, but due to the size of the head, it cannot be retracted completely. For this reason, it has developed rigid scutes on the top and sides of the head. The Madagascar big-headed turtle is a medium-sized turtle, reaching up to 20 inches in length. Because of their large head size, their head cannot completely retract. Its shell is dark brown and rigid. Some individuals have golden yellow coloring near the ridge of their shells and on the scales of their head.
Reproduction
After the female lays the spherical to elongate-shaped eggs with leathery shells, she covers them with a small amount of soil and heads back to the water. After mating in the water, the female comes ashore to lay her eggs. No parental care or protection is provided to the young.
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Other Madagascar Turtles
Madagascar is home to a diverse array of turtles, with eight species found on the island. Among the most well-known is the radiated tortoise (Astrochelys radiata), easily identified by its highly domed shell adorned with star-like patterns. This herbivorous tortoise is threatened by habitat loss and poaching, with both export and transport within Madagascar being prohibited. Another notable species is the angonoka tortoise (Astrochelys yniphora), characterized by its imposing carapace. The spider tortoise (Pyxis arachnoides) is a prized species often targeted by smugglers. Pyxis feed on insects, dried leaves and dead fruits and they can get by on very little food. The flat-tailed tortoise (Pyxis planicauda) is another protected species, active during periods of moderate heat and humidity. The African helmeted turtle (Pelomedusa subrufa) is a widespread aquatic turtle found in small pools and ponds across the African continent. In Madagascar, this animal is not subject to conservation measures. The blackish pelus has a carapace and skin that becomes black or dark brown. The yellow-bellied turtle has a dark brown carapace on the upper side and a slightly yellowish-yellowish carapace on the underside. However, it is subject to legal and controlled export to Asia. Sea turtles, including the loggerhead (Caretta caretta), green turtle (Chelonia mydas), hawksbill turtle (Eretmochelys imbricata), olive ridley (Lepidochelys olivacea), and leatherback (Dermochelys coriacea), also inhabit the waters surrounding Madagascar.
Diet of Platysternon megacephalum
Populations of the big-headed turtle Platysternon megacephalum are declining at unprecedented rates across most of its distribution in Southeast Asia owing to unsustainable harvest for pet, food, and Chinese medicine markets. Research on Asian freshwater turtles becomes more challenging as populations decline and basic ecological information is needed to inform conservation efforts. Studies in North America showed that freshwater turtles can considerably influence ecosystem processes (Sterrett, Maerz & Katz, 2014) through movements of seeds and nutrients from aquatic to terrestrial habitats (Moll & Jansen, 1995) and potentially by enhancing seed germination (Braun & Brooks Jr, 1987).
Study of Diet Composition in Hong Kong
Fecal samples collected from P. megacephalum in five streams in Hong Kong were examined to quantify the diet, and the germination success of ingested and uningested seeds was compared. Fruits, primarily of Machilus spp., were most frequently consumed, followed by insects, plant matter, crabs and mollusks. The niche breadth of adults was wider than that of juveniles. Diet composition differed between sites, which may be attributable to the history of illegal trapping at some sites, which reduced the proportion of larger and older individuals.
Impact on Seed Germination
Digestion of Machilus spp. fruits by P. megacephalum enhanced germination success of seeds by about 30%. However, most digested seeds are likely defecated in water in this highly aquatic species, which limits the potential benefit to dispersal.
Conservation Implications
The results of this study can be used by conservation-related captive breeding programs to ensure a more optimal diet is provided to captive P. megacephalum.
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Study Area and Methods
The study was conducted in five streams in the Hong Kong Special Administrative Region, China (22°09′-22°37′N, 113°50′-114°30′E). Elevations of the study sites ranged from 300-800 m above sea level, and riparian vegetation was mainly secondary forest dominated by Machilus spp. Among the five study streams, four were located in national parks and are accessible by the public and one is in a private, fenced and patrolled conservation area. Turtles were captured as part of a mark-recapture study (Sung, Karraker & Hau, 2013) carried out between September 2009 and June 2011, which included wet (April-September) and dry (October-March) seasons. Basic morphometric data on captured turtles were collected, including straight-line carapace length (CL) using calipers, and body mass using a spring scale. Passive implant transponder tags were inserted and marginal scale notching following a system developed by Cagle (1939) was used to mark and identify turtle individuals. Turtles were sexed by examining secondary sexual characteristics, including distance of cloaca from the edge of the plastron and thickness of the tail base above the cloaca, and all turtles smaller than 105 mm in CL were considered to be juveniles (Sung, Karraker & Hau, 2013). Fecal analysis was used to examine the diet following Demuth & Buhlmann (1997). Fecal samples were collected from captured turtles that were kept in plastic enclosures with approximately two cm of water for 20-24 h. The samples were sorted under a dissecting microscope (MZ8; Leica Microsystems, Wetzlar, Germany) and diet items were identified to order or lower taxonomic levels if possible. Seeds of Machilus spp. were collected from fecal samples and were assessed for level of damage following digestion. Seeds that exhibited a spherical shape similar to undigested seeds were considered undamaged and were retained for the germination experiment.
Germination Experiment
To investigate the effects of gut passage on germination success and rate of Machilus seeds, three experimental treatments were established: undamaged digested seeds, undigested seeds with fruit pulp intact, and undigested seeds with fruit pulp removed. Seeds representing each treatment were planted in seed trays placed in a shaded area in a greenhouse. Seeds were watered approximately three times per week, depending upon ambient temperature and drying of the potting soil. Seeds trays were checked at least three times per week for six months and germination was documented.
Data Analysis
The frequency of occurrence of each diet item was calculated as percent of individuals that contained a given diet item (Bowen, 1983). Niche breadth of female, male and juvenile turtles was calculated using the Shannon index. Logistic regression analyses were conducted with occurrence of seeds and animals as response variables, and carapace length of turtles as predictor variables. Multivariate analysis was also conducted to compare diet composition among seasons, sites, and age and sex groups based on presence of diet items with non-metric multidimensional scaling and analysis of similarity using Bray-Curtis similarity index. Germination success of seeds collected from fecal samples and control seeds with and without pulp was compared using a generalized linear mixed model with a binomial error variance (Zuur et al., 2009).
Results of the Study
141 fecal samples were collected, of which 89 contained at least one item, from 61 individual turtles (31 females, eight juveniles and 22 males). 356 diet items belonging to 11 categories were identified. Diet items most frequently recovered from fecal samples were fruits, insects and mollusks. All fruits recovered belonged to the genus Machilus, except one sample that contained seeds of Turpinia arguta. Identifiable remains of insects consisted of terrestrial adults and larvae belonging to seven orders (Coleoptera, Homoptera, Hymenoptera, Isoptera, Lepidoptera, Mantodea, Orthoptera) and aquatic larvae belonging to four orders (Diptera, Ephemeroptera, Odonata and Tricoptera). All mollusks found were Sulcospira hainanensis.
Niche Breadth and Diet Composition
Based on the Shannon and evenness Indices, niche breadth in the wet season was broader than that in the dry season, and niche breadth of adult turtles was wider than that of juvenile turtles. Diet composition was similar between seasons (R = 0.074, P = 0.115), and sexes and ages (R = − 0.009, P = 0.570), but differed among sites (R = 0.344, P < 0.001). In pairwise comparisons, diet composition of turtles in KF differed from that of other sites (P < 0.020), whereas the diet of turtles in MS, TN and TO was similar (P > 0.332). Diet of turtles in SH was different from that in MS (P = 0.024) but similar to diet of turtles in TN (P = 0.332) and TO (P = 0.075). Fruits, insects, crabs and other plant matter contributed the most to the dissimilarity between sites. Frequency of occurrence of fruits was the highest in KF and that of insects was the lowest, whereas frequencies of occurrence of fruits and crabs were the lowest in SH.
Seed Damage and Germination
Of seeds consumed by turtles, 64% were damaged, either by mastication or the digestion process.
Dietary Habits and Ontogenetic Shifts
P. megacephalum have long been regarded as carnivorous (Bonin, Devaux & Dupre, 2006), but we found that fruits were frequently consumed, and we believe that this fruit is consumed within the stream channel. Fruits occurred in at least 62.5% of fecal samples of females, juveniles and males. P. megacephalum exhibits ontogenetic shift in diet, becoming increasingly frugivorous with increases in body size. Shifts in diet from largely carnivorous to largely herbivorous have been documented in a number of freshwater turtles (Chen & Lue, 1998; Parmenter & Avery, 1990; Spencer, Thompson & Hume, 1998). However, a high proportion of adult P. megacephalum consumed a diversity of animals, including larger prey, such as frogs, fish, and crabs, upon which smaller juveniles are incapable of predating and thus contributing to the narrower niche breadth of juveniles. It was surprising that fruits of Machilus occurred in 62.5% of fecal samples of juveniles, including the smallest juveniles with carapace length of 48 mm, indicating that fruits may be an important diet item across turtles of all sizes. Diet composition of P. megacephalum differed between study streams, and this may be associated with demographic differences among sites, which have been shaped by a history of illegal trapping.
Communication and Behavior
Turtles are predominantly solitary animals, so little is known about their communication. They do, however, use physical communication when breeding season approaches. The people of Madagascar both fear and respect these turtles because of their strength and aggressive nature. They are often featured in the folklore of the Malagasy cultures. These highly active turtles enjoy both shallow and deeper water, and make fine tank turtles,. They do well in groups, but may out compete milder species. Madagascar Big Headed Side-necked Turtles are an impressive and fascinating species to keep.
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