Platysternon megacephalum is a monotypic turtle, meaning that it is unique enough to be placed in its own family. Some scientists have grouped Platysternon with the Chelydridae (snapping turtles) but evidence gathered from examining chromosomes of various species indicates that the Big-headed turtle most closely resembles the batagurine group of turtles (the Old World pond turtles including such turtles as Chinemys and Cuora). Although portions of the Big-headed turtle's anatomy resemble portions of other turtles, these are only superficial resemblances and do not indicate an underlying relationship.
Currently there are five subspecies of P. megacephalum recognized: P. m. megacephalum, P. m. peguense, P. m. vogeli, P. m. tristernalis, and P. m. shiui. Big-headed turtles were first described in 1831 by J. E. Gray, with various subspecies advanced in 1870, 1969, and 1984. Ernst and McCord described the most recent subspecies, P. m. shiui, in 1987. Carl Ernst is investigating the relationships between the subspecies. Some subspecies may be invalid, as the diagnostic characteristics used to separate them might be too variable to adequately differentiate between subspecies.
In appearance Big-headed turtles look as if they have been assembled using mismatched and poorly sized parts left over from other turtles. The most distinctive characteristic is an oversized triangular head - it can be up to half the width of the carapace itself. Due to the size of its head, the turtle is incapable of withdrawing it into its shell. To make up for this lack of protection, the top and sides of the turtle's head are covered with a large horny scute, similar to the scutes on the carapace and plastron. As further protection, the turtle's skull is solid bone; unlike many turtles Big-headed turtles have no openings in the roof of their skull. Their jaws have a wide but sharp beak, while the edges of both jaws are lined with another tough horny sheath. Only a narrow band of unprotected skin remains on the cheeks and running from the turtle's eyes to the corner of its mouth. The eyes themselves are not particularly recessed, although they appear so because of the surrounding horny shielding. Each eye has a black pupil and a golden iris, occasionally with a brownish bar running through it horizontally. The head of P. megacephalum gives the impression, at least superficially, of a Snapping turtle head; it is not surprising that they have been regarded as close relatives.
The carapace of the Big-headed turtle is roughly rectangular with a squared-off front and a more rounded and notched or even serrated posterior. It is not highly domed; in fact it can be quite flattened, especially when compared to the size of the turtle's head. The plastron, unlike that of Snapping turtles or Mud turtles, is well-developed and lacks a hinge. While the carapace can range from yellow to a dark brown, occasionally with a darker radiating pattern, the plastron is generally yellow. Some subspecies (P. m. peguense, P. m. vogeli) have a dark edging to the seams on the plastron. P. m. shiui is the most strikingly marked of the subspecies, with traces of red or pink on its shell and head.
Big-headed turtles have slightly webbed toes with strong claws. Their feet resemble those of Wood turtles or Spotted turtles (Clemmys insculpta or C. guttata) in appearance. The legs are covered with large scales, as is the very long and muscular tail. The tail itself is not very heavily built; instead it is thin and whiplike. It is capable of a large range of motions, and seems quite useful to the turtle. Individuals have been observed using the tail as a balance when in the water, allowing the turtle to balance on two back legs only. When swimming Big-heads occasionally will hold their tail up in an arc over their back, almost coming to resemble a scorpion. Big-heads can use their well-armored tail as a prop; while attempting to climb a smooth vertical wall a turtle was seen to support its own weight entirely with its tail.
Platysternon megacephalum hails from Southeast Asia. Specimens have been obtained as far south and west as Burma, and as far north and east as southern China. Various subspecies are found in different parts of the turtle's range: P. m. megacephalum is found in southern China, while P. m. peguense occurs in southern Thailand and Burma. The most colorful subspecies, P. m. shiui, is located in northern Vietnam. In all of these locales, the primary habitat of the Big-headed turtle remains the same. It prefers to live in fast-moving mountain streams filled with boulders and broken rock. Although Big-heads are located in countries that most people consider to be tropical, water temperature in the mountain streams favored by the turtles can reach lows of 53 degrees F (12 C). Turtles spend the day underwater, burrowed into gravel deposits or hidden in crevices in the rocks. In the evening they emerge to search for food along the stream bottom and out of water along the sides of the stream. Big-heads are not well-equipped for swimming; instead they are much more adept at walking and climbing. The rapid water flow in the streams they frequent makes the ability to cling to rocks more advantageous than skill in swimming.
The climbing ability of the Big-headed turtle should not be underestimated. The bridge between their carapace and plastron is somewhat reduced, allowing the turtle a large range of motion with its feet. The heavy head of P. megacephalum places the turtle's center of gravity well forward compared to other turtles. This shift, coupled with the wide range of motion in its legs, allows the turtle to climb over anything it can reach with its claws. As noted previously, they are capable of using their tail as a prop, greatly extending their reach. Reports indicate that they also use their beaks in climbing. Given all these adaptations, it is not surprising that Big-headed turtles have been reported to climb trees and bushes. Their surprising climbing ability is reminiscent of that of Pancake tortoises, as are a number of the Big-headed turtle's habits and some features of its anatomy.
Captive maintenance of P. megacephalum requires consideration of their normal ecological niche and the adaptations they have made to their environment. As stream-dwellers, Big-headed turtles should be maintained in an enclosure that allows access to both land and water. The water section cannot be too deep - Big-headed turtles are ill-equipped for swimming, so there is a possibility of drowning. Their climbing abilities must be considered when designing the land area. If possible, the top of the enclosure should be screened to prevent escapes. As the water area cannot be too deep, the total area of the cage should be as large as possible to give the turtles adequate room. Often this can be accomplished by building a multi-layered habitat. For example, an aquarium can be designed with a suspended land section in the middle, permitting the turtle to use the whole tank as a water area yet still providing a land area. Arrangements of this type allow the turtles to utilize all of the space in the container. Do not allow the temperature in the enclosure to rise too high - although P. megacephalum originates from tropical countries, it lives in water that ranges from 53 to 72 degrees F (12-23C). If allowed to get too warm, there is a chance of overheating.
Be careful when handling Big-headed turtles. When threatened, their normal defensive reaction is to retract their legs and to move their heavily scaled tail under the rim of the carapace. They also drop their chin down, presenting the armored top of their head to the threat. However, they are capable of delivering a severe bite with their heavy jaws and hooked beak and some individuals are quite apt to do so when provoked!
Big-headed turtles in their natural environment are almost entirely carnivorous. Their strong beaks and hard-edged jaws allow them to consume crustaceans and mollusks, and these prey items form a large portion of their diet, although they will feed on a number of other items. In captivity they will eat most meats, fishes and insects with which they are presented. Also, Big-heads can be trained to take commercial turtle foods. Captive individuals are quite capable of capturing prey on land, however they prefer to enter the water before consuming it and so should be fed in water.
The behavior of P. megacephalum in its natural environment, especially details concerning mating and reproduction, is almost totally unknown. Captive individuals appear to be fairly intelligent, adapting quickly to feeding routines for example. They can live for a long time in captivity; the Chicago Zoological Park maintained a male Big-headed turtle for over 25 years. P. megacephalum is slightly sexually dimorphic. Male Big-heads possess concave plastrons while their vents are located beyond the rim of the carapace, comparable to many other turtle species. One or two white eggs are laid at a time. Unlike most turtle eggs, they are not spherical or lozenge-shaped. Instead, the eggs are ellipsoidal and resemble bird eggs. Juvenile Big-heads are more brightly marked, the serrations at the rear of the carapace are more pronounced, and the tail is proportionately longer than in the adults.
The Big-headed turtle, Platysternon megacephalum, is a unique chelonian for a number of reasons. In form it appears to have been assembled from parts of many turtles, while its behavior and natural environment are equally exotic. Although currently not believed to be threatened, its small clutch size and specialized habitat leave the Big-headed turtle vulnerable. The IUCN Species Survival Committee on Tortoises and Freshwater Turtles recommended in their 1991 action plan that the status of Platysternon megacephalum be evaluated to determine if it is in danger in its natural habitat. Hopefully more research will be devoted to this turtle, to answer some of the numerous questions concerning its natural behavior and lifestyle.
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