The Family Platysternidae, and its sole member: Platysternon megacephalum, the Big-headed Turtle

David T. Kirkpatrick, Ph.D


Basic Taxonomy

Family status

Platysternon megacephalum is a monotypic turtle, meaning that it is unique enough to be placed in its own family, the Platysternidae, and has no close relatives. However, the exact placement of Big-headed turtles has been the subject of some controversy over the years. Agassiz originally grouped this turtle with the snapping turtles, but subsequent authors placed Platysternon in its own group. More recently, Eugene Gaffney grouped Platysternon with the Chelydridae (snapping turtles) based on a number of morphological features of the skull (Gaffney, 1975). 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) (Haiduk and Bickman, 1982), as chromosomes vary more between Platysternon and the chelydrids than they do between Platysternon and the batagurines. 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. Peter C. H. Pritchard, in his book on Alligator Snapping turtles (Macroclemys temminckii) discusses these differences in form between Alligator Snappers and Big-headed turtles (Pritchard, 1989).

Species status

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 (Table 1). Ernst and McCord described the most recent subspecies, P. m. shiui, in 1987. Some subspecies may be invalid, as the diagnostic characteristics used to separate them might be too variable to adequately differentiate between subspecies. Carl Ernst is investigating the relationships between the subspecies (Ernst and McCord, 1987).

Currently the subspecies are designated based on differences in a number of characteristics, including the patterns on the plastron and carapace, the presence of growth annuli and posterior serrations on the carapace, and the shape and coloration of the head and jaws. 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 (Iverson, 1992).

Habitat

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. 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) and generally don't exceed 72 degrees F (23 C).

Physical Characteristics

In appearance Big-headed turtles look as if they have been assembled using mismatched and poorly sized parts left over from other turtles (see Figures 1 through 3). 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 extending 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, depending on specific subspecies. 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 counterweight when in the water, allowing the turtle to balance on two back legs only. When swimming Big-heads occasionally will curl 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 megacephalum is the nominate subspecies, and can be distinguished by its unpatterned plastron, lack of growth annuli and posterior serrations on the carapace, and a radiating pattern of narrow lines on the head with yellow mottling of the jaws. P. m. tristernalis is similar to P. m. megacephalum except for the presence of three small scales at the intersection of the gular and humural scutes. P. m. peguense is readily distinguished from the previous two subspecies, as it has a dark pattern along the seams of the plastron. In addition, this turtle has pronounced growth rings on the carapace, while the posterior portion is serrated. The head of this chelonian has a dark stripe behind the eyes and an unpatterned jaw. The subspecies P. m. vogeli also has a dark plastral figure, but the carapace is smooth and lacks serrations. Finally, the most recently described subspecies, P. m. shiui, is distinguished by its coloration; it is heavy spotted with pink, orange or yellow on its limbs, head, shell and tail. It has a smooth carapace with respect to both growth annuli and posterior serrations (Pritchard, 1979 and Ernst and Barbour, 1989).

Sexing

There is no marked difference between the sexes in Platysternon. The plastron of male turtles tends to be more concave than that of females. Also, the vent is located posterior to the edge of the carapace. However, these differences are rather slight, and a series of turtles should be examined to correctly identify the sexes, especially if a breeding program is planned.

Reproduction

The behavior of P. megacephalum in its natural environment, especially details concerning mating and reproduction, is almost totally unknown. 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. Eggs measure approximately 37 mm by 22 mm (Ernst and Barbour, 1989). Incubation times for the eggs have not been reported. Similarly, it is unclear if the sex of the hatchling turtle is determined by the incubation temperature, or if it is temperature-insensitive.

Growth and Development

Juvenile Big-heads are more brightly marked than adults. Also, juveniles have more pronounced serrations at the rear of the carapace, while the tail is proportionately longer than in the adults.

Lifespan

As with many semi-aquatic turtles, Platysternon megacephalum is moderately long-lived in captivity. Kate and Frank Slavens' Reptiles and Amphibians in Captivity: Breeding, Longevity and Inventory, 1994 edition, mentions a male P. m. megacephalum acquired by the Chicago Zoological Park in 1959 as an adult that died over 25 years later. The Columbus Zoological Park has a male that has been in captivity over 27 years (Snider and Bowler, 1992). With reasonable care, wild-caught Big-headed turtles certainly are capable of surviving decades in captivity.

Selection and Acquisition

Big-headed turtles are relatively uncommon in captivity. Slavens' 1994 Inventory lists only 14 institutions and individuals possessing specimens, although this figure undoubtedly is low, as many individual turtle keepers do not contribute to the inventory. Platysternon megacephalum is available occasionally from importers or secondary reptile dealers. Even more rarely they may be encountered in pet stores. Virtually all Big-headed turtles offered for sale are imported from their country of origin. Turtles should be examined for the presence of fresh wounds, abrasions or scute infections. The head plates also are a potential site for infection, and should be examined closely. Imported turtles may have old wounds, including (but not limited to) missing toes or the tip of the tail.

Locality data usually are unavailable when purchasing from a reptile importer, making subspecies identification more difficult. If at all possible, attempt to ascertain the country of origin, as this information will help in determining the identity of the turtle. The purchase of a number of turtles at the same time from the same shipment should be considered if the goal is to breed Platysternon megacephalum, as the chances of obtaining turtles from the same geographic region are increased.

Behavior

Captive individuals appear to be fairly intelligent. They adapt quickly to feeding routines, for example. Specimens observed by the author generally demonstrate an interest in their surroundings. They even seem capable of recognizing the existence of glass walls, a capacity beyond that of many other semi-aquatic turtles. In their natural environment, 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 (Figure 4). 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 (Obst, 1988). Given all these adaptations, it is not surprising that Big-headed turtles have been reported to climb trees and bushes. Their climbing ability is reminiscent of that of Pancake tortoises (Malacochersus tornieri), as are a number of the Big-headed turtle's habits and some features of its anatomy.

Handling

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, guarding the unprotected throat area and 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! A Big-head's neck is only of moderate length, and so cannot reach as far as a Snapping Turtle (Chelydra serpentina), especially directly back over the carapace. Platysternon megacephalum's claws are sharp and they have powerful leg muscles befitting a rock-climbing turtle. They can deliver a scratch or launch themselves from an unwary handler's grip, with potentially disastrous results.

Housing

Captive maintenance of P. megacephalum requires consideration of their normal ecological niche and the adaptations they have made to that 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 (see Figure 5). The land area must be deep enough to allow sufficient space for the burial of eggs by females, if captive reproduction is possible or desired.

A minimum size for an enclosure containing a single Big-headed turtle is 1' wide by 2' long by 1' high, if a screened top is used to prevent escapes and the land area is a suspended one as described above. If you are maintaining more than one turtle together, a larger area is a necessity. Separate underwater hiding places must be provided for all turtles kept together, as each turtle will select a spot in which to sleep during the day. Provisions must be made for housing turtles individually, as individual animals may be incompatible and require separation.

Platysternon megacephalum is crepuscular or nocturnal in habits, but a light source should still be provided, and attached to a timer to provide a normal day/night cycle for the turtle. The light source does not need to be a heat source, in fact, 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.

Feeding/Water requirements

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 P. megacephalum's 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 such as Tetra's Reptomin, Wardley's ReptileTen, and Purina Trout Chow. 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.

Waste Management

The accustomed habitat for Platysternon megacephalum is rushing mountain streams. In captivity, water cleanliness must be maintained for the health of the animal. For adequate water filtration, a large canister filter is recommended. In addition to providing the maximum filtering capacity, large units also discharge the water with some force, mimicking a mountain stream. Big-headed turtles benefit from having rocks on which to climb, however, these rocks often interfere with aquarium upkeep. Piles of rocks provide areas in the tank where dirt can accumulate. Feeding the turtles outside of the tank in a secondary container prevents uneaten food debris from collecting among the rocks in the aquarium. As turtles often will defecate after eating, external feeding aids in reducing the accumulation of fecal material also.

Preventive Health Care

As most specimens of Platysternon megacephalum will be imported, wild-caught individuals, newly acquired turtles should be examined for the presence of internal and external parasites. Treatments, if necessary, are dependent on the nature of the parasites detected, and follow the usual protocols for semi-aquatic turtles. A veterinary examination is highly recommended if you are inexperienced in dealing with chelonian parasites. Regardless of the result of the examination, all new arrivals should be quarantined for at least a month to prevent the potentially lethal spread of disease to other turtles.

Turtles should be examined periodically for the presence of cuts, bites, or abrasions, especially if more than one individual is kept in a single enclosure. Minor wounds can be treated with topical antibiotic ointments or gels. Over-the-counter polysporin ointments have worked well on mild cuts on Big-headed turtles maintained by the author. Imported individuals may develop infections under the scutes, including the horny layers around the jaw and on top of the head. Routine monitoring will detect these infections before they become life-threatening. Periodic supplementation of the turtle's food with vitamins and calcium may help prevent deficiencies due to undetected inadequacies in diet. Captive turtles in the author's collection are fond of calcium blocks made of plaster of paris with added vitamin and calcium powder. The turtles use their powerful jaws to break the blocks apart and rapidly consume them. In addition to the benefits from the calcium and vitamins, the blocks also help keep a turtle's jaws worn down.

Common Problems

Platysternon megacephalum appears to be a relatively easy turtle to maintain in captivity if the basic environment of the turtle is kept in mind. Most problems arise from a failure to provide an appropriate environment. For example, if the area is kept too warm, or the water quality and cleanliness are not adequate, the health of the turtle may suffer.

Another problematic area involves interactions between turtles. Although they have been reported to be unaggressive towards turtles kept in the same enclosure (Ernst and Barbour, 1989), fights have occurred between three juvenile Big-heads maintained by the author (see Figure 6). The level of aggression necessitated separation of the three into individual aquaria. The powerful jaws of P. megacephalum make any fight potentially injurious to the animals involved. Careful monitoring of specimens will prevent most difficulties.

Bibliography/Recommended Reading

Ernst, CH; Barbour, RW: Turtles of the World. Smithsonian Institution Press, Washington, DC, 1989, 313pp.

Ernst, CH; McCord, WP: Two new turtles from southeast Asia. Proc. Biol. Soc. Wash., 1987; 100(3): 624-628.

Haiduk, MW; Bickman, JW: Chromosomal homologies and evolution of testudinoid turtles with emphasis on the systematic placement of Platysternon. Copeia, 1982(1): 60-66.

Gaffney, ES: Phylogeny of the chelydrid turtles: a study of shared derived characters in the skull. Fieldiana:Geol. 33(9): 157-178.

Iverson, JB: A Revised Checklist with Distribution Maps of the Turtles of the World. 1992, Privately printed, Richmond, IN, 363pp.

Obst, FJ: Turtles, Tortoises and Terrapins. St. Martin's Press, New York, NY, 1988, 231pp.

Pritchard, PCH: Encyclopedia of Turtles. T.F.H. Publications, Neptune City, NJ, 1979, 895pp.

Pritchard, PCH: The Alligator Snapping Turtle: Biology and Conservation. Milwaukee Public Museum, Milwaukee, WI, 1989.

Slavens, FL; Slavens, K: Reptiles and Amphibians in Captivity: Breeding, Longevity and Inventory. 1994, Slaveware, Seattle, WA, 534pp.

Snider, AT; Bowler, JK: Longevity of Reptiles and Amphibians in North American Collections. 1992, Herpetological Circular No. 21, series editor: Collins, JT; Society for the Study of Amphibians and Reptiles, 40pp.

This chapter copyright © 1997 by David T. Kirkpatrick. Originally published in The Biology, Husbandry and Health Care of Reptiles, Volume II: The Husbandry of Reptiles, Lowell Ackerman, DVM, editor, published by T.F.H Publications, 1997, pg 438-446.

Reprinting of this chapter for non-profit purposes is permitted provided that it is unaltered and appropriate attribution, including copyright information, is included. Please notify the author of any reprinting.