Dermatemys mawii

Gray, 1847
Central American river turtle

Dermatemys mawii is large (to 65 cm), almost totally aquatic, and has a row of inframarginals on the bridge. The oval, uniformly olive-gray, flattened adult carapace is only slightly domed, not marginally serrated, and lacks a well-defined vertebral keel. Its nuchal bone contains well-developed lateral extensions (costiform processes) in hatchlings, but these, with age, are apparently covered by costals and peripherals. Neurals 6 and 7 are reduced, and consequently the posterior costals meet along the midline. The shell is thick, and in old adults the bones become so tightly fused that the sutures are obliterated. Vertebral scutes are longer than broad in adults, the opposite in juveniles. Thinness of the scutes allows the shell surface to be easily injured by abrasion, and suture lines may disappear with age. Twelve pairs of marginals are present, and the 10th dorsal rib does not contact the costal bones. The cream-colored bridge is broad and covered by three to six (usually four or five) inframarginal scutes; the inguinal is the largest, the axillary the smallest in this series. The solid, hingeless, cream-colored plastron is well-developed, but the buttresses are not greatly developed. Posteriorly there is a wide plastral notch. Either 11 or 12 scutes are present on the plastron, as the gular may be single or divided. Occasionally, small extra scutes may lie along the mid- or lateral seams. The plastral formula is usually: abd > pect > fem > hum > an > gul. For such a large turtle, the head is relatively small. It contains a slightly upturned, somewhat tubular snout, and is olive gray laterally and lemon yellow to reddish brown dorsally. Some dark vermiculations may occur laterally, and the lower jaw is white. The skull has the temporal region cut away posteriorly so that the squamosal is separated from the parietal and broad postorbital bones. Frontal bones enter the orbits. The jugal articulates with both the pterygoid and palatine bones, but the quadratojugal does not touch the maxilla, and the quadrate does not surround the stapes. A concavity separates the premaxilla from the rest of the triturating surface, which contains a strong medial ridge; there is a moderately developed secondary palate. Several skull features are absent: the stapedio-temporale foramen, the stapedial artery and the structures housing it, and the postlagenar hiatus. The unpatterned limbs are dark gray, and the toes are webbed; a conspicuous fringe of large scales occurs on the outer edge of each foot. On the femur, the trochanteric fossa is widely open, a primitive condition. In the pelvic girdle, the pubes and ischia contact those from the opposite side ventrally, but the pubic and ischiadic symphyses are widely separated.
Dermatemys mawii has a diploid number of 56: 7 pairs of metacentric to submetacentric macrochromosomes, 5 pairs of telocentric or subtelocentric macrochromosomes, and 16 pairs of microchromosomes (Carr et al., 1981).
Males have long tails which extend beyond the carapacial rim; females have very small tails that barely reach the rim. Also, the dorsal surface of the head is yellowish to reddish brown in males, olive gray in females.

Dermatemys ranges from central Veracruz eastward through Tabasco, northern Chiapas, southern Campeche, and southern Quintana Roo in Mexico (it is absent from most of the Yucatán Peninsula) to northern Guatemala and Belize. There is also a single record from Tetela, Oaxaca (Smith and Smith, 1979) and it possibly occurs in northwestern Honduras.

As its common name implies, Dermatemys mawii lives primarily in large rivers, lagoons, and lakes. It has also been taken at sites ranging from deep, clean, permanent holes in these water bodies to muddy backwaters, oxbows, and temporary seasonal pools. Apparently, as long as an abundance of aquatic food plants exist, it can live in almost any freshwater body within its range. Brackish water can be tolerated, and, evidenced by the occurrence of barnacles attached to the shell of some specimens, it may spend some time in the tidal areas near the mouths of large rivers.

Natural History
Since this turtle is so aquatic, it is no surprise that much of its life history is unknown. The courtship and mating acts have not been described, but may involve aggressive behavior since members of the opposite sex often fight when kept together in captivity.
Vogt and Flores-Villela (1992b) reported a nesting season from September to March in Mexico, with a maximum of three clutches per year. In northern Belize, Dermatemys mawii nests during the latter part of the rainy season, from late September through December, and may produce up to four clutches per year (Polisar, 1996). According to Smith and Smith (1979), fall nestings are aided by flooding, which allows the female to select secluded areas away from the normal river channel that she could not reach by walking. However, they stated that nests are excavated at the very margin of inhabited waters, or but a few meters from it. Eggs may be deposited in a shallow excavation or merely covered with decaying vegetation. Nests are capable of surviving prolonged immersion (Polisar, 1996).
In Belize, 8-14 (2-20), white, brittle-shelled, ellipsoidal (54.1-72.0 x 32.4-49.8 mm) eggs are laid (Polisar, 1996). The sex of the hatchling is determined by the incubation temperature (temperature sex determination); more males result from clutches incubated at temperatures below 28°C, while a higher percentage of females result from eggs incubated at temperatures above 28°C (Vogt and Flores-Villela, 1992a).
Juveniles have a brownish-olive carapace which bears a well-developed vertebral keel and a highly serrated posterior rim. Their olive-colored heads have a yellow stripe extending posteriorly from each orbit; in some this stripe may also run anteriorly from the orbit to the nostril.
Wild adults are almost entirely herbivorous (a low percentage of insects is consumed; Moll, 1989), although captives sometimes eat fish; they consume both submerged and emergent plants, predominantly Paspalum peniculatum, fallen leaves, and fruits (Alvarez del Toro et al., 1979; Moll, 1989). Wild juveniles also eat large quantities of Paspalum peniculatum (Moll, 1989), but captive juveniles more readily accept animal foods, leading to speculation that they may be naturally more carnivorous than adults.
This species is almost entirely aquatic, and it spends long periods of time under water. Apparently it takes sufficient dissolved oxygen from the water through the nasopharyngeal lining, as submerged individuals continually suck in water through the mouth and then expel it from the nostrils. This turtle is most active at night and spends the daylight hours under water or floating (perhaps basking) at the surface.
Dermatemys is so adapted to a buoyant liquid medium that its limbs cannot well support its weight out of water, and it has difficulty walking any distance on land or holding its head off the ground.

IUCN Red List Status (1996)
Endangered (A1abcd+2bcd, B1+2cde). These turtles are highly prized for their meat and can be found in most markets close to large river bodies. They are hunted to the point of overexploitation. Alvarez del Toro et al. (1979) commented:

Dermatemys is capable of converting otherwise unused aquatic vegetation into high quality protein. Carefully harvested on a sustained yield basis, it could continue to be a valuable protein source; it has considerable economic potential for southern Mexico and other countries where it occurs. To let it become extinct is senseless.

As late as 1984, Dermatemys was common to abundant in sparsely populated areas of Belize, but declining where accessible to hunting (Moll, 1986). Although exploitation occurs year-round, it peaks during the last two months of the dry season, April and May, when low water levels facilitate captures (Polisar, 1994). Some villages concerned with conservation of Dermatemys developed a request for local regulations on its harvesting which was submitted to the Ministry of Natural Resources of Belize. This and survey reports and recommendations for northern Belize were taken into consideration and resulted in a nationwide comprehensive legislation protecting and managing Dermatemys mawii. It is hoped that this will successfully control its exploitation and bring about an increase in the population size of the species.