(Gray, 1830, in Gray, 1830-1835)
River terrapin
Recognition
Batagur baska is one of the largest emydid turtles, reaching a carapace length of at least 60 cm. The carapace is somewhat domed, has smooth scutes, and is not serrated posteriorly. There is a low, interrupted vertebral keel in juveniles which disappears with age. All vertebrals are broader than long, and the underlying neural bones are elongated, hexagonal, and short sided anteriorly. Vertebral 4 only covers three neurals. The carapace is uniformly olive gray or olive brown. The plastron is well-developed, but smaller than the carapacial opening, and has only a shallow posterior notch. The bridge is broad and the plastron is extensively sutured to the carapace. Plastral buttresses are greatly enlarged, extending almost to the neural bones; the axillary buttress is connected to the 1st rib while the inguinal buttress connects between the 5th and 6th costals. On the bridge, the inguinal scute is larger than the axillary. Both the anterior and posterior plastral lobes are shorter than the bridge. The plastral formula is: abd > pect >< fem > hum > an > gul. Plastron and bridge are uniformly yellow or cream in color. For such a large turtle, the head is small to moderate in size with an upturned, pointed, projecting snout, and a medially notched upper jaw. The back of the head is covered with small scales. In the skull, the temporal arch is complete with the quadratojugal touching both the jugal and postorbital. The frontal bone may contribute to the orbital rim. The maxilla does not touch either the parietal or the squamosal. The orbito-nasal foramen is large, and there is a dorsal ridge on the palatine bone which may help brace the palate against the cranium. The triturating surfaces of the jaws are broad with two strongly denticulated ridges; the sides of the jaws are serrate. The head is olive gray dorsally, but lighter gray laterally and ventrally with light-colored jaws. Forelimbs are somewhat paddle shaped with four claws. All toes are webbed and there are large transverse scales on the limbs. Skin is olive gray. Interestingly, during the mating (monsoon) season, the male's head, neck, and legs turn black and his iris changes color from yellowish cream to pure white (Moll, 1978). After the breeding season these parts revert to their original colors.
In addition to the sexual dichromatism, males can be told from females by their longer, thicker tails; also adult males are somewhat smaller (to 50 cm) than adult females (to 60 cm).
Distribution
Batagur baska is known from the Irrawaddy River in Myanmar southward through southern Thailand to southern Vietnam, and through Malaysia to Sumatra. Khan (1982) confirmed its presence in Bangladesh.
Geographic Variation
Nutphand (1979) described the form Batagur baska ranongensis from Thailand as differing by its flat, round carapace, as opposed to the elongated, narrower carapace of normal adult B. baska. However, this probably just represents an abnormal growth pattern, as all juvenile B. baska are rounded, but become more elongated with age. Edward O. Moll has told us that B. baska from the east coast of Malaysia differ in coloration from those of the west coast.
Habitat
Batagur baska lives in tidal areas of the estuaries of large rivers, often associated with mangrove thickets. During the nesting season, it ranges far upstream in these rivers.
Natural History
Presumably, mating occurs just prior to and in the beginning of the monsoon season when the males are in breeding color. Courting males at the Bronx Zoo, New York swayed their heads and pumped their throats during courtship, and were in breeding coloration from October to February (Blanco et al., 1991).
Communal nesting occurs from late December to early March on sandbars and banks far upstream from the normal estuarine habitat, and some females may swim over 80 km to reach these sites. At the mouth of the Irrawaddy River in Myanmar, females nest on adjacent sea beaches. A body pit up to 90 cm deep is dug first before the actual 15-30 cm-deep nest cavity is constructed. Digging is slow and deliberate and may take several hours.
Females nest up to three times a season, laying 5-60 eggs each time. Sometimes these eggs are placed in two or three nest cavities dug during one night. A typical egg is oblong (66-70 x 40-45 mm), and has either a hard expandable shell or a parchmentlike shell. Natural incubation takes from 68 to 112 days. Hatchlings are olive gray and have round, slightly serrated carapaces with a low medial keel.
These turtles follow the high tides upstream to forage, retreating back to the estuaries as the tide lowers. Davenport et al. (1992) showed that under laboratory conditions, B. baska eats plant material at salinities below 19.8รค, but refuses to eat and drink at higher salinities. This species is predominantly herbivorous, feeding on leaves, stems and certain mangrove fruits, but also taking mollusks, crustaceans, and fish (Moll, 1978, 1980). Captive adults will eat green vegetables, such as kale, spinach, bok choy, dandelion and lettuce, and Purina Turtle Chow, earthworms, and fish. Hatchlings will accept mixed chopped greens, crickets, waxworms, mealworms, macerated newborn mice, chopped clams and prawns (Blanco et al., 1991).
IUCN Red List Status (1996)
Endangered (A1bcd). Batagur baska has decreased drastically in numbers owing to the harvesting of its eggs, slaughter of adults, and loss and deterioration of its habitat. Conservation programs began in Malaysia in the 1960s, but the outlook for the species' survival still is in doubt. In addition to captive breeding program, a conservation program has been started in West Bengal that includes monitoring reproduction in wild, and the determination of ecological critical parameters (Bhupathy, 1997).