(Schlegel and Müller, 1844)
The oval carapace (mostly under 60 cm, but may reach 76 cm) is somewhat flattened and smooth in adults, but may be slightly serrated posteriorly in hatchlings and juveniles. Juveniles have a well-developed continuous medial keel and an interrupted lateral keel on the pleural scutes of each side. These keels become lower and disappear with age. Vertebrals 1-3 and 5 are broader than long; the 4th is about as broad as long and is smaller than the 3rd. A projection on the anterior border of the 4th vertebral fits into a posterior concavity on the 3rd. The underlying neural bones are elongated, hexagonal, and short sided anteriorly. The 4th vertebral covers three neurals (4-5 in Kachuga). The adult carapace is light brown to olive with three broad, black longitudinal stripes. The plastron is well-developed, but smaller than the shell 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 neurals; 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 > fem > pect > an >< hum > gul. Both plastron and bridge are uniformly yellow or cream colored. The head is small to moderate in size with an upturned, pointed, projecting snout, and a shallow medial notch on the upper jaw. The back of the head is covered with small scales. In the skull, the temporal arch is complete and the quadratojugal touches both the jugal and postorbital. The frontal bone does not enter the orbital rim. The maxilla does not touch either the parietal or the squamosal. The orbito-nasal foramen is large. Triturating surfaces of the jaws are broad with a single medial denticulated ridge; the sides of the jaws are serrate. The limbs have enlarged transverse scales. There are five claws on the forefeet, and all toes are heavily webbed. Limbs and other soft parts are normally olive to gray.
Diploid chromosomes total 52; 28 macrochromosomes (18 metacentric or submetacentric; 10 telocentric or subtelocentric) and 24 microchromosomes (Carr and Bickham, 1986).
The head of the female is olive, that of nonbreeding males charcoal gray. During the mating season the male's head becomes white and a red stripe develops between the eyes (Moll et al., 1981). In addition to the sexual dichromatism, males are shorter (to 39 cm) than adult females (41+ cm) and have longer, thicker tails.
Callagur ranges from extreme southern Thailand southward through Malaysia to Sumatra and Borneo.
Adults live mainly in estuaries and tidal reaches of medium to large rivers. Juveniles and hatchlings are less tolerant, and probably inhabit freshwater portions of these same rivers. However, Dunson and Moll (1980) demonstrated that the hatchlings can survive for at least two weeks in 100% sea water. Apparently this allows them enough time to make the migration from their sea-beach nests, some of which may be as far as 3 km from the mouth of a river, to fresh water.
Nesting occurs from June to August. In extreme southwestern Thailand females migrate up rivers to exposed sand bars to nest. In peninsular Malaysia, nesting occurs on the same sea beaches as Dermochelys coriacea, Chelonia mydas, and Lepidochelys olivacea (Dunson and Moll, 1980). The female emerges from the sea, selects a site, and then rapidly digs a 30-cm-deep hole, lays her eggs, covers them, and returns to the sea. Eggs are ellipsoidal (68-76 x 36-44 mm) with pliable shells; about 12 are laid in each clutch. Hatchlings are rounded with dark spots on each vertebral and one on the posterior border of each pleural scute (De Rooij, 1915).
Adults are almost entirely vegetarian; any animals consumed are probably taken accidentally.
McDowell (1964) thought Callagur borneoensis to be most closely related to Kachuga trivittata, differing from that species in its elongated neural bones, narrower triturating surfaces, and greater size. He suspected C. borneoensis might be the end representative of a single K. kachuga-K. trivittata-C. borneoensis superspecies.
IUCN Red List Status (1996)
Critically endangered (A1bcd).