(Grandidier, 1867)
Madagascar big-headed turtle
Recognition
Erymnochelys madagascariensis has an oval carapace (to 43.5 cm) which is flattened dorsally, lacks a vertebral keel, and is unserrated posteriorly in adults. Hatchlings and juveniles have a medial vertebral keel. Vertebrals are broader than long; the shortest is the 5th which is flared posteriorly. No cervical scute is present, and the posterior marginals may be somewhat flared or upturned over the tail and legs. Rugose radiations are usually present on the surface of the carapacial scales, although the scales of very old turtles may be worn smooth. The carapace is olive to grayish brown. The plastron is long and narrow, not completely covering the carapacial opening. Its anterior lobe is rounded anteriorly and is broader than the posterior lobe, which tapers toward the rear and has a shallow anal notch. The intergular scale is small and does not completely separate the gulars. The plastral formula is: abd > pect > fem > an > intergul > hum > gul. The broad bridge is approximately equal to the width of the plastral posterior lobe. Plastron and bridge are yellow to brown. The large, broad head has a protruding snout, a slightly hooked upper jaw, and a hooked lower jaw. Only a single weak ridge is present on the triturating surface of the maxilla. In the skull, the jugal and quadrate are in contact, but the quadratojugal extends to the parietal separating the postorbital and squamosal bones. Premaxillae extend backward to separate the maxillae and usually reach the choanal rim; the incisive foramen lies completely within the premaxilla. Also, the fossa housing the geniculate ganglion is larger than in other podocnemines. No groove occurs between the eyes, and the interparietal scale is large but does not separate the parietals. Usually only one chin barbel is present, but some individuals may have two. The head is brown to reddish brown dorsally, yellow laterally, and has yellow jaws. The neck is olive, gray, or brown dorsally, but yellow ventrally. Limbs are olive or gray with webbed toes. Three large scales occur on the posterior border of the hind foot.
The karyotype consists of 28 chromosomes; 6 large to medium-sized metacentrics and submetacentrics, 4 large to mediums-sized subtelocentrics, and 18 small to very small metacentrics and submetacentrics (Rhodin et al., 1978; Bull and Legler, 1980).
Males have longer, thicker tails than do females.
Distribution
Erymnochelys is restricted to the island of Madagascar, from the Mangoky River in the southwest to the Sambirano basin in the north.
Habitat
Erymnochelys lives in slow-flowing rivers, streams, swamps, lagoons, and marshes.
Natural History
Nesting has been reported in July (Tronc and Vuillemin, 1973), September-October (Jenkins et al., 1990), October-November (Kuchling, 1988) and January (Tronc and Vuillemin, 1973). Females lay up to three clutches totaling more than 60 eggs (Glaw and Vences, 1994). The eggs are approximately 34-40 x 20-30 mm (Kuchling, 1993a and literature cited in that publication). A study by Kuchling (1993c) indicates that this species has a biennial ovarian cycle, unusual among freshwater turtles.
E. madagascariensis is omnivorous, feeding mainly on the snail Melanoides tuberculata, shoots and roots of the reed Phragmites mauritianus and (dead) fish, but also on insects, shrimps, nuts, seeds and leaves (Kuchling, 1993a). Kuchling's data suggests that at low water levels, females and juveniles are mainly carnivorous, but they apparently switch to an omnivorous diet when the water level rises; males seem to be largely carnivorous throughout the year. Local fishermen told Kuchling (1993a) that E. madagascariensis is less active in the months May to August/September.
Remark
Although often included in the genus Podocnemis, this species differs from those turtles, and also from Peltocephalus, in having an additional central bone in its foot and an iliac-carapacial connection contacting the suprapygal. Frair et al. (1978) showed that E. madagascariensis has a distinct blood chemistry.
IUCN Red List Status (1996)
Endangered (A1cd+2d). Although it is fully protected on Madagascar, E. madagascariensis in many parts of its range is rapidly declining as it is often caught (and slaughtered) as a by-product of fishing; an additional threat to its survival is the conversion of its habitat into agricultural area (Kuchling and Mittermeier, 1993). Competition with the recently expanding Pelomedusa and Pelusios, by some authors regarded as an additional threat, seems unlikely, as these species occupy different niches (Kuchling, 1993a).