Podocnemis expansa

(Schweigger, 1812)
South American river turtle

This is the largest living Podocnemis. Its oval (to 89 cm) adult carapace is flattened, broadest behind the center, and has a smooth posterior rim. At best there is only a weak cervical indentation. A medial keel is usually absent, but if present, it appears as a raised area on the 2nd (and very rarely the 3rd) vertebral. Keeling is more prominent in juveniles. Vertebrals are broader than long in juveniles, but in adults the 2nd is longer than broad. The 4th is the smallest, and the 5th is posteriorly expanded. Posteriorly, the marginals are flared over the hindlimbs. The surface of the carapacial scutes usually lacks ridges or raised annuli. The carapace is olive to dark gray or brown and may have some dark spots and a light border in younger individuals. The plastron is large, but does not completely cover the carapacial opening. Its anterior lobe is broader than the posterior lobe and rounded anteriorly. The posterior lobe tapers toward the rear and contains a posterior notch. The broad bridge is wider than the plastral posterior lobe. The plastral formula usually is: abd > pect > fem > intergul > an > gul > hum. The interhumeral seam may be very short. The intergular is long and narrow; it completely separates the gulars and almost separates the humerals. Plastron, bridge, and undersides of the marginals are yellow. The head is broad with a protruding snout and an upper jaw which is neither notched nor rounded, but rather is squared-off (Williams, 1954a). An interorbital groove is present. Two or three ridges occur on the triturating surface of the maxilla. Premaxillae are short, do not separate the maxillae or reach the choanal rim, and have foramina incisiva at the posterior margin. There is no vomer bone. The tympanum is wider than the orbit. The large interparietal scute tapers posteriorly and may or may not separate the parietals. Subocular scales are absent. Usually two chin barbels are present. The head is gray brown with yellow markings. Two yellow spots occur on the interparietal scutes, and one on each side of the head; these spots fade with age. Jaws are tan; the chin yellow. The neck is gray dorsally and yellow ventrally. Small rounded scales or tubercles occur on the dorsal surface of the neck. Two or three enlarged scales occur on the posterior margin of the hind foot; limbs are gray.
P. expansa has 28 diploid chromosomes; 6 large to medium-sized metacentric and submetacentric chromosomes, 4 large to medium-sized subtelocentrics, 14 small to very small metacentrics and submetacentrics, and 4 small acrocentrics and subtelocentrics (Ayres et al., 1969; Rhodin et al., 1978).
Males have longer, thicker tails, deeper anal notches, and more obtusely rounded heads than do females. Males also retain the juvenile head markings.

Podocnemis expansa occurs in the Caribbean drainages of Guyana and Venezuela and in the upper Amazon tributaries in northern Bolivia, northeastern Peru, Colombia, Venezuela, and northern Brazil. It is also occasionally found on Trinidad, especially after floods of the adjacent mainland Orinoco River.

Podocnemis expansa lives in the larger rivers and their tributaries (both blackwater and whitewater) and in adjacent lagoons and forest ponds.

Natural History
Nesting occurs at night, but may extend past sunrise during the dry season. Vanzolini (1977) reported nesting occurs in Brazil in September and October on the Rio Purus, October on the Rios Trombetas and Tapajos, and in December on the Rio Negro. Pritchard (1979) stated the nesting season is February to April in Venezuela, and Soini (1996) reports the months July-October for the Reserva Nacional Pacaya-Samiria, Peru. The nests are dug in low sand beaches and bars which are vulnerable to flooding. The nesting process involves several distinct phases (Roze, 1964; Alho and Padua, 1982). Suitable nesting sites are apparently rare, and large congregations of Podocnemis expansa gather there at the beginning of the dry season.
Copulation occurs in the water, and the females then begin a period of basking for as much as six or more hours a day, presumably hastening egg development. After several weeks of this, they enter phase two where they retreat into the water after sunset but only for a few hours. Then they emerge to lay eggs in groups; the first nights may only be used for exploring the beach with actual nesting occurring a few nights later. A body pit 80-100 cm deep, similar to that of sea turtles, is dug first and then the actual nest is excavated in the bottom of this pit. Nesting beaches may become so crowded that several females may utilize the same nest cavity. The flask-shaped nest may be 75-80 cm deep (Alho and Padua, 1982).
From 61 to 172 eggs are laid in each nest (Soini, 1996) and possibly more than one clutch is produced each season; the average clutch size in over 11,000 Brazilian nests was 105 eggs (Soares, 1996). Contrary to the eggs of other Podocnemis species, those of P. expansa are spherical (32-54 mm) and leathery shelled. Hatching occurs in an average of 59 days (Soares, 1996). Hatchlings have carapace lengths of 40-45 mm and are more brightly marked than adults.
In the wild, P. expansa is predominantly herbivorous, eating fruits, flowers, roots, and soft vegetation of aquatic plants and also those of the flooded riverine forests during the wet season. They seem to fast during the dry season. In captivity some will take beef and fish. Rhodin et al. (1981) reported that juvenile P. expansa sometimes skim fine particulate matter from the water's surface by neustophagia.

IUCN Red List Status (1996)
Lower risk: conservation dependent. This species is now endangered since it has been greatly overexploited for its meat, oil, and eggs.