(Duméril and Bibron, 1835)
This turtle represents the terminal southward extension of the Trachemys scripta series in the Americas. Its oval to elongated (to 26.7 cm) carapace is moderately domed (more so in females), contains a low medial keel (more pronounced in juveniles), and is serrated posteriorly. Vertebrals are usually broader than long. The carapace is brown to olive with variously shaped red, orange, or yellow markings on each scute. These marks are usually dark bordered and range from elongated, straight or curved streaks to rounded, light-centered ocelli. Each marginal has a light-colored vertical bar. The plastron is large and contains an anal notch. Its formula is: abd > gul > pect > an > fem > hum. The plastron is yellow or orange with an intricate, broad, dark, seam-following pattern. In juveniles, this pattern contains numerous hollow light areas which become progressively darker with age until the entire plastral surface is almost covered with dark pigment. Males become melanistic until the carapaceandplastron are totally black. The head is of moderate size with a somewhat projecting, pointed snout and a slightly notched upper jaw. Skin of the head is green to brown, with numerous yellow to orange, black-bordered stripes. The rather wide supratemporal stripe does not touch, or only narrowly touches, the orbit. Below this on the side of the head are usually three narrower stripes running from the orbit to the neck, and then another wide stripe running from the lower surface of the orbit downward and backward to the neck. The snout contains several narrow stripes on each side. On the chin there are several longitudinal stripes, the middle of which is wide and forks as it passes backward toward the neck. At each corner of the mouth is an elongated black-bordered spot. Limbs and neck are green to brown with yellow stripes.
Males have elongated foreclaws and long, thick tails.
Trachemys dorbignyi has been found at São Luís, Maranhão, Brazil, and in the Rio Guaíba drainage near Pôrto Alegre in Rio Grande do Sul, Brazil. It also occurs throughout Uruguay and in the Paraná and Uruguay drainages of northeastern Argentina.
Two subspecies have been described. The southern Orbigny's slider, Trachemys dorbignyi dorbignyi (Duméril and Bibron, 1835), occurs in northeastern Argentina and Uruguay. This turtle is generally brown with orange stripes, and the supratemporal stripe is not enlarged. Ground color of the plastron is orange. T. d. brasiliensis (Freiberg, 1969), the northern Orbigny's slider, inhabits the Rio Guaíba of Rio Grande do Sul, Brazil, and has also been found at São Luis, Maranhão, in northern Brazil. It is green with yellow or pinkish red stripes and a broad supratemporal stripe. The plastron is yellow to green, and, according to Freiberg (1969), males may lack plastral markings.
This species occurs in various bodies ranging from ponds, lakes, marshes, swamps, streams, and rivers. It seems to prefer waters with slow to moderate currents, soft bottoms, abundant aquatic vegetation, and a prevalence of basking sites.
Size and age at maturity and the gametic cycles of both sexes are unknown. Courtship and mating have not been described. According to Freiberg (1981), nesting takes place in December, and the nest chamber is about 30 cm deep. Freiberg (1967) reported that Argentine females lay several clutches each season, and Vanzolini (1997) dissected a female collected in Brazil on 30 October that contained two sets of oviducal eggs, probably representing two different clutches. The first set of oviducal eggs consisted of 16 shelled, ellipsoidal (37-41 x 23-27 mm) eggs. The 25 unshelled eggs composing the second set were almost spherical in shape and 9.1-20.4 mm in diameter. The female's ovaries also contained 138 enlarged, vitellogenic follicles and some smaller ones. Krause et al. (1982) excavated 14 Brazilian nests, and reported the clutches in them had 1-16 (mean 10.9) eggs.
Trachemys dorbignyi supposedly is an opportunistic carnivore (Cei, 1993), feeding on various invertebrates, mollusks, fish, and amphibians, but probably it will also take some aquatic plants. Orbigny's slider is an avid basker, often forming large assemblages at good sites.
Moll and Legler (1971) and Pritchard (1979) considered Trachemys dorbignyi to be a subspecies of T. scripta. Williams (1956) and Wermuth and Mertens (1961, 1977), however, retained it as a full species. It is certainly related to T. scripta, especially T. s. ornata and T. s. callirostris, but so are the West Indian Trachemys, which are equally as divergent but still considered separate species. We feel it is best to consider T. dorbignyi as a full species since it is allopatric and no evidence of reproductive compatibility with T. scripta has been demonstrated.
IUCN Red List Status (1996)