Afro-American river turtles

(Podocnemididae)

Class Reptilia

Order Testudines

Family Podocnemididae

Thumbnail description
Large sideneck turtles with four claws on the hind feet, a complete series of seven neural bones posterior to which the pleurals meet at the midline, mesoplastral bones present, and the pelvis is fused to the plastron

Size
Up to 42.1 in (107 cm) carapace length

Number of genera, species
3 genera; 8 species

Habitat
Freshwater rivers, streams, flooded forests, lakes, and ponds

Conservation status
Endangered: 2 species; Vulnerable: 4 species; Lower Risk/Conservation Dependent: 1 species

Distribution
Madagascar and northern South America, although fossils have been discovered in freshwater, marine, and terrestrial habitats nearly worldwide

Evolution and systematics

Podocnemididae is most closely related to Pelomedusidae, but diverged from that family at least by the Cretaceous (at least 110 million years ago). Although the living forms occur in fresh water in South America and Madagascar, this family, including freshwater and marine forms, has an extensive fossil record in North and South America, Europe, Asia, and Africa. The largest turtle that ever lived, Stupendemys geographicus, at up to 7.5 ft (2.3 m) shell length, belongs to this family.

Physical characteristics

These large sideneck turtles have a complete series of seven neural bones, posterior to which the pleurals meet at the midline; mesoplastral bones are present; the pelvis is fused to the plastron; there is no cervical scute; an intergular scute touches the dorsal rim of the plastron; and there are four claws on the hind feet.

Distribution

Madagascar and Atlantic drainages of northern South America, although fossils are known from freshwater, marine, and terrestrial habitats nearly worldwide.

Habitat

Most Podocnemididae are riverine (living on the banks of a river) or large lake species, although one species inhabits ponds and streams.

Behavior

The behavior of these turtles is highly variable. Some species are almost totally aquatic, migratory, and riverine, leaving the water only to nest colonially on seasonally inundated midstream sandbars. Others inhabit smaller tributaries or flooded forest pools, nesting in groups on sandy river banks; and one species (Podocnemis vogli) inhabits streams and ponds, migrates considerable distances overland to nest, and even estivates underground when the aquatic habitat dries up. Some species (and mainly females) are known to bask.

Feeding ecology and diet

These turtles are primarily herbivorous, consuming mainly the fruits of riparian trees, but also leaves, stems, and grasses. They occasionally eat freshwater sponges, mollusks, crustaceans, insects, and fish.

Reproductive biology

The courtship and mating of the Podocnemididae have not been described. The more aquatic species may migrate considerable distances to nesting beaches. The females nest predominantly on sandbars and river banks, often in large numbers. The leathery eggs range from nearly spherical in the largest species, to oblong in the others. Clutch size ranges from five to 20 in the smallest species, to as many as 156 in the largest. Incubation is completed in 40–149 days. All studied species exhibit temperature-dependent sex determination,

with warm temperatures producing females, and intermediate temperatures producing males. In some species still cooler temperatures may again produce mostly females.

Conservation status

All species are exploited by humans for their meat or their eggs. At one time nearly 124,000 of these turtles nested on a single beach during a six-week season. However, an estimated 33 million eggs were once harvested annually from just three beaches, and an average of over 6,000 adult females were slaughtered annually from a single beach. Because of this harvesting intensity, all species have been reduced in numbers, four to the point of being listed at Vulnerable to extinction on the IUCN Red List, and two more considered Endangered. Efforts are being made to protect the remaining colonial nesting beaches.

Significance to humans

These turtles are exploited primarily for their eggs and their meat.

Species accounts

List of Species

South American river turtle

Podocnemis expansa

taxonomy

Emys expansa Schweigger, 1812, "America meridionali" (South America). No subspecies are recognized.

other common names

English: Arrau, giant South American river turtle; French: Podocnémide élargie; German: Arrauschildkröte; Spanish: Arrau.

physical characteristics

South American river turtles are large sideneck turtles, up to 42.1 in (107 cm) in shell length, with a broad, flat carapace that is wider posteriorly than anteriorly, usually having two barbels on the chin, a broad skull, and with the front of the upper jaw squared off (as opposed to notched or rounded). Juveniles have large lemon-yellow spots on the head within which are one or two black spots.

distribution

Orinoco and Amazon River basins of northern South America.

habitat

These turtles primarily inhabit large rivers, but will venture into flooded areas adjacent to rivers during high water.

behavior

This species migrates considerable distances up or down rivers to localized, colonial nesting beaches. Two to three weeks prior to nesting, females bask on sand beaches in the morning and the late afternoon.

feeding ecology and diet

This primarily herbivorous species apparently feeds predominantly on the fruits of riparian trees. Leaves and stems are also taken, as are freshwater sponges and occasional insects.

reproductive biology

Courtship has not been described, but DNA studies reveal that eggs in the same clutch are often fertilized by different males. As water levels subside after the peak of the rainy season, females migrate to the main river channels and upstream or downstream to sandbars to nest. The nesting season is short, lasting only 10–60 days (typically 25–45) in February and March on the Orinoco, and September to October (or even December), at various Amazon River branches. Most nesting occurs at night, after midnight. Females use all four limbs to excavate a body pit about 3.3 ft (1 m) in diameter and 1.6 ft (0.5 m) deep. They then excavate the actual nest chamber in the bottom of this pit, using only the hind limbs. Following egg laying, the female covers the chamber and usually fills the body pit, in either case using her hind limbs. This species is unique in its family in producing eggs that are nearly spherical, averaging 1.6 in (4 cm) in diameter. However, females occasionally produce one or two "giant" eggs, which can be as large as 3.2 in (8 cm) in longest diameter. Clutch size ranges from 48 to 156 eggs, although around 80 is typical. Larger females lay larger clutches of larger eggs, which are buried deeper than those produced by smaller females. Only a single

clutch is laid per year. Incubation is rapid in the very warm sand of the nesting beaches, requiring only about 45 days. Over the following two to three days, the hatchlings dig out as a group, generally emerging at night or in the early morning to avoid lethally high ground temperatures. This species exhibits temperature-dependent sex determination, with temperatures above 90.7°F (32.6°C) producing females, and temperatures below that producing mostly males; however, females may be produced again at still cooler temperatures. The pivotal temperature of 90.7° (32.6°C) is the highest known for any turtle.

conservation status

This species is listed as Lower Risk/Conservation Dependent by the IUCN. Exploitation by humans has virtually eliminated this turtle from most of the upper Amazon River basin, and populations across the range are much reduced, in spite of national legal protection and concerted efforts to protect the remaining nesting beaches from disturbance, harvesting, predation, and flooding.

significance to humans

Adults and even hatchlings are harvested for their flesh, and eggs are collected for the oil that can be extracted from them. This harvesting is technically illegal and at the local subsistence level, but it remains to be seen whether this species can ever recover from the last four centuries of overexploitation.

Yellow-spotted river turtle

Podocnemis unifilis

taxonomy

Podocnemis unifilis Troschel, 1848, "im Rupununi und Takutu" (rivers in Guyana). No subspecies are recognized.

other common names

English: Terecay, yellow-headed sideneck, yellow-spotted Amazon turtle; Spanish: Tracaja.

physical characteristics

Yellow-spotted river turtles are large sideneck turtles, with a maximum shell length of 26.8 in (68 cm), an oval carapace bearing a low keel on the second and third vertebral scutes, and a slight medial indentation anteriorly. Only a single barbel is usually present under the chin. Juveniles have yellow-orange spots on the head.

distribution

These turtles occur in the tropical lowlands of northern South America, including the Orinoco and Amazon River basins.

habitat

Yellow-spotted river turtles inhabit freshwater rivers, streams, lakes, ponds, and flooded forests.

behavior

These turtles frequent rivers and large lakes during times of low water, but during the rainy season they migrate far into flooded forest areas. Females migrate to sand beaches along the main river courses to nest, and bask on the shoreline prior to nesting season.

feeding ecology and diet

Primarily herbivorous, this species feeds on the fruits of riparian trees, water hyacinths, and grasses. They also occasionally eat clams and dead fish.

reproductive biology

Nesting season is timed with the period of low water and hence varies geographically, occurring in late January to early March in Venezuela, July to December in Colombia, October to February in Peru, and June to July, September to October, or during December at Brazilian sites. Females typically emerge to nest on sandbars just after dark, and excavate and cover their shallow nests only with their hind limbs. The eggs are elongate and average about 1.8 by 1.2 in (4.5 by 3.0 cm), and 0.9 oz (25 g). Clutch size ranges from four to 49 eggs, 20–30 being usual, with larger females producing larger clutches. At least two clutches may be produced each season. This species exhibits temperature-dependent sex determination, with warm temperatures (greater than 89.6°F [32°C]) producing females, and cool temperatures producing males.

conservation status

Although listed as Vulnerable, this species is still exploited by local people. Females are captured on nesting beaches, fished for with hooks, and speared from the water, and eggs are harvested from nests.

significance to humans

Adults and eggs are harvested for human consumption.

Madagascan big-headed turtle

Erymnochelys madagascariensis

taxonomy

Dumerilia madagascariensis Grandidier, 1867, "Mouroundava Tsidsibouque flumina in occidentali insulae Madagascar littore" (Morondava and Tsidibou rivers on the western coast of Madagascar). No subspecies are recognized.

other common names

None known.

physical characteristics

Madagascan big-headed turtles are medium-sized, sideneck turtles, with a maximum shell length of 19.7 in (50 cm), and a flattened oval carapace that lacks keels. The connection of the pelvis to the carapace contacts the suprapygals.

distribution

These turtles occur only in the western drainages of Madagascar.

habitat

This species inhabits slow-moving rivers, streams, ponds, and swamps.

behavior

Very little is known about the behavior of Madagascan bigheaded turtles. They apparently often spend the dry season buried in the mud.

feeding ecology and diet

These turtles are generally omnivorous, with adult females feeding primarily on the shoots of Phragmites, and the adult males feeding mainly on snails and fish. Juveniles eat both plant and animal material, including insects and crustaceans.

reproductive biology

Nesting occurs primarily during October and November, but may extend beyond that month. Females lay one to three clutches in a season, but apparently reproduce only every other year. Clutch size ranges from 10 to 30, and eggs average 1.5 by 0.9 in (38 by 24 mm), and 0.4 oz (10.2 g). The effects of temperature on sex determination have not been studied.

conservation status

This species is cited as Endangered on the IUCN Red List.

significance to humans

These turtles are captured by humans for food in baited hoop nets or traps, on hook and line, with spears or harpoons, by diving for them, and on nesting beaches. The eggs are also taken from the nesting beaches.

Resources

Periodicals

Escalona, T., and J. E. Fa. "Survival of Nests of the Terecay Turtle (Podocnemis unifilis) in the Nichare-Tiwadu Rivers, Venezuela." Journal of Zoology 244 (1998): 303–312.

Kuchling, G. "Possible Biennial Ovarian Cycle of the Freshwater Turtle Erymnochelys madagascariensis." Journal of Herpetology 27, no. 4 (1993): 470–472.

——. "Biologie und Lebensraum von Erymnochelys madagascariensis (Grandidier, 1867) und Vergleich mit den andern Wasserschildkröten Madagaskars." Salamandra 28, nos. 3 and 4 (1993): 231–250.

Thorbjarnarson, J. B., N. Perez, and T. Escalona. "Nesting of Podocnemis unifilis in the Capanaparo River, Venezuela." Journal of Herpetology 27, no. 3 (1993): 344–347.

Valenzuela, N. "Multiple Paternity in Side-Necked Turtles Podocnemis expansa: Evidence from Microsatellite DNA Data." Molecular Ecology 9 (2000): 99–105.

——. "Constant, Shift, and Natural Temperature Effects on Sex Determination in Podocnemis expansa Turtles." Ecology 82, no. 11 (2001): 3,010–3,024.

——. "Maternal Effects on Life-History Traits in the Amazonian Giant River Turtle Podocnemis expansa." Journal of Herpetology 35, no. 3 (2001): 368–378.

John B. Iverson, PhD