The giant river turtle is, as its name suggests, the largest of all 25 or so species of freshwater turtles
found in the Amazon River basin. Also called the arran (or arrau) turtle (scientific name Podocnemis
expansa), an adult can weigh more than 100 pounds (45 kg), measuring over 3 feet (91 cm) long. The
carapace (shell) is dome-shaped but somewhat flattened to streamline the shape and help the turtle swim against
river currents where necessary. The shell is greenish brown, perhaps due to algae that live on the surface.
This is an example of commensalism, where one organism
benefits without any effect on the other.
The head may be mottled yellowish-green so that young arrans can be confused with the yellow spotted Amazon
river turtle (P. unifilis). This species belongs in the side-necked turtle family (Pelomedusidae). It
has a long neck compared with those turtles that withdraw the head vertically into the shell. Rather this
group of turtles swings the head to the side to retract it in between the top and bottom half of the
carapace. Evidently this offers less protection than if it were vertically retracted.
HABITAT AND DISTRIBUTION
This species is found in northern South America from the Orinoco River in Venezuela to the southern Amazon
River basin in Brazil. It prefers low-lying lakes and ponds and other areas with open water, and during the
wet season ventures deep into flooded forest areas.
FEEDING AND DIET
The giant river turtle is herbivorous and forages among the half-submerged trees of the flooded forest when
the river rises due to seasonal rains. Its diet comprises mostly fruits, flowers, roots, and the soft
tissues of aquatic plants. As the rains taper off, the turtle's feeding declines and until the waters
begin to rise again it feeds very little if at all.
As with other aquatic reptiles, the female giant river turtle is committed to returning to land in order to
lay her eggs. And, as with other turtles, this species lays its eggs in sandy riverbanks or beaches. The
female uses her back flippers to excavate a hole where she lays between 60 and 150 eggs. (See Vanzolini 2003.)
Incubation is among is very rapid for a turtle of this size. Eggs hatch 40 to 50 days after laying. This
reflects selection for fast incubation due the unpredictability of rising river levels. The risk of the
nest being swamped favors as short a time as possible between laying and hatching. Part of the reason
for the rapid development of the egg may be the high nest temperatures, reaching over 100°
F (40° C), the highest of any turtle species. As with most other turtles, the nest temperature determines
the sex of the developing turtle.
Depending on food availability the female may lay one or two clutches in a single season. If the sandbank is
already home to overcrowded nests, it is inevitable that she will accidentally expose other clutches,
whereupon the predators move in. Vultures (others?) saunter nonchalantly, always ready to take advantage of
mistakes caused by the female turtle's inexorable instinct.
When the hatchlings emerge, they are vulnerable to a host of additional predators: raptors, wading birds,
predatory mammals, caimans and large fish. It's a lucky turtle that outgrows the threat to begin the
The giant river turtle was first listed as endangered in 1982 due to over-hunting for its meat, shell and
eggs, and loss of habitat. The meat is reputed to be delicious and several projects have attempted to
establish populations in captivity. Historically, Indians are said to have captured the animals and kept
them in pens for later use, but they do not breed in captivity (Kricher 1997).
Indeed, John Kricher (1997: 211) cites studies that show turtles could yield 400 times the amount of meat
compared with cattle raised on the same amount of pasture converted from rainforest. If turtles could be
raised for food, pressure on wild populations would be relieved, and the prospects for long-term survival of
the species would be much improved.
In the 1970s, the Brazilian government created a government agency to protect the turtles and outlawed the
hunting of turtles or eggs. The government even assigned armed guards to protect the nests from egg hunters.
As a result, the population rebounded, showing that the species has the reproductive capacity to rebound
from serious declines. However, the loss of habitat has a longer-term consequence. The beaches where the
females lay their eggs are being rapidly gobbled up by human settlement, farming and mining projects.
Millions of these animals would nest each year but the lack of suitable nest sites is a loss from which they
can never recover. The future of this species remains in doubt.