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: Arrauschildkrote; Spanish: Arrau.
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.
Orinoco and Amazon River basins of northern South America.
These turtles primarily inhabit large rivers, but will venture into flooded areas adjacent to rivers during high water.
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.
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.
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.
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.
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. ♦
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