Travelers to the islands of the Galápagos are far more likely to see a sea turtle in the wild than a tortoise, but in many ways the experience can be more rewarding. Green sea turtles (Chelonia mydas) are found throughout the warm waters of the world, and between 1,200 to 3,000 females nest each year on six of the Galápagos islands. Typically they weigh between one and two hundred pounds, though some have been recorded up to 330 pounds; females tend to be larger than males. Their hard shell varies in color from black to green to yellow, regardless of sex.

Though the variety found in the islands is superficially browner than its continental cousins, ther is evidence of interbreeding between the two varieties, so speciation has not occurred. Sea turtles are able swimmers, and the local variety has been found as far as 1500 miles from the Galápagos along the South American coast. They feed primarily on seaweed, and spend large periods of time underwater, "sleeping" on shallow sandy bottoms. They are usually seen by tourists from boats as they raise their heads above water to gulp air before disappearing for another long dive.

The nesting behavior of the sea turtle can be quite spectacular. There is no pair bonding -- a female may mate with several, if not dozens of males, a process which can be exhausting. Female turtles are often seen hauled upon on the beach just above the surf line, taking a break from male attention. When she is ready to lay eggs, the female drags herself well above the high tide line, digs a large pit in the loose sand, and lays about 75 eggs at a time. These egg-laying excursions take place up to eight times during a breeding season, each session separated by a couple of weeks.

The high proportion of eggs for each individual -- about 600 to 1 -- is necessitated by the low survival rate of turtle eggs, due to climatic and tidal conditions as well as natural predators such as egg beetles, crabs, hawks, mockingbirds and frigatebirds. In the Galápagos, sea turtles also have "unnatural" predators -- feral pigs and rats are serious problems in some areas, though the relative low density of human population has so far delayed direct threat.