Evolution of Sea Turtle Migrations

Each year around Christmas time, green turtles (Chelonia mydas) leave their shallow feeding grounds along the coast of Brazil and embark upon a 2000 km journey to their nesting grounds, the beaches of Ascension Island, in the mid-Atlantic. The journey takes a little more than 2 months in both directions, and is a miracle of navigation. Biologists have long wondered how the turtles manage the feat, and also why they bother to do it at all.

Fifteen years ago two researchers, Archie Carr and Patrick J. Coleman, advanced an ingenious hypothesis that appeared to take the mystery out of the migration. The answer, they said, was continental drift. The turtles were assumed always to return to the beach at which they were born: a behavior that biologists call natal homing. For the ancestors of the modern turtle species, this was not a major problem 40 million years ago because the mid-Atlantic was then on Brazil's geological doorstep.

However, with the distance between Brazil and the volcanic islands of the mid-Atlantic ridge widening by some 2 cm a year through sea floor spreading, the passage of 40 million years transformed a short journey into an heroic odyssey. Locked into the need to nest on the beaches of Ascension Island (and its forerunners) by the biological imperatives of natal homing, the green turtle gradually adapted to an ever longer migration. Carr and Coleman acknowledged that theirs was an audacious hypothesis, but it also seemed persuasive.

Roger Lewin, Sea Turtle Migrations and Continental Drift

• evolution