It’s night, on a tropical beach, and a 300-pound green turtle is hauling herself up a beach. When she passes the high-tide line, she starts scooping sand away with her hind flippers, creating a deep hole. She fills it with eggs—100 to 200 leathery, white spheres, which she then buries. After an hour or two of hard work, she returns to the sea.
And then, she turns around.
A single female will lay her eggs at several places within the same nesting ground—a reproductive spread-bet that prevents her from losing an entire generation to, say, a storm or an industrious predator. Scientists have assumed that green turtles lay an average of 3.5 clutches each, and counting these clutches helps scientists estimate the global turtle population. But after tagging green turtles in the Indian Ocean, Nicole Esteban from Swansea University has shown that each female lays around twice that number. And if that holds true across other nesting beaches, it means that we might have overestimated the population of this endangered and declining animal by a factor of two.
Sea turtles are enigmatic animals, and for much of their lives, their whereabouts are a mystery. This means that if you want to count the world’s population, an underwater census is a non-starter. The best approach is to wait till they come ashore to lay their eggs. Scientists can canvas these nesting beaches and count the tracks of the females. If you divide that by the number of nests that each female makes, you get the total female population. For example, if you get 300 tracks, and you assume three nests per turtle, you get a total of 100 females. “But if you think the number of nests per individual is six, it’s a very different story,” says Esteban.