Climate turns up heat on sea turtles

The ancient mariners need beach temperatures that are just right to hatch their eggs. If it's too warm, only females are born – and a species could vanish.

By Moises Velasquez-Manoff | Contributor to The Christian Science Monitor

from the June 21, 2007 edition

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Tortuguero National Park, Costa Rica - Every three days, researchers from the Caribbean Conservation Corporation walk the beaches here counting sea turtle nests. Four species nest in its black sands – leatherback, green, loggerhead, and hawksbill – each at a different time of year, at a different area on the beach, and at a different depth in the sand. The researchers record whether the female laid eggs or just poked around and, unsatisfied, returned to the sea. And they note if humans, the greatest threat to these ancient mariners, have dug up any eggs.

Sea turtles have navigated the world’s oceans for some 200 million years. They saw the dinosaurs come, and they saw them go. But of the seven species of sea turtles, six are endangered. While conservation efforts have led to some rebound in the Atlantic and Caribbean, the picture is much grimmer in the Pacific. Last year, 46,784 green turtles visited an 11-mile stretch of Tortuguero that researchers have monitored since 1971 – a 450 percent increase. But between 1980 and 2002, female leatherbacks nesting along the west coast of the Americas dropped from an estimated 91,000 to less than 2,000. [Editor's note: The original version of this story misstated the number of green sea turtles at Tortuguero National Park. The figure quoted refers only to an 11-mile stretch of beach researchers have been monitoring since 1971, not the entire beach at the park.]

Scientists worry that climate change, on top of ongoing stresses, could deal the final blow to these creatures. Ongoing threats include habitat loss, poaching, and being caught in nets and on hooks intended for other prey. Now, in a warming world, scientists also foresee stronger storms increasing erosion of the sandy beaches the turtles use to nest. Rising seas will inundate existing beaches, even as human development halts natural beach migration inland and upland. Shifting currents may alter the ocean's upwelling patterns, which turtles depend on for food.

Finally, there's global warming's most direct effect – more heat.

Turtles lack sex chromosomes. Their genes do not directly determine whether a hatchling comes out male or female. Instead, buried eggs take their cue from the ambient temperature. For leatherbacks, temperatures below 29.4 degrees C (85 degrees F.) produce a clutch that is mostly male; above that, it's mostly female. With a mere 2 degree C (3.6 degrees F.) increase, a nest will produce all females. A few degrees higher yet, and the "boiled" eggs don't hatch at all.

In order to maintain a viable breeding population, a cool, male-producing year has to come at least once every five to 10 years, says James Spotila, a professor of environmental science at Drexel University in Philadelphia. If male years begin to come only every 20 years because of climate change, the turtle could become extinct.

Beaches at Playa Grande on Costa Rica's Pacific coast are already producing clutches that are 70 to 90 percent female, depending on the year, Dr. Spotila says. At Junquillal (Hoon-ki-YAL) Beach on the Pacific coast, where it is often too hot for eggs to hatch at all, scientists have begun moving eggs to nurseries – essentially holes dug to a certain depth on cooler areas of the beach. When the hatchlings emerge, rangers chaperone them from the nest to the water, protecting them against human and animal predators alike.

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