99 Percent of Great Barrier Reef Green Sea Turtles Are Hatching Female

Australia's 1,200-mile (2,000 kilometer) Great Barrier Reef is an enormous climate change experiment that's not happening in the safe isolation of a laboratory. Instead, the warming waters off the east coast of the continent have a profound real-world effect on thousands of miles of coral, as well as the animals that live there. For decades, scientists have suspected that increases in oceans temperatures would affect sex ratios in certain animals, and new research shows that exactly what's happening to the Pacific green sea turtle.

In most of Earth's creatures, gender is determined during the fertilization process. That's not true of animals like turtles, crocodiles, and alligators, which rely on a concept called temperature-dependent sex determination (or TDS) to dictate the sex of their offspring. In the case of turtles, warming waters and sands are altering the TDS process.

During the breeding season, turtles flop ashore and bury their eggs in the sand. The temperature of that sand determines whether baby turtles will wind up with blue or pink flippers, figuratively speaking. If the incubation temperature is below 81.86 degrees Fahrenheit (29.7 Celsius), the little turtles will hatch as males; above 87.8 degrees F (31 C), the babies will be females.

To see how varying temperatures might affect turtle populations, scientists compared sex ratios of turtles near multiple breeding grounds near the Great Barrier Reef. They used blood tests and laparoscopy to determine the sex of these animals, which can grow to nearly 500 pounds (227 kilograms), with a shell diameter of 4 feet (1.22 meters).

At the southern edge of the reef, near Brisbane, water temperatures are cooler, and female turtles outnumber males by a ratio of 2 to 1 (about 65-69 percent female). However, about 1,200 miles north, in the largest and most critical sea turtle rookery in the Pacific Ocean, warmer sea and air temperatures are having a dramatic affect – 99 percent of hatchlings are females.

Although each male can mate with more than one female during a breeding season, a severe imbalance in sex ratios doesn't bode well for temperature-sensitive species like sea turtles. Furthermore, once the incubating sand becomes too warm, it outright kills the developing organism, further threatening turtle populations.

"Our study highlights the need for immediate management strategies aimed at lowering incubation temperatures at key rookeries to boost the ability of local turtle populations to adapt to the changing environment and avoid a population collapse—or even extinction," the researchers write. The study was published in January 2018 in the journal Cell Biology.