More Female Sea Turtles Born as Temperatures Rise

Male sea turtles are disappearing from Australia’s Great Barrier Reef.

A new study of gender ratios found that 99 percent of immature green turtles born in the northern part of the reef are female. Among adult turtles, 87 percent are female, suggesting that there has been a shift in gender ratios over the last few decades.

A sea turtle’s sex is determined by its nesting environment. As sands warm, more females will hatch relative to males; if the sand temperature tops 84.7 degrees during incubation, only females will emerge.

The gender shift suggests that climate change is having a significant effect on one of the biggest green turtle populations in the world, said Michael Jensen, lead author of the new study, published in Current Biology.

“We’re all trying to wrap our heads around how these populations are going to respond to those changes,” said Dr. Jensen, a marine biologist with the National Oceanic and Atmospheric Administration in San Diego.

The gender shift has been noticed before by people who study hatchlings, said Jeanette Wyneken, a sea turtle expert and professor at Florida Atlantic University, who was not involved in the new research.

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But it wasn’t clear until this study that the shift was so dramatic and happening in such a large population across time, she said.

“This is the first paper that’s shown this multigenerational effect,” influencing the gender of juveniles, older adolescents and adults, Dr. Wyneken said.

It takes 35 to 40 years for a green sea turtle to reach sexual maturity, she said.

“These animals are teenagers for an awfully long time,” Dr. Wyneken said. “We won’t see the effects of what’s happening today for several decades.”

David Owens, a professor emeritus from the College of Charleston in South Carolina, was not involved in the new study, but said he’s dreamed of doing such research for years. He praised the way the study team — which included a wide range of expertise — was able to link temperature with turtle gender.

“We’ve had little pieces of this story come out before, but this is the first time anyone’s been able to orchestrate the whole story to bring in global climate change along with feminization,” Dr. Owens said.

A green sea turtle on a reef in Hawaii. If male sea turtles are declining in Australia, researchers say the same is probably happening to sea turtles elsewhere. CreditDavid Fleetham/VW PICS/UIG, via Getty Images

And if the green sea turtles of Australia are seeing this shift, the same thing is probably happening to many other sea turtles and other animals whose gender is determined by temperature, he said.

Sea turtles tend to lay their eggs on the same beaches where they were hatchlings decades earlier, so they can’t quickly adapt to warming temperatures, Dr. Owens said.

It’s possible that they could lay their eggs earlier in the season, instead of waiting for the hottest weeks of the year, but Dr. Jensen said there’s no evidence to suggest that such a shift has or could occur.

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