A warmer external temperature means 99 percent of baby green sea turtles are turning into females, endangering the worldwide population

The temperature outside the growing egg, not its chromosomes, determines the sex of a baby green sea turtle. And hotter external temperatures mean female sea turtles. This biological mechanism, according to scientists and reported by the Washington Post, is endangering the long-term survival prospects of the entire species.

What’s going on with green sea turtles? 

A green sea turtle’s sex is a product of the environment, and some populations are already so lopsided that almost all members are female, according to a report in the journal Current Biology.

“They have temperature-dependent sex determination,” said Camryn Allen, a National Oceanic and Atmospheric Administration endocrinology researcher and co-author of the new study to the Washington Post.

“It’s not genetics. It’s actually the temperature.”

At the so-called “pivot-temperature,” turtles hatch in a mixture of males and females. The temperature for green sea turtles is 85 F (29.3 C). If the temperature is a few degrees lower, turtles are born male; a few degrees higher, and the baby turtles are all females.

“That transitional range, from 100 percent males to 100 percent females, spans a very narrow band of only a couple of degrees,” said NOAA marine biologist and study co-author Michael Jensen to the Washington Post. 

And with hotter air and water temperatures becoming the norm worldwide, researchers are concerned that the population of green sea turtles is at risk. 

Research in Australia 

Raine Island on Australia’s east coast holds prime turtle-nesting grounds. Around 200,000 turtles lay their eggs on the beaches here, making it one of the world’s largest gathering spots for green sea turtles and a prime area for research. During their research, the scientists collected turtles over the course of several weeks, scientists took plasma samples and released the animals, using a new technique to reveal their sex by studying their hormones.

The study’s authors used historical records of sea and air temperatures in the island’s breeding grounds, dating from 1960 to 2016 to determine if weather was at play. By the 1990s, they found, the sand temperatures were consistently higher than the aforementioned pivot temperature for green sea turtles. They agreed that climate change was the culprit.

Scientists confirmed that predictions from previous studies — that green sea turtles and other temperature-dependent reptiles might be changing in response to a warming climate — are coming true. But none of them had seen the effect played out quite so dramatically, finding that more than 99 percent of newly hatched turtles here were female and 87 percent of mature turtles were female. For every juvenile male, there were 116 female turtles.

What are the study’s implications?

A crash in the green sea turtle population is not imminent, as long as the multitudinous female population can find males to fertilize their eggs. Luckily, turtles do not need a 50:50 ratio of males to females. “A few males can go a really long way,” Jensen said to the Post. “Male turtles mate more frequently” than female turtles do.

Scientists went on to highlight another nesting population of green sea turtles, 1,000 miles south of Raine Island. Here, the water temperature stays consistently cooler, and the turtle sex ratio is one male turtle to two female turtles.  “These two populations at opposite ends of the reef have massively different sex ratios,” said Jensen.

As turtles do not migrate to breed, returning instead to the beaches they hatched on, management strategies are essential to make sure this particular population does not die out. The Australian government, working with the Raine Island Recovery Project is working to help protect and monitor the turtles, and it’s hoped that they can adapt before it’s too late.

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