Green sea turtles do not develop into males or females due to sex chromosomes, like humans and most other mammals do. Instead, the temperature outside a turtle egg influences the sex of the growing embryo.
And this unusual biological quirk, scientists say, endangers their future in a warmer world.
Already, some sea turtle populations are so skewed by heat that the young reptiles are almost entirely female, according to a new report in the journal Current Biology.
"This is one of the most important conservation papers of the decade," said biologist David Owens, a professor emeritus at the College of Charleston who was not a part of this research.
It will not be long, perhaps within a few decades to a century, until "there will not be enough males in sea turtle populations," he warned.
The sex of a green sea turtle is a result of its environment.
"They have temperature-dependent sex determination," said Camryn Allen, a National Oceanic and Atmospheric Administration endocrinology researcher and co-author of the new study. "It's not genetics. It's actually the temperature."
At what biologists call the pivot temperature, turtles hatch as a mixture of males and females.
For green sea turtles, this temperature is 29.3 degrees Celsius (85 Fahrenheit). A few degrees below 29.3 C, all the sea turtles are born male. Heat up the eggs and only females are born.
"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.
Head toward the equator along Australia's east coast and, near the continent's tip, you will arrive at prime turtle nesting grounds.
Some 200,000 turtles lay their eggs at the beaches of Raine Island and nearby cays. It is one of the largest gatherings of green sea turtles in the world.
Green sea turtles play critical roles in their ecosystems. They graze sea grass beds like cattle at pasture, and the turtles' nibbles appear to keep plants healthy.
Where the turtles feed is pristine, Jensen said, untouched by human civilisation. Dugongs and tiger sharks cruise by. And there are loads and loads of turtles.
"It really is the ideal place to study turtles," he said. "There's 30 years of knowledge about this population along the east coast of Australia."
That's where the study authors were waiting. For several weeks, the scientists collected turtles, took plasma samples and released the animals.
It is difficult to distinguish young male turtles from females. Their external features are unhelpful — you cannot simply flip a subadult turtle over and inspect the undercarriage.
In the past, researchers cut open juvenile turtles to inspect their gonads. But the researchers want to minimise their impact on the population; the International Union for Conservation of Nature lists these animals as endangered.
And laparoscopic turtle surgery is an untenable proposition at scale.
Allen worked on a new technique to reveal the turtles' sex through their hormones. The scientists examined the plasma samples in a California lab.
"You can't use genetic tests," Allen said, because "they don't have sex chromosomes like humans do."
Biologists discovered the first turtle thermometer gene in snapping turtles in 2016. Not all turtles regulate their sexes this way, but snapping turtles are like green sea turtles in this regard.
Why turtle sex is linked to temperature remains unclear, though some biologists have a hypothesis: Turtles that develop in colder conditions grow larger, and it might benefit a turtle species if the larger ones are males.
Previous studies had predicted that green sea turtles and other temperature-dependent reptiles might be changing in response to a warmer climate.
Turtles around the world "are absolutely being affected right now," said Owens, who has collaborated with some of the study authors in the past.
"Many of the other species and populations my colleagues are studying are already showing 90 percent or more female populations."
But no one had seen anything quite to this extent. ("Holy moly" is how Allen described her initial reaction to the results of the lab tests.)
If Raine Island is a baby factory, the assembly-line switch has been thrown in one direction. It churns out female turtle after female turtle.
More than 99 percent of young turtles are female, the scientists found, and 87 percent of mature turtles are female. For every juvenile male, there are 116 female turtles.
Doom will not come for these turtles tomorrow. In fact, the overall turtle population might briefly increase, as long as the more numerous female turtles can find males to fertilise their eggs.
Turtles do not need a 50:50 ratio of males to females.
"A few males can go a really long way," Jensen said. "Male turtles mate more frequently" than female turtles do.
"It's hard to say whether it's good or bad but it's big and it could have a lot of cascading consequences," said Rory Telemeco, a biologist at California State University Fresno, not affiliated with this research, who studies temperature and reptile development.
"Though it does seem a little scary."
To estimate sand temperatures, study authors used historical sea and air temperatures in the breeding grounds between 1960 and 2016. By the 1990s, the sand temperature estimates were consistently higher than the pivot temperature.
The researchers were convinced climate change was the hand on the factory lever. "If it's not climate change," Allen said, "then what is it?"
Owens agreed. "Climate change is clearly the culprit," he said.
There is another nesting region, more than 1,000 miles (1,600 km) to the south of Raine Island, which consistently stays cooler. In southern Australia, the sex ratio is less skewed, with 1 male turtle to 2 female turtles. "These two populations at opposite ends of the reef have massively different sex ratios," Jensen said.
It does not seem likely, the study authors said, that these populations would interbreed. Turtles, like salmon, imprint on their birth areas. The reptiles return to their home shores to find mates. "Breeding will happen in the vicinity of these nesting beaches," Jensen said.
Researchers estimate that green sea turtles can live for 60 to 70 years. "Oh yes, there are a few males remaining, and there will be for decades to come," Owens said.
"But they will eventually die off. I predict that very soon the [northern Great Barrier Reef] population will start to see reduced fertility at the nesting beach if it is not already happening."
The good news, according to Jensen and Allen, is that management strategies are possible. Shading the beaches or pouring water on the sand can cool the nesting areas.
The Australian government, through the Raine Island Recovery Project, is working to monitor and protect these animals.
"We've got time on our hands," Allen said. The turtles still have ways to adapt, she said. It's just a matter of whether they adapt quickly enough.
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This article was originally published by The Washington Post.