The turtle wranglers landed on Ingram Island thinking about sex and heat.

Pacific green sea turtles spend years cruising this northern Australia feeding ground, fattening up on sea grasses before heading to nesting areas to mate and lay eggs. The scientists simply wanted to know: which of these reptiles were male and which were female?

You can't always tell a sea turtle's sex by looking, so researchers kicked off a "turtle rodeo." They stood atop skiffs and raced toward swimming turtles and launched themselves like bull wrestlers onto the animals' carapaces. After gently steering each turtle to shore, they took DNA and blood samples, and made tiny incisions to inspect turtle gonads.

Since the sex of a sea turtle is determined by the heat of sand incubating their eggs, scientists had suspected they might see slightly more females. Climate change, after all, has driven air and sea temperatures higher, which, in these creatures, favors female offspring. But instead, they found female sea turtles from the Pacific Ocean's largest and most important green sea turtle rookery now outnumber males by at least 116 to 1.

"This is extreme—like capital letters extreme, exclamation point extreme," says turtle scientist Camryn Allen, with the National Oceanic and Atmospheric Administration in Hawaii. "We're talking a handful of males to hundreds and hundreds of females. We were shocked."

New research published in Current Biology Monday by Allen and her colleagues is just the latest to suggest that rising temperatures around the world can turn sea turtle populations female. But it is the most detailed look to date at just how significant this problem is already, and raises new questions about the risks globally for marine turtles, as well as for other temperature-dependent species—from alligators and iguanas to inland silversides, an important fish in many streams and estuaries.

"You work on one of the biggest turtle populations in the world and everyone tends to think that means things are good," says marine biologist Michael Jensen, the new study's lead author and a research fellow with NOAA's Southwest Fisheries Science Center in La Jolla, California. "But what happens in 20 years when there are literally no more males coming up as adults? Are there enough to sustain the population?"

“It was way worse than we thought”

Eastern Australia's green sea turtles, which can reach 500 pounds with heart-shaped shells that spread 4 feet or more in diameter, primarily nest in just two places—a collection of islands near Brisbane along the southern Great Barrier Reef, and a remote teardrop of sand and grass 1,200 kilometers to the north called Raine Island. A few years after offspring hatch from either of these places, they mingle and swim about in shallow waters in small stretches of the Coral Sea, where they may stick around for a quarter-century or more before peeling off and heading back to one of these two regions to mate. They will return to the same feeding grounds again and again for decades.

Jensen wanted to know if climate change had already altered the ratio of male hatchlings to females. By using genetic tests, he'd figured out that he could trace turtles of all ages from one feeding area back to specific nesting sites. Still, his demographic data would lack an important detail: sex. Only after a turtle matures is it possible to tell its sex from the outside. (Mature males have slightly longer tails.) By then turtles can be decades old. So scientists often use laparoscopy, sending a thin tube into each animal to view its organs. But that's invasive, and not so practical if you're hoping to examine hundreds of creatures. Jensen was stumped.

At a turtle conference in Mexico, he bumped into Allen, a former koala researcher. Allen had used testosterone levels to track pregnancies in the tree-loving marsupials. She went on to perfect ways of deciphering the sex of marine species based on hormone levels. All she needed was a little blood.

The pair teamed up with others, including Australian turtle expert Ian Bell, and drew blood from Great Barrier Reef turtles. They performed a few laparoscopy exams to confirm the accuracy of Allen's methods. They compared their results with temperature data for nesting beaches. And they examined turtles of varying age. The results caught them by surprise.

"We immediately said, 'Holy Smokes!'" Allen says. "It was way worse than we thought."

It appears that Raine Island has been producing almost exclusively female turtles for at least 20 years. This is no small thing. Eighty-acre Raine and its associated coral cays host one of the largest green sea turtle rookeries on Earth, where more than 200,000 turtles come to nest. During high season, 18,000 turtles may settle in at once. And those are just the females.

Since scientists also were able to determine rough ages for the turtles they sampled, they also made another discovery. Along that stretch of the northern Great Barrier Reef, where increasing heat had led to significant coral bleaching in recent years, the ratio of females to males had grown more severe with time. Turtles that hatched there around the 1970s and 1980s were also mostly female, but only by a ratio of 6 to 1.

"This is groundbreaking work," says Brendan Godley, a sea turtle expert and professor of conservation science at the University of Exeter. He was not affiliated with the study. The scope—encompassing the length of the Great Barrier Reef—and the multidisciplinary approach make the research highly valuable, he says.

Equally important is what Jensen and Allen found down south. There, turtles hatching from the southern reef near Brisbane—where temperatures have not increased as significantly, and where corals remain quite healthy—fare far better. There, female turtles today outnumber males by only 2 to 1.

"This combined with some neat modeling shows that cooler beaches in the south are still producing males, but that in the more tropical north, it's almost entirely females hatching," Godley says. "These findings clearly point to the fact that climate change is changing many aspects of wildlife biology."

View Images More work needs to be done to assess the changing sex ratios of green sea turtles in other parts of the world, such as this animal in the Galapagos Islands. Photograph by Chris Newbert, Minden Pictures/Nat Geo Image Collection

But how widespread is this phenomenon—and how consequential?

“Temperatures are changing incredibly fast”

For the moment, no one knows.

Since male sea turtles often mate with more than one female, and males typically mate more frequently, a slight female bias may be beneficial. A recent look at 75 sea turtle rookeries around the world showed the ratio of females to males was roughly 3 to 1. In fact, some turtle populations produced fewer males than females even a century ago. The question, though, is: how much has it changed, and how much is too much?

Marine turtles have been around for 100 million years and temperatures have risen and fallen during that time. Plus, after decades of decline from hunting, poaching, pollution, disease, development, habitat loss, and bycatch in commercial fishing, many populations around the world recently have shown signs of improvement.

"But now temperatures are changing incredibly fast," Jensen says. "Evolution requires many generations for animals to adapt. But these are animals that live for 50 years or more, and things are changing dramatically just in their lifetimes."

Just on Raine Island alone, for example, rising seas have inundated nest sites, drowning eggs. Beach erosion is creating mini-cliffs, causing adult greens to fall onto their backs and die, unable to right themselves. Australian authorities are spending millions of dollars restoring the island to improve life for turtles.

Even so, scientists have been predicting for at least 35 years that the male-female balance of all seven sea turtle species—greens, loggerheads, leatherbacks, hawksbills, flatbacks, olive, and Kemp's ridleys—would be exceedingly vulnerable to climate change. The reptiles are so temperature-sensitive that a rise of just a few degrees Celsius could in many places eventually produce entirely female offspring. That could wipe out whole populations. If temperatures climb too high, things actually get worse; eggs literally cook in their nests.

Before the latest research, however, most studies suggested excessive feminization wouldn't pose a threat until late in the 21st century, and scant work had been done to examine what may be happening already. In research two years ago on a small collection of green sea turtles in San Diego, Allen found 65 percent were female—but among the young that figure rose to 78 percent. Meanwhile, some leatherbacks in Costa Rica and loggerheads from Florida and a few other places, such as West Africa, have shown an increasing female bias. But none of that work examines populations on a scale that even comes close to Jensen's and Allen's work.

Even then, determining when the number of males may drop too low is difficult. The answer can change by species and location. In addition, the very thing that determines sex—temperature—can itself be impacted by local factors. In the Chagos Archipelago in the west Indian Ocean, heavy bouts of sand-cooling rains, shade from the leaves of coastal trees, and narrow beaches that force hawksbills to nest close to water help maintain a healthy ratio of male hatchlings. In the Caribbean, scientists warn that sea turtles are at risk from logging because it reduces shade that keeps beaches cool enough to produce males.

"What's really scary”

All of that makes the Great Barrier Reef research that much more compelling, says sea turtle scientist Nicolas Pilcher, who was not part of the study. There, most beaches offer no shade, so the link between climate and sex ratios is clearer. And the numbers of turtles affected is likely in the hundreds of thousands. No study has shown so disproportionate a ratio in so important a place—in part because no one until now had figured out how to do so.

"It's unique in that the Raine Island population is so large that the impact of (any potential) loss will be huge, and that the authors have ‘backtracked’ the data showing that in the earlier days there was a more even distribution of sexes," Pilcher says.

What worries Allen is what her research suggests about thousands of sea turtle populations around the world that have yet to be studied in this way, which is virtually all of them. She and Jensen plan to continue applying their techniques to new nesting places and already have collected samples in Guam, Hawaii, and Saipan.