Photograph by Brian Skerry, Nat Geo Image Collection
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Green sea turtles mate in the protected waters of Buck Island’s lagoon in St. Croix—though the species is seeing big impacts from warming water.

Photograph by Brian Skerry, Nat Geo Image Collection

Sea turtles are being born mostly female due to warming—will they survive?

Climate change is causing a crisis in sea turtle sex ratios. But there are signs of hope.

She started out studying tree-climbing marsupials, but only after she applied what she knew to marine reptiles did Camryn Allen actually get worried.

Allen, a scientist with the National Oceanic and Atmospheric Administration in Hawaii, had spent her early career using hormones to track koala bear pregnancies. Then she started using similar techniques to help colleagues quickly answer a surprisingly hard question: whether a sea turtle is male or female.

You can't always tell which is which just by looking. That often requires laparoscopy, viewing the turtle's internal organs by inserting a thin camera. Allen figured out how to do it using blood samples, which made it easier to check lots of turtles quickly.

That mattered because the heat of sand where eggs are buried ultimately determines whether a sea turtle becomes male or female. And since climate change is driving up temperatures around the world, researchers weren't surprised that they'd been finding slightly more female offspring.

But when Allen saw results from her research on Raine Island, Australia—the biggest and most important green sea turtle nesting ground in the Pacific Ocean—she realized how serious things might get. Sand temperatures there had increased so much, she and a team of scientists reported last year, that female baby turtles now outnumber males 116 to 1.

"I can't deny it: seeing those results scared the crap out of me," Allen says.

Sea turtle life is hard enough on its own, and humans were already making it even harder.

Dropping survival odds

Seven species of sea turtle crisscross the tropical and temperate oceans. From the start, there's is a risky world.

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A leatherback sea turtle lays its eggs at Sandy Point National Wildlife Refuge on St. Croix.

Sea turtles lay eggs on sandy beaches, but many of them never hatch. The incubating creatures may be killed by microbes, dug up by wild dogs, or exposed by other turtles scraping out new nests. Those breaking free from their delicate shells have to race past vultures and racoons to make it to the ocean. There, fish, crabs, and all manner of marine life wait, eager for a meal. Just a fraction of 1 percent of baby hatchlings ever make it to adulthood.

Once they're mature, adults face few natural predators. An adult Kemp's ridley may weigh less than most teenagers, while a leatherback can outweigh a bison. Either way few wild creatures aside from tiger sharks, jaguars, and orcas ever really try to eat them.

Humans, however, have lowered their survival odds considerably.

We build condominiums on their nesting beaches. We poach eggs for sale on the black market, carve up adults for their meat, and use their skin to make boots and handbags. We transform the fiery gold and red carapace of hawskbills into bracelets, eyeglasses, hairbrushes, and jewelry boxes. (Read more about how much bigger this problem is than first thought.)

Fishing boats accidentally snare turtles in their nets or with longlines. Container ships hit them and crack open their shells.

"The general trajectory is that sea turtles are depleted worldwide, across all species," says Bryan Wallace, who oversees a committee that evaluates sea turtle status for the International Union for the Conservation of Nature's endangered species redlist. "I'd have to think hard to find a population that is way better off than it was 100 years ago."

Currently, six of the seven species are considered threatened or endangered. The seventh, Australia's flatback, is merely lacking information for scientists to say one way or the other.

New research—new hope?

Still, when Allen and NOAA colleague Michael Jensen began examining sea turtle feminization, there wasn't much reason to fear a large or immediate impact on turtles. In one earlier study, Allen found a small population of green sea turtles outside San Diego had gone from 65 percent female to 78 percent female over time as nesting sands warmed. Loggerheads from West Africa to Florida also had shown similar trends.

No one had ever examined a population as important or as large as Raine Island.

More than 200,000 sea turtles nest on or near Raine, a tiny 80-acre curl of sand along the northern edge of the Great Barrier Reef, the portion hardest hit by warming waters. The other portion of that sea turtle population nests further from the equator, near Brisbane, where temperature increases have not been as dramatic.

What Allen and Jensen discovered was significant. Older turtles that had emerged from their eggs 30 or 40 years earlier were also mostly female, but only by a 6 to 1 ratio. But younger turtles for at least the last 20 years had been more than 99 percent female. And as evidence that rising temperatures were responsible, female turtles from the cooler sands near Brisbane currently still only outnumber males 2 to 1.

Six weeks after Allen and Jensen published their results, another study from Florida looking at loggerheads revealed that temperature is just one factor. If sands are moist and cool, they produce more males. If sands are hot and dry, hatchlings are more female.

But new research in the last year also offered rays of hope.

Long-term resilience?

Sea turtles have been around in some form for more than 100 million years, weathering ice ages and even the extinction of the dinosaurs. They may have developed lots of coping mechanisms. One, it turns out, could be altering how they mate.

Using genetic tests to examine a small group of critically endangered hawksbills in El Salvador, Alexander Gaos, a turtle scientist who works with Allen, found male sea turtles mating with multiple females. That population was producing 85 percent female hatchlings.

"We found that, hey, wow, this strategy is being employed in small, endangered, highly reduced populations," Gaos says. "We think they were just responding to a lack of options for females."

It's not clear whether this move is something that has increased over time, or whether it could help stave off a population-scale turtle crisis.

"Is there potential for this to help compensate for more females being born?" Gaos asked. "We don't know. Just the fact that the males can actually do that is fairly new. We might only be seeing the very beginning."

Meanwhile, other researchers in the Dutch Caribbean figured out that providing nesting beaches with more shade from palm leaves actually cooled down the sand. That could help alter the male-to-female ratio of baby turtles considerably.

Allen, for one, finds the new developments reassuring. Sea turtles may be more resilient than once thought.

"We may lose some smaller populations, but sea turtles are never going to go away completely," Allen says. "I think turtles, out of all the other species, might actually have a pretty good shot."

They might just require a little more help from us.

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