For more than a hundred million years, female sea turtles have dug nests on beaches, filling them with papery eggs before covering them up and bidding adieu.
But in Australia’s northern Great Barrier Reef, something in this time-honored process has gone awry: For 20 years, almost all of the green turtle hatchlings that dug themselves out of the sand to flipper-flop into the ocean have been female.
That’s the conclusion of research published recently in Current Biology, led by researchers at the National Oceanic and Atmospheric Administration. The scientists came up with a new method for counting males and females of a breeding population—leading them to a discovery that no one predicted.
"This is extreme—like capital letters extreme, exclamation point extreme," NOAA’s Camryn Allen, a research biologist one of the lead authors, told National Geographic. "We're talking a handful of males to hundreds and hundreds of females. We were shocked."
Sea turtle sex is determined by the nest temperature where the eggs mature. Colder temperatures produce more males, with more and more females hatching as the sand heats up. Scientists have known this for decades, and have considered the implications in a world beset by climate change. But the details are hard to come by.
The green turtles of the northern Great Barrier Reef form one of the largest populations of sea turtles in the world, with an estimated 200,000 breeding females. The extreme sex ratio is particularly surprising given how well this group of animals is faring overall.
The near-absence of males among juveniles and hatchlings brings urgency to discussions among scientists and conservationists around what humans should do to intervene. If the adult males that remain die out before a fresh crop of mature males enter the population, these turtles could see dramatic collapse or even extinction.
"This is quite an important paper because it's really sounding the alarm," Jacques-Olivier Laloë, an associate research fellow at Deakin University who was not part of the study, told The Daily Beast. “This is happening right now."
The sex organs of sea turtles aren’t externally visible until they become adults, which for green turtles is at about 25 years of age. Sexing hatchlings means killing them first, which is why scientists tend to take temperature readings at the nest sites and make estimates based on those.
The Great Barrier Reef’s green turtles are particularly confounding, since there are two distinct breeding populations that share foraging grounds, but return to separate nesting beaches, some to the north and some to the south. The researchers tested hormone levels in blood samples, in addition to physical exams, to determine the sex of turtles in their study, also performing DNA analysis on skin samples to tell where they called home.
The northern turtles come from a piece of the Earth that has been particularly ravaged by warming. Extreme water temperatures in 2016 and 2017 caused massive coral die-offs. In some stretches more than half of the coral is now dead.
“Our study highlights the need for immediate management strategies aimed at lowering incubation temperatures at key rookeries to boost the ability of local turtle populations to adapt to the changing environment and avoid a population collapse—or even extinction,” the authors write.
Climate change hurts sea turtles in more ways than just stealing all the male babies. Sea level rise and bigger storms erode nesting beaches, making it harder for turtles to find safe spots to lay their eggs. And at extreme temperatures, you can go from exclusively female broods to no hatchlings at all. “You have entire nests that fail because the temperatures of the nests are too warm,” Laloë said.
Keeping nests cool is, in theory, straightforward. "Two of the natural ways that nest temperatures become reduced is through rain and through shade,” Jacob Hill, a PhD candidate at Mississippi State University who was not involved with the recent study, said. As a master’s student at Indiana University—Purdue University Fort Wayne, Hill completed research demonstrating the feasibility of watering and shading to cool sea turtle nests in Costa Rica.
When too much heat becomes a problem, reversing the trend is theoretically simple: Just add water or shade to ensure more males, and less mortality. If you can track the nest temperature in real time, it shouldn’t be too hard to keep the heat in the optimum range.
Surprisingly few studies consider these options, perhaps because, to a point, a female skew is good for the population overall. Females only mate every two-to-four years because of the energetic costs of producing so many eggs. Males can mate every year with multiple females. The net effect is that few males and more females means more eggs, and more babies. We still don’t know what the ideal ratio is, or the minimum number of males required to stave off collapse.
Hill recommended careful research and consideration before messing with any turtle eggs. It’s important for scientists to test methods like watering and shading in the specific environment where they plan to use them. "The same technique can have different results based off local climatic conditions," he said. For example, if shading structure keeps rain off, nests might end up hotter than if there was no such shading structure.
Just a few nests full of mainly male hatchlings could make a significant difference for populations like this one where the feminization is so extreme, and there’s a risk of most of the adult males dying off before a new generation comes up. Once that risk is well understood, the scientific priority turns from research to action.
That’s good news not just for turtles, but for entire ecosystems where they live. Green turtles munch on seagrass, preventing it from becoming overgrown and maintaining a healthy habitat for fish and other marine life. Hawksbill turtles eat sea sponges, which would otherwise crowd out coral in reef ecosystems. All species improve coastal ecosystems by planting their eggs on beaches, which feeds both plants and animals. "They are what we call keystone species, where they have an important role within their ecosystem. If you remove them, then the whole ecosystem is at risk of declining,” Laloë said.
Between climate change and other human impacts on their ecosystems, today’s turtles have a lot do deal with. But they’ve dealt with a lot before. "They've been around for millions of years, and they survived the catastrophic event that wiped out the dinosaurs, so I have a lot of faith in them,” Laloë said.