Global Warming Could Skew Reptile Sex Ratios and Lead to Extinctions
NEWS | 18 December 2025
The sex of many turtles, crocodilians, and other reptiles is determined by the temperature at which their eggs incubate. Global warming could doom them Under no light but the stars, a green sea turtle hauls herself out of the surf and onto the familiar sand of Alagadi Beach on the northern coast of Cyprus. She doesn’t notice any predators as she makes her way up the beach; tonight will be the night. When the turtle reaches a satisfactory spot, she nestles into the warm sand and begins excavating a deep pit. Nothing can distract her; she’s gone into a kind of trance. She pushes out 100 wet, leathery eggs into the pit. The turtle won’t move until she has completed her task, even if humans creep close to measure her shell and tuck a temperature logger in among her eggs. She finishes laying in about 20 minutes, but her work isn’t done. Still focused, she spends another few hours laboriously scooping the sand over her eggs. Then she turns around and crawls back into the ocean. In about two months her babies will emerge from that sand and make a mad dash to the water. They’ll have to fend for themselves—their mother is done caring for them. She’ll never know the curious fate that befalls her offspring: nearly all of the hatchlings will be female. On supporting science journalism If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. Many reptiles differ from typical vertebrates in that their sex is not determined by their genes. They lack sex chromosomes such as the X and Y inside human cells. Instead the temperature of their nest pushes them toward becoming male or female. For green sea turtles, if the temperature is about 29 degrees Celsius (84 degrees Fahrenheit) during a critical mid-incubation window, the babies will hatch as a half-and-half mix of females and males. But the hotter the nest, the more they will skew female. In the dark sand of Alagadi Beach, the eggs incubate at a toasty 33 or 34 degrees C, resulting in broods that are overwhelmingly female. The reptiles that have temperature-dependent sex determination—most turtles, as well as all crocodilians (crocodiles, alligators and their kin), some lizards, and a unique creature from New Zealand called the tuatara—belong to lineages that have survived Earth’s climatic ups and downs for millions or even hundreds of millions of years. Yet the present day brings a confluence of problems they haven’t ever encountered in their long history, including anthropogenic habitat loss and a planetary thermostat gone haywire. How in the world reptile biology came to rely on temperature to decide the sex of babies is largely a mystery. Global warming will affect various kinds of reptiles in different ways. Whereas hotter temperatures turn developing turtles female, they have the opposite effect on crocodilians, producing more male hatchlings. Scientists have predicted nearly single-sex generations of alligators by the year 2100. If rising temperatures mean entire generations of sexually reproducing reptiles will be dramatically skewed male or female, it’s not hard to see how they could doom species: mating opportunities will decline; populations might become inbred. Surviving members of a species that’s already dwindling from other pressures might not be able to find a partner with whom to make babies. Some species seem to be adapting to the shifting conditions by altering their nesting behaviors. But as climate change accelerates, their continued survival may depend on whether they can keep up. Temperature-based sex determination is an admittedly strange phenomenon—one that scientists have struggled to explain. Consider the American alligator. Around the summer solstice the alligator mother shoves vegetation from her wetland habitat into a pile to create a nest. She then deposits her eggs in this “glorified compost heap,” as Benjamin Parrott, an ecologist at the University of Georgia, calls it. The eggs are warmed by the slowly decomposing pile. If they incubate at a little below 32 degrees C, they’ll hatch as an even mix of males and females. If they incubate above this so-called pivotal temperature, more hatchlings will be male, although if the temperature gets hot enough, the ratio will skew back toward females. “A couple of degrees Celsius makes all the difference,” Parrott says. How in the world reptile biology came to rely on temperature to decide the sex of babies is largely a mystery. Some of Parrott’s research suggests that this type of system can evolve in a species if one sex survives better at warmer temperatures. Sex determination based on temperature has evolved in some fishes, too. It makes sense for parents to use information about their environment to give their kids an edge. But unlike, for example, the Atlantic silverside—a fish with temperature-based sex determination whose offspring grow up within a year—reptiles such as alligators and turtles mature very slowly. American alligators in South Carolina, for instance, won’t start laying eggs until they are 16 years old. Sea turtles might take as long as 40 years to mature. Why should one warm summer dictate the fate of your offspring when they’ll need to survive another 16 summers or more before they reproduce? “It’s hard to wrap your head around,” says Rachel Bowden, an ecological physiologist at Illinois State University who studies freshwater turtles. Nor do researchers understand why incubating eggs at their pivotal temperature produces a mix of males and females, because all the eggs have experienced the same conditions. Bowden is investigating the hormones and other molecules that might turn differences of a few degrees into ovaries or testes. Newly hatched green sea turtles climb across the sand at Alagadi Beach on the island of Cyprus. The high temperatures of the turtle nests there produce broods that are almost entirely female. Laura Boushnak/AFP via Getty Images Further complicating the research is the fact that even experts can’t always tell whether a baby gator or turtle is a girl or boy. “There’s no easy way to sex them,” Parrott says. An alligator’s external genitalia won’t start to look distinctive for about six months. In the laboratory, scientists have euthanized hatchlings and examined them internally to learn that certain steady incubation temperatures produce certain ratios of males to females. But they usually have to estimate the sex ratios of hatchlings that emerge from nests in the wild under naturally fluctuating temperatures. Sex determination is even more complex in lizards. Among the several thousand lizard species on Earth, some have temperature-dependent sex determination, whereas others rely on genes. But even in species with sex chromosomes, temperature can influence whether offspring develop into males or females. In the snow skink, a little copper-colored lizard that lives in Tasmania and bears live young, populations that dwell at high altitude have genetically determined sex, whereas low-altitude populations depend on temperature. In the central bearded dragon, native to Australia, warm temperatures can turn chromosomally male lizards into females. Against this backdrop of unknowns, researchers are racing to learn just how flexible reptiles can be in their nesting habits. During their many millions of years on Earth, reptiles have survived dramatic climate shifts, living through ice ages and intense heat. Alligators and their relatives, for example, managed at least in part by migrating to more favorable climes: The fossil record shows that they moved toward the equator when it was cooler and toward the poles when things heated up. At one point, crocodilians lived in Alaska. But in our human-dominated world, Parrot says, migration might not be a feasible solution. “I don’t think people in D.C. are going to tolerate gators in the Potomac,” he says. In theory, today’s reptiles might be able to keep their eggs cool and their sex ratios steady by nesting earlier in the year or in shadier places or by digging deeper in the ground. But that would depend on the animals perceiving the temperature shift—and having the capacity to do things differently. During the summer nesting season, Sophie Davey becomes nocturnal. She leads the sea turtle monitoring project for the Society for the Protection of Turtles. Nearly every night of the week, she goes to Alagadi Beach at about 7:30 in the evening. Every 10 minutes a team member will walk up and down this beach to check for laying turtles. Both loggerheads and green sea turtles nest here, and this past summer “they’ve been coming thick and fast,” Davey says, especially the greens. To avoid disturbing the nesting mothers, the team members do as much as they can in the dark. They use a red light if they need it for something like entering data in a form. Otherwise they have learned to rely on their other senses. The scent of freshly dug sand is often their first clue that a green sea turtle has arrived. “As soon as that wind hits you and you smell earth, it’s like, okay, there’s a turtle on the beach somewhere,” Davey says. In contrast, loggerheads, with algae and barnacles clinging to their shells, have a seaweedy smell. Sounds offer more hints to what’s happening in the dark. Fat plops of sand hitting the beach mean a turtle is using her big front flippers to carve out a depression for her body, whereas more delicate scoops indicate she’s using her flexible back flippers to dig the egg chamber. During the turtles’ nesting rituals, the team members gather data. They measure the animals, give them identification tags and microchips, and take tissue samples that let them study genetics as well as where the turtles have been living and what they eat at sea. Afterward the workers cage the nest to keep predators away. As part of its mission, the project is helping to answer questions about how turtles might adapt as the world gets hotter. Since the 1990s green sea turtles have been laying their first nests at Alagadi Beach almost one day earlier each year. Marine conservation biologist Annette Broderick of the University of Exeter in England, who is one of the scientific advisers of the turtle-monitoring project in Cyprus, says the older and more experienced females, who lay multiple clutches in a nesting season, are driving the trend. Nests that are laid earlier stay cooler throughout their incubation. Loggerheads exhibit a similar trend, with experienced loggerhead moms arriving about half a day earlier each year. In other words, the sea turtles seem to be adjusting their habits in response to warming. “But that can happen only to a point,” Broderick says. The seagrass that the turtles eat, hundreds of kilometers away, may not be available early enough in the year for them to fill up on before heading to the nesting beach. The nesting beaches themselves may pose their own constraints. Even though the mothers who nest at Alagadi Beach are arriving earlier and earlier, their offspring continue to be almost all females. With its dark, hot sand (and, Davey points out, a constant influx of potentially heat-retaining plastic trash), Alagadi Beach just might not be able to maintain temperatures cool enough to produce males no matter how early the mothers arrive to lay their eggs. Elsewhere on Cyprus there are cleaner beaches with deep, white sand, where a few more males might hatch. Still, all the island’s beaches skew highly female. In any case, a 50–50 ratio of females to males in a brood isn’t necessarily the goal. Sea turtles seem to naturally have female-biased nests, Broderick says. Things may balance out because a female will lay eggs only every two to four years, whereas a male can mate every year. In the near term, then, temperatures that produce extra females could actually help sea turtle populations grow. Baby American alligators bask in Florida’s Big Cypress National Preserve. Scientists have predicted that entire generations of alligators could be nearly single-sex by 2100. Troy Harrison/Getty Images “Undoubtedly, there will be some positive effects,” Broderick says. “But there will also be some negatives.” Besides producing more female turtles, warm temperatures tend to make reptiles grow faster—so those females might be ready to reproduce sooner. But hot enough nesting temperatures, in addition to leaving those females with almost no potential mates, could make entire clutches fail to hatch. Ocean-dwelling turtles aren’t the only ones changing their habits as the climate warms. Freshwater turtle mothers seem to be adapting their behavior, too. Populations of several different North American turtles have shifted their nesting earlier in recent decades. Slider turtles in Illinois nest more than three weeks earlier in the spring than they did in the 1990s. In one study, Jeanine Refsnider, an evolutionary ecologist at the University of Toledo, brought wild painted turtles from New Mexico, Illinois, Nebraska, Iowa and Washington State to research ponds in Iowa. All of the turtles were pregnant females. They could have sought the ideal nesting conditions for their home climates: females from hot New Mexico choosing shade, those from chilly Washington looking for relatively warm spots, and so on. Yet in Iowa everyone acted the same. The mothers chose similarly shady conditions to build their nests, and their offspring hatched with essentially the same sex ratios. “They do seem to adjust their nesting behavior quite quickly in response to the current conditions they find themselves in,” Refsnider says. She notes, however, that “if they don’t have the habitat availability to express that kind of innate flexibility, then they’re in trouble.” In other words, it’s not enough for turtle moms to be perceptive and flexible; they also need better options for nesting. Refsnider is now studying the nesting habits of endangered spotted turtles, which live in swamps and nest in damp, mossy mounds or rotten logs. If researchers can understand turtles’ preferences about nesting conditions, then humans can try to manage habitats to help the turtles: adding shade cover, for instance, or hauling in rotting logs so the turtles can maintain a healthy sex ratio as the climate warms. “I don’t think it’s hopeless,” Refsnider says of the fate of turtle populations on a warming planet. “But it is concerning.” The situation is more acute for species that are also facing pressure on other fronts. The tuatara is a charismatic medium-size reptile with a Muppet-like face and a row of spines running down its back. It looks a lot like its closest living relatives, the lizards. But it’s actually the lone survivor of a separate order of vertebrates that split off from the lizards around 250 million years ago, before mammals had evolved from our reptilian ancestors, the therapsids. Lately its survival hasn’t been easy. Predators introduced by waves of human settlers have eliminated tuatara in much of their original habitat, which once spanned all of mainland New Zealand. Today they live only on about 32 small islands, and climate change is threatening those scattered survivors. Tuatara share the sex-determination pattern of the crocodilians in that warmer temperatures create males, although unlike in crocodilians, no one has found an incubation temperature warm enough to shift the sex ratio of the developing tuatara young back toward female. Predicting how climate change will affect adult tuatara sex ratios is difficult because the reptiles mature so slowly, explains Alison Cree, an emeritus professor at the University of Otago in New Zealand, who has studied the biology and ecology of tuatara. Their eggs may stay in the ground for a year or longer. The adults don’t reproduce until age 13 or so. But because the remaining tuatara populations all live on islands, it’s clear that their options for shifting their habitat or habits are limited. On a tiny, treeless bit of land called North Brother Island, the population is becoming increasingly male. Researchers warn that the combination of habitat restriction and skewed sex ratio is setting up the North Brother Island tuatara for a downward spiral known as an extinction vortex that will be very difficult, if not impossible, to escape. It doesn’t help that tuatara tend to return to the same nesting sites they’ve used before—that behavior is ingrained. But they may be more flexible when it comes to timing. A study at another site, on Stephens Island, found that tuatara nested earlier in years with warmer springs. That shift could help keep their eggs cooler, as it does for turtles. The authors of that study predicted that at least in the medium term, the reptiles could keep their female-to-male ratios steady. If tuatara sex ratios reach a crisis, Cree suspects it will be a symptom of a larger emergency that’s already underway. Once temperatures have climbed high enough to turn most tuatara male, the creatures will also be facing drought, flooding, and a rising sea eating away at their islands. In that case, we will be losing far more than a charismatic species. Tuatara play significant roles in their ecosystems, Cree explains. If they went extinct, their unique quarter-billion-year-old lineage would disappear, too. “From a perspective of studying reptile evolution, that’s a huge loss,” she says. Tuatara are also culturally significant to the Maori, the Indigenous people of New Zealand. Commonly featured in Maori stories, tuatara may signify knowledge or warn of a coming calamity. Still, next spring the tuatara will do what they always do. The pregnant females will gather in open, sunny areas. Each expectant mother in the shared rookery will use her strong limbs and claws to dig a burrow for her eggs, working across several nights. She’ll lay a small clutch, only about 10 eggs. She’ll cover her burrow with dirt and vegetation until it’s undetectable. For another two weeks or so, the tuatara mother will stay and guard her nest. Then she’ll walk away, having done everything she can.
Author: Kate Wong. Elizabeth Preston.
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