Some like it hot—and no one knows this better than a female green turtle.
The green sea turtle (Chelonia mydas) is one of several species with a temperature-dependent sex determination system. Green turtle eggs incubated above about 84 degrees Fahrenheit tend to hatch females, while eggs chilling below this temperature usually yield males. This delicate system has supported these creatures for millennia—but climate change is upending the balance.
According to a study published today in the journal Global Change Biology, elevated temperatures might turn up to 93 percent of the hatchlings in one West African population female by the year 2100. Though sex ratios this skewed might initially foster a population boom, they could also make these turtles more vulnerable in the long term, especially as the increasingly intolerable heat threatens the viability of nesting sites.
“Something we humans see as a minor increase in temperature can make for huge changes to ecosystems and wildlife,” says conservation biologist Jeanine Refsnider of the University of Toledo, who was not involved in the new study. “We are likely to start seeing extinction if we can’t get it under control.”
In the report, a team of scientists led by Rita Patrício, a marine ecologist at the University of Exeter in the United Kingdom, studied a large population of green turtles that nest on the protected Bijagós archipelago of the West African nation of Guinea-Bissau. The researchers then combined their data with temperature and sea level rise estimates to predict the turtles’ wellbeing through the year 2100.
According to the researchers’ model, as global temperatures continue to climb, females may comprise over 90 percent of Bijagós hatchlings by the end of the 21st century. This so-called “feminization” of the turtle population has already been observed in populations that nest in Australia. But the full effect of feminization remains murky. Many female-skewed turtle populations are currently thriving: As the egg-bearing half of the population, female turtles can get by as long as there are some males around to father their young. In fact, the new study estimates that the number of nesting females will actually increase in the next century, potentially bolstering sea turtle populations.
But Patrício doesn’t think the growth should necessarily put researchers at ease. The consequences of climate change are extremely complex—and feminization isn’t the whole picture.
“There’s more to the story than, ‘It’s getting warmer, they’re all going to be girls,’” says Bryan Wallace, a marine ecologist at Conservation Science Partners who wasn’t involved in the new study.
Female-heavy populations, while high in reproductive potential, can come with significant setbacks. For one thing, having only a few males in the group could compromise genetic variation, resulting in more inbreeding over time. Additionally, as temperatures creep upward, high birth rates may give way to sharp declines as turtle eggs cook to death in the scorching sands.
According to Patrício’s findings, swelling sea levels could also counteract any temporary boosts in turtle numbers: Up to 43 percent of the currently available nesting areas stand to be swallowed by higher-than-average tides. And as beaches disappear, females may find themselves competing for space, even resorting to unearthing or destroying the nests of fellow turtles, explains Rachel Bowden, an expert in turtle physiology at Illinois State University who was not involved in the study.
“Increases in population size doesn’t mean the turtles are necessarily off the hook,” Patrício says. And the problem with these booms, she adds, is that they can make the inevitable declines that much harder to predict.
For the moment, however, the current sex ratio of green sea turtles in Guinea-Bissau is at roughly 50/50. And Patrício is hopeful that even as the landscape around them is being permanently altered, these turtles might deploy a few behavioral strategies to beat the heat.
Many of Guinea-Bissau’s island shores are cradled by dense patches of forest, and females have long sought shelter beneath the foliage to lay their eggs. Nests in these more shaded sites reliably hatch male offspring; an inland shift might also save eggs from tidal flooding. Similarly, more males are born when females nest during the cooler spring and fall months, rather than the sweltering heat of summer.
But researchers still don’t fully understand the cues females use to determine when and where to nest, Bowden says, so there’s no telling if turtles will actually be able to capitalize on these opportunities. There are likely multiple factors at play—and with global habitats disappearing, turtle populations may soon reach a point where even extreme changes in behavior won’t be enough to offset the effects of climate change.
Of course, predictive models offer no guarantee of what the world will look like in 100 years’ time. But as the climate changes at an unprecedented rate, Wallace says, the million-dollar question is whether evolution can keep pace. Sea turtles can only be resilient if humans give them the space to do so, he adds.
“Studies like this remind us... that nature is far more complicated than we ever imagined,” says Warren Porter, who studies turtle ecology at the University of Wisconsin-Madison, but did not contribute to the study. “Perturbations in one spot might be analogous to touching a spider web in one location. But the impact is felt throughout the entire web.”