Sea turtles may be more resilient to global warming than we thought

Sea turtles may be better able to cope with climate change than we thought. Biologists fear the reptiles could face extinction because warmer conditions would encourage most turtle eggs to develop into females. But it turns out that animals have a genetic safety net that could help them maintain a more even balance between the sexes, even as temperatures rise.

“We think we have discovered the ability of turtles to adapt to the environment they find themselves in,” says Chris Eizaguirre of Queen Mary University of London.

The sex of baby sea turtles is determined not by a sex-determining chromosome – as is the case in many animals, including humans – but by the temperature inside the nest. Laboratory studies have shown that at lower nesting temperatures more hatchlings will be males and at higher temperatures more females, raising concerns that global warming will cause ever more turtles to hatch as females.

For example, a 2018 genetic study found that about 99 percent of young green turtles (Chelonia mydas) aged approximately 4 to 20 years and originating from the warmer nesting sites of Australia’s Great Barrier Reef, were female. Modeling based on such findings has raised concerns that without enough males, sea turtle populations could collapse.

However, the actual situation at hatching is a mystery because you cannot determine the sex of a turtle until it is several months old, unless you kill it to check. Field data on the sex of newborns are therefore rare.

To get around this problem, Eizaguirre and his colleagues conducted laboratory and field experiments with loggerhead sea turtles (Caretta caretta).

In the first part of their work, they collected a total of 240 eggs from seven loggerhead nests on the beaches of Palm Beach County, Florida. They put the eggs in artificial incubators at one of three temperatures: 27°C (81°F), a temperature favoring the male; 30°C (86°F), a “watershed temperature for equal numbers of males and females; and 32°C (90°F), which should give rise to females.

When the hatchlings were 1 to 3 days old, the team collected blood samples, then raised the turtles in captivity for months until they were large enough to allow gender verification via keyhole surgery and a laparoscopic camera.

Comparing genome sequencing data collected from the blood samples with sex identification revealed that, regardless of the temperature at which the eggs had been incubated, male and female turtles each exhibited different patterns in the activity of hundreds of genes due to an epigenetic process known as DNA methylation. Some 383 genes were hypermethylated in women – meaning they were less active than expected – and 394 were hypermethylated in men. Many of these genes play a documented role in sexual development. This meant that researchers could determine the sex of a baby turtle simply from a blood sample.

The team used this knowledge in a field study by collecting 29 clutches of newly laid loggerhead sea turtle eggs from the beaches of Sal Island in Cape Verde, off the coast of West Africa. They divided each clutch, burying one half in a protected area at a depth of 55 centimeters – where it would be cooler – and the other half 35 centimeters lower, where it would be warmer, and monitored the temperatures.

When the researchers sequenced blood cell samples from 116 hatchlings, half from “cool” depths and half from “warm” depths, they found more males than expected given the temperatures the eggs had been subjected to. In fact, models based on incubation temperature overestimated female production by 50 to 60 percent.

This suggests that in addition to providing a tool for determining the sex of baby turtles, the work shows that there are molecular mechanisms that help turtles cope with climate change by altering the sensitivity of their developing sexual organs to temperature, Eizaguirre says.

“We are not saying that there is no feminization because it exists, and we are not saying that climate change does not exist because it is there and it is accelerating,” he says. “What we’re saying is that when populations are large enough, when there’s enough diversity, then it appears that the species [can] evolve in response to the climate in which they live.

This work confirms recent evidence from a team including Graeme Hays of Deakin University in Australia, showing that more male sea turtles hatch than expected if one assumes that temperature is the only factor in sex determination. These results indicate how the pivotal temperature at which the turtle sex ratio is 50:50 can be adapted to local conditions, says Hays.

Turtles also have other mechanisms to mitigate the impacts of warming, he says. These include nesting earlier in the year and migration patterns to breeding areas reducing the impact of feminization. “Female turtles don’t typically breed every year, but males travel to breeding grounds more often than females,” says Hays. “Thus, the reproductive sex ratio is more balanced than the actual adult sex ratio.”

Such behavioral adaptations are good, Eizaguirre explains, but newborns are still exposed to extreme heat, leading to lasting changes in DNA methylation. Signs of molecular adaptation are therefore better news for these vulnerable reptiles.