Surprising Resilience: How Sea Turtles May Thrive Amid Global Warming

Recent research indicates that sea turtles may be more resilient to climate change than previously believed. Concerns have been raised that rising temperatures could lead to the extinction of these reptiles, as a majority of turtle eggs tend to develop into females. However, scientists have discovered a genetic safety net that maintains a more balanced sex ratio even as temperatures increase.

According to Chris Eizaguirre at Queen Mary University of London, “We believe we have uncovered the ability of turtles to adapt to the environment they find themselves in.”

The gender of baby sea turtles is temperature-dependent rather than determined by chromosomes. Laboratory studies show that cooler nest temperatures favor male hatchlings, while warmer conditions promote female hatchlings. This raises concerns that global warming could result in significantly more female turtles.

For instance, genetic research conducted in 2018 revealed that around 99% of young green sea turtles (Chelonia mydas) aged 4 to 20 years in a nesting area off Australia were female. This finding contributed to alarming predictions about male shortages which could lead to a population collapse.

However, due to the challenges of identifying a turtle’s gender before it reaches maturity, field data regarding hatchling sex ratios have been limited.

To address this gap, Eizaguirre and colleagues conducted both laboratory and field experiments focused on loggerhead sea turtles (Caretta caretta).

In one phase of the study, they collected 240 eggs from seven loggerhead nests along Florida’s Palm Beach County coast. These eggs were incubated at three different temperatures: 27°C (81°F) suitable for male hatchling production, 30°C (86°F) for an equal sex ratio, and 32°C (90°F) to promote female hatchlings.

After one to three days, blood samples were taken from the hatchlings, which were kept until mature enough for sex determination via keyhole surgery or laparoscopic imaging.

By comparing genetic data from the blood samples, researchers found distinctive activity patterns in hundreds of genes that indicated sex, attributable to an epigenetic process called DNA methylation. In females, 383 genes were hypermethylated, while males had 394 hypermethylated genes, many of which are known to play roles in sexual development.

Utilizing these findings, the team conducted field research on Sal Island, Cape Verde, collecting 29 newly laid loggerhead sea turtle eggs. The eggs were divided, with half buried in a cooler area and the other half in a warmer spot, and monitored for temperature variations.

Analysis of blood samples from 116 hatchlings revealed a higher number of males than predicted, suggesting previous models had overestimated female hatchling production by 50-60%, likely due to previously unrecognized biological adaptations.

“This discovery highlights that molecular mechanisms exist that help turtles adapt to climate change by modulating the sensitivity of sexual differentiation to temperature,” Eizaguirre explains.

“While feminization is a concern and does occur due to climate change, we are suggesting that if populations are robust and genetically diverse, species can adapt to their environmental conditions,” he adds.

These findings are supported by recent evidence from Graham Hayes at Deakin University, which showed that more male sea turtles are hatching than originally expected if temperature were the sole factor in sex determination. Hayes notes that turtles can adapt their crucial temperature-related sex ratios to local conditions.

In addition, turtles employ other strategies to mitigate the impacts of climate change, such as nesting earlier in the season and adjusting their migration patterns to breeding grounds to counteract feminization effects. “While females may not breed annually, males migrate to breeding grounds more frequently, contributing to a more balanced reproductive sex ratio,” Hayes explains.

Despite these behavioral adaptations, Eizaguirre warns that hatchlings still face threats from excessive heat, which can lead to lasting changes in DNA methylation—an indication of molecular adaptation that is promising for these vulnerable reptiles.