Hotter sands could mean more female offspring for endangered sea turtles

January 26, 2021

Male sea turtles are scarce as hatching-ground sand temperatures rise. (Ga. Dept. of Natural Resources via AP)

It's possible that endangered sea turtle hatchlings born on important coastlines of the Red Sea are now mostly female, according to new research, and climate change is poised to push scorching nesting-site temperatures even higher.

Sand temperatures at nesting sites around the region where long-living hawksbill and green sea turtles can lay clutches of 100 to 130 eggs have already exceeded the threshold at which hatchlings are generally thought to become predominantly female, researchers from King Abdullah University of Science & Technology said in a study published in Conservation Science and Practice in September.

Because the sex of sea turtle hatchlings depends on the temperature of the nesting site at the time of incubation, scientists have expressed growing concern that climate change could cause feminization at unsustainable rates. Studies set in Australia, Turkey and Brazil have found that up to more than 90% of green hatchlings were female, for example. But just a handful have examined the Red Sea area, and less is known about how turtles there are coping.

For this study, electronic data loggers collected sand temperatures every 15 minutes from each species’ average nesting depth at five sites in Egypt and Saudi Arabia between May and September 2018. 

At four sites, sand temperatures across that period were well above 29.2 degrees Celsius, or 84.56 degrees Fahrenheit, generally the critical point for maintaining populations comprising an equal proportion of males and females, which is important for breeding. This “pivotal temperature” varies slightly between species and geographic locations, however, and it still must be empirically confirmed for this region, the authors say.

“Does this mean they’re all females?” asked Lyndsey Tanabe, the study’s lead author and a Ph.D. student at KAUST’s Red Sea Research Center in Saudi Arabia. “No one yet has looked at the sex ratios of these turtles.”

Tanabe and her colleagues used an established maximum likelihood model to predict expected sex ratios at the sites, and suggested they reached up to 97% female at the hottest ones.

Additionally, temperatures hit a high of 36 and 35.3 degrees Celsius, respectively, at hawksbill and green turtle nesting sites. For all species, prolonged periods at 33 degrees Celsius is thought to cause hatchling deformities and mass mortality.

Those highs also mean temperatures are reaching the commonly cited maximum thermal tolerance for marine turtles of 25 to 35 degrees Celsius. Beyond that point, embryonic development and hatching becomes unlikely. 

While the authors note the possibility that the turtles are adapting to these high temperatures, a recent study found "no signs of localized thermal adaptation" among green turtles. 

“Future studies need to take a closer look and see if these turtles in this region are adapted to higher temperatures,” Tanabe said in an interview with The Academic Times. “If these turtles could adapt to these hotter temperatures, can turtles elsewhere?”

The Red Sea, which contains some of the hottest seawater and nesting beaches in the world, is home to five of seven marine turtle species. The green and hawksbill varieties that are the focus of the new research are classified as endangered and critically endangered, respectively. They are also the most common species, the authors note, with about 1,500 green and between 450 and 650 hawksbill nesting in the region each year.

Proposed mega-developments could further strain these populations, the authors say. Enormous “giga-projects,” including one called NEOM that’s set to span the northern coast of Saudi Arabia, parts of Egypt and Jordan, would encompass areas where turtles could be nesting. Such developments are trialing conservation elements like no-take zones, Tanabe explained, which could protect local flora and fauna and the economic benefits they could bring as Saudi Arabia seeks to become a major tourist destination.

As human-driven global warming continues to affect different species, it is also important to explore at what point high temperatures may be causing mortality because turtles are unable to hatch, Tanabe said. Another vulnerability of sea turtles to climate change is their late sexual maturity.

“There do need to be a lot of future studies to figure out how to protect them best, but this is a really good start, to find out just where they’re nesting and what the temperatures are there,” she said.

Just one of the five sites examined in the study, Small Global Island in the northern Red Sea, did not experience sand temperatures above the pivotal point. That could make it a priority area for conservation efforts, the authors suggested.

The next step for this research, which was part of Tanabe’s master’s program, is logging temperatures within the nests themselves, which would enable scientists to better understand hatching success rates and how they relate to heat. That work was interrupted by the COVID-19 pandemic but is set to continue this year.

Now in her Ph.D. program, Tanabe aims to establish other baselines for turtles in the area, too, including nesting locations, heavy metal pollution and plastic pollution and consumption.

The study “Potential feminization of Red Sea turtle hatchlings as indicated by in situ sand temperature profiles,” published Sept. 4 in Conservation Science and Practice, was authored by Lyndsey K. Tanabe, King Abdullah University of Science and Technology; Joanne Ellis, King Abdullah University of Science and Technology and Waikato University; Islam Elsadek, Egyptian Environmental Affairs Agency and Michael L. Berumen, King Abdullah University of Science and Technology.

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