Rising temperatures pose long-term risk to sea turtles

By Julia John

A pair of loggerhead sea turtles swim in the waters around Cape Verde. ©Kostas Papafitsoros

Heat intensifying due to climate change could eventually prevent sea turtle eggs from hatching, decimating their already threatened populations, according to recent research from the Cape Verde islands off the Atlantic coast of Africa.

“The threat is that sand temperatures will become so warm that nests will fail and this will cause population decline,” said Jacques-Olivier Laloë, first author on the paper in Global Change Biology. “There is a risk of seeing at least some local extinctions if turtles aren’t able to adapt to anthropogenic climate change.”

Every summer, turtles crawl out of the sea and lay about 100 eggs per nest a couple meters into the shore, said Laloë, a marine biologist at Deakin University in Australia. For nests to successfully produce at least 70 hatchlings each, he said, incubation has to occur between 25 and 35 degrees Celsius, or 77 and 95 degrees Fahrenheit. Because the temperature at which embryos develop determines their sex, on the lower end of the temperature range, a nest will generate more males. On the higher end, it will generate more females.

To understand how warming temperatures would impact both sex ratios and hatching success, Laloë studied loggerheads (Caretta caretta) on Cape Verde, one of the planet’s largest nesting sites for the species. During nesting seasons from 2008 to 2014, his team recorded hourly sand temperatures across the beach 45 centimeters below the ground, the depth at which the turtles lay their eggs. The biologists also walked the beach at night to search for nesting turtles and count their eggs. Around hatching time two months later, the researchers excavated nests and determined the percentage of eggs that produced hatchlings that made it out of each nest. Then Laloë and his colleagues predicted how these numbers would look as temperatures increased, based on local projections by the Intergovernmental Panel on Climate Change and literature on hatchling survival at different incubation temperatures at various global nesting sites.

“When temperatures warm, you’ll have more females produced on the beach, hence more eggs and turtles,” Laloë said. “That benefits, to a certain extent, the sea turtle population because it increases their natural growth rate.”

Ultimately, however, it will get too hot for embryos to develop, fewer hatchlings will emerge and more nests will fail, he said.

“Around the turn of the century, it’s likely nest numbers will decrease at our field site because of increased hatchling mortality,” he said. “So warming temperatures are detrimental to turtles in the long run.”

Although Cape Verde loggerheads experience an 85 percent hatchling rate — a high success rate— at the moment, Laloë said, this will no longer be the case as temperatures rise and become unfavorable to egg development in the vulnerable species.

“Sea turtles have been around for hundreds of millions of years,” he said. “They’ve survived temperatures warmer than current temperatures, but the climate is changing so quickly now, it’s not sure they’ll have time to adapt.”

Temperature’s influence on reptile sex determination was discovered in the 1960s, Laloë said, and since the 1980s, people have been concerned about the potentially low viability of female-biased sea turtle populations in a warming future. But because males can breed with multiple females in a season, he believes skewed sex ratios are not the main problem.

“The effect of warming temperatures on hatchling survival is far more worrying,” he said. “If you don’t have hatchlings surviving at all, it doesn’t matter whether they’re male or female. It’s not good for sea turtles in the long-term.”

If turtles do not acclimate through natural selection of individuals with higher heat tolerance or through finding cooler places or times to nest, Laloë said, conservationists could help promote optimal temperatures for successful incubation and hatchling survival. People could plant endemic vegetation on the beach to reduce sand temperatures beneath the leaves, he said, or relocate eggs to controlled shaded areas. Such conservation efforts are currently being tested in Cape Verde, he said.

“It’ll help this generation of turtles and the next, but you’re not tackling the real problem, which is climate change,” Laloë said. “Warming temperatures will bring more threats to sea turtles in the future. It’s important to be aware of this already so we can plan for it and hopefully prevent the local extinction of sea turtle populations.”

Julia John is a science writer at The Wildlife Society. Contact her at jjohn@wildlife.org with any questions or comments about her article.

Read more of Julia's articles here.