Imagine a volcanic island in the middle of the Atlantic Ocean, over a thousand kilometres from the nearest land, where cinder cones rise over barren plains of once-molten lava, and a cloud forest cloaks the highest peak in lush green vegetation that is home to tree-climbing crabs. Where seabirds gather in the millions and feast on the abundance of fish within one of the world’s largest marine protected areas.
This is Ascension Island, a UK Overseas Territory that few people have even heard of, but one which is uniquely important for tropical wildlife. It’s home to at least 71 endemic species, and around 25,000 Green Sea Turtle nests are laid there every year, making it one of the largest nesting sites in the world!
But sadly, these Green Sea Turtles have found themselves on the frontlines of climate change. So, today’s article is looking at a study carried out during the 2022 nesting season, which set out to understand how rising sand temperatures are affecting Green turtle survival, and to test a simple but potentially game-changing solution. But first, we need to dive into some sea turtle biology…
Turtles lay their eggs in deep chambers of sand, which they dig out using their back flippers as a spade. They can lay over 150 eggs in one clutch, and will very carefully cover them up with loose, aerated sand that allows the developing embryos to get oxygen, and enables the babies to dig their way to the surface once they hatch.
The entire nesting process is truly remarkable, but perhaps the most fascinating part is that, unlike mammals whose sex is determined at conception, turtles (and many other reptiles) have “temperature dependant sex determination”, which means the sex of the hatchling depends on the temperature that the egg was during the critical middle third of incubation.
For Green Turtles, the “pivotal temperature” at which sex flips from male to female is 29ᵒC, so eggs incubated at colder temperatures than this will produce males, warmer will produce females, and a clutch that is incubated at 29ᵒC will produce a balanced sex ratio of male and female hatchlings. Super interesting, but you can see how climate change might lead to some issues here!
As a result, all over the world, turtle populations are beginning to show “female-skewed sex ratios”, meaning that more females than males are hatching due to climate–change–driven increases in temperatures at nesting beaches. This may eventually mean there won’t be enough males to sustain a healthy population.
On Ascension Island, this problem is particularly severe because there is no vegetation or natural shading on the nesting beaches, so nests are exposed to the full heat of the sun. And, even worse than the skewed sex ratios, is that turtles also have an “upper thermal tolerance” which means that if they are incubated at temperatures above 34ᵒC the embryos will likely die.
So, with that in mind, our team of scientists on Ascension Island came up with a way to help the turtles maintain a healthy population. They decided to try re-locating some nests to a purpose-built hatchery, which was shaded from the sun, where the eggs could develop at a lower temperature and produce more male hatchlings to balance out the sex-ratio of the Ascension population.
The scientists went out to nesting beaches overnight (when turtles feel safest coming onto land to lay their eggs) and randomly chose 3 nesting females per night. One was the “in situ control” whose nest was set up with a device called a TinyTag, which sits inside the egg chamber and records temperature throughout the incubation period. The remaining 2 nests were both “relocated”, meaning they were very carefully dug up, and moved to man-made holes at the back of the beach, which were dug to the exact measurements of a natural turtle egg chamber.
One of these nests was left uncovered, and the other was shaded by a net screen, which blocked around 50% of direct sunlight from hitting the sand. Both the relocated nests also received a special TinyTag temperature logger. Repeat this for 20 nights, and you end up with some extremely sleep-deprived scientists and a nice big sample of nests!
The scientists then had an excruciating 6-8 week wait for the eggs to hatch. Once the nests got close to hatching, they would be checked every day for signs of the babies beginning to emerge.
First the sand above the egg chamber becomes dipped where the hatchlings are moving around inside the nest. Then you might see tiny flipper-tracks in the sand leading to the sea, or even be lucky enough to be down on the beach overnight or at dawn when a nest is “erupting” and hundreds of teeny turtles are making a mad dash for the safety of the ocean. Once a nest had been confirmed to have hatched, the scientists needed to leave it another painful 72 hours, just to make sure any slower hatchlings had plenty of time, before the nest could be excavated (or “exco’d” to use the turtle conservationist lingo).
An exco is a very sweaty, sandy, smelly kind of data collection that allows scientists to determine how successful a turtle nest has been. Scientists dig up the eggs, and count how many eggshells are left behind (a sure sign of successful hatchings), as well as opening any unhatched eggs to find out why they didn’t make it. The TinyTags were also collected and their temperature data downloaded.
The scientists found that there was no negative impact of relocating the clutches to hatchery sites, which was a sigh of relief and showed that their careful egg handling methods had paid off. They also found that on the hottest black sand beach, shading the nests resulted in 23% more hatchlings surviving- an incredible result! On the cooler white sand beach, temperatures were not so high that embryos were dying, but the shading did make a big difference to sex ratios, with 12% more male babies being produced from the shaded nests!
These results prove that, even with climate change causing very real and severe threats to sea turtles around the world, human interventions have the potential to make a really positive impact and change the tide for threatened turtle populations. Despite our world becoming increasingly hostile to many of the beautiful animals that call it home, we have the power to create change and bring endangered species back from the brink.
Article written by Maddie Kirby
Primary paper:
Wiggins, J. et al. (2023) ‘Efficacy of artificial nest shading as a climate change adaptation measure for Marine Turtles at Ascension Island’, Wildlife Society Bulletin, 47(4). doi:10.1002/wsb.1497. Available at: https://wildlife.onlinelibrary.wiley.com/doi/full/10.1002/wsb.1497
Other references
Weber, S.B. et al. (2014) ‘Recovery of the South Atlantic’s largest green turtle nesting population’, Biodiversity and Conservation, 23(12), pp. 3005–3018. doi:10.1007/s10531-014-0759-6. Available at: https://www.researchgate.net/publication/264166591_Recovery_of_the_South_Atlantic’s_largest_green_turtle_nesting_population
Photo credits: Maddie Kirby, 2024, featuring the Ascension Island Government Conservation and Fisheries Directorate 2024 turtle team.
