Nesting turtles starting to feel the heat

Climate change is forcing sea turtles to nest earlier.

Researchers monitoring nesting green and loggerhead turtles on the Mediterranean island of Cyprus have discovered that they are returning to their regular nesting spots earlier each year to compensate for rising temperatures.

“Migratory marine species are some of the most endangered species globally, and among this group, sea turtles are the most threatened” writes lead author, Dr Mollie Rickwood, of the University of Exeter in the UK. 

The paper is published in Proceedings of the Royal Society B.

Turtle gender is set by nest temperature, as these reptiles lack sex chromosomes. On Queensland’s Heron Island, when nest temperature rises above 29.1^oC Green Sea Turtle hatchlings are all female.

Western Australian loggerhead turtles’ pivotal temperature (PT) is 28.7^oC — the PT is the nest temperature at which you’ll get equal numbers of males and females — higher temperatures mean more females.

 “In far North Queensland we have seen the world’s largest green turtle rookery producing a 99 per cent female population due to a warming climate, it also has very poor hatching success,” says Charles Darwin University Senior Research Fellow, Dr Rachel Groom.

Turtles also return to nest at the same beaches from which they hatched, which makes populations vulnerable to global heating.

University of Exeter researchers were able to explore questions of temperature’s effect on nesting responses of Green Sea Turtles through analysis of their 31-year data set focussed on a turtle population which nests every year on a North Cyprus beach in the Mediterranean.

The dataset contained information on more than 600 individual turtles, collected during nesting seasons from mid-May to mid-August from 1992 to 2023. Females had been tagged, and numbers of clutches and numbers of eggs recorded. Sea surface temperatures were recorded and temperature loggers put into nests when the females were laying their eggs and retrieved once the turtles hatched.

Modelling had revealed that there would be few Cypriote turtles produced by 2100 unless they moved their nesting season forward to counter the rising temperatures, writes Rickwood.

And the dataset showed that turtles had indeed been responding — nesting earlier —since 1993. 

Coauthor, Professor Annette Broderick says: “This is a bit of good news, as we’ve shown that these turtles are responding to the elevated temperatures brought about by climate change by shifting to cooler months to nest.

Read more: how turtles in WA behave, forage and breed

Individual turtles, particularly more experienced females, were laying eggs around 6.5 days earlier for every 1^oC rise in ocean temperature. And those turtles laying more clutches were also nesting earlier. 

Nesting season sea surface temperature rose by about 1.5^oC over the study period, write the researchers.

Rickwood says that the earlier nesting — referred to as ‘behavioural plasticity’ — may have been possible because egg development rate is dependent on temperature. Higher local sea temperatures may have increased egg maturation rates, allowing earlier nesting after mating, she adds.

“There is no guarantee that they carry on doing this though – it’s very much dependent on how much the temperatures rises, and also what they are eating.

“If the timing of production in terms of where their food’s coming from shifts, then they could start to be disconnected ecologically between where they forage and where they breed,” says Broderick.

Cypriote green sea turtles are herbivorous and forage for algae and seagrass in marine meadows off Turkey, Libya and Egypt, so annual patterns are dependent on the stability of food sources, whether that’s temperature-related changes in abundance and distribution, as Rickwood says, or other human-related impacts, such as pollution and dredging. 

Dr Damla Beton, from the Society for Protection of Turtles (SPOT), added: “Although our turtles appear to be coping with current rising temperatures, it is unclear how long they may be able to do this before conditions in Cyprus are no longer suitable, but cooler locations in the Mediterranean may become available for them to nest.”