Green sea turtles are adjusting their nesting habits in response to rising global temperatures. Individual females are laying their eggs earlier in the season to cope with warmer conditions, researchers report in the February Proceedings of the Royal Society B.
Scientists have long known that the sex of most turtle species is determined by incubation temperature — higher temperatures give life to females, and lower ones produce males. As climate change drives up temperatures, more females and fewer males are being born, potentially weakening populations. Extreme heat can also be lethal for the eggs.
To understand how turtles are adapting, conservation ecologist Annette Broderick and colleagues analyzed three decades of nesting data from around 600 tagged green turtles (Chelonia mydas) on the beaches of Northern Cyprus. The data included the number of successful hatchlings in each nest and temperatures during incubation. The team found that individual females nested earlier as temperatures rose, laying eggs just over six days earlier, on average, for each 1-degree-Celsius increase.
This is “the first time anyone looked at individual turtles and looked at how they’re changing,” rather than studying nesting behavior at a population level, says Broderick, of the University of Exeter in England.
While previous research has shown the nesting activity of whole turtle populations moving earlier, the new study confirms that individual turtles are actively adjusting their behavior in response to climate change. “This may seem like a foregone conclusion. It really isn’t,” says study coauthor Mollie Rickwood, a marine biologist at Victoria University of Wellington in New Zealand. That’s because population-wide shifts in nesting behavior could be happening for multiple reasons.
“For example,” Broderick says, “each female may be consistent in the time she nests each year, but new recruits to the population may lay earlier than older nesters, therefore making the average earlier.” By looking at individual animals, the researchers showed that all females are adjusting their timing.
Along with temperatures, breeding experience and number of clutches laid in a year equally influenced the timing of nesting. Other studies suggest that the earlier egg-laying seen now in some turtle species seems to be enough to offset the effects of warming temperatures on the eggs.
“This gives them a much better outlook in the face of climate change,” Rickwood says.