Out of her shell: Rising temperatures mean more female turtles.
Sea turtles are likely to be beneficiaries of a warming climate as hotter incubation conditions trigger a rising share of female hatchlings that could lift natural rates of population growth, new research to be published in Nature Climate Change on Monday shows.
But gains will be temporary if temperatures keep rising and nudge populations towards becoming all female, or exceed levels at which developing embryos die, the study found.
It will be end of story without human intervention.
''There'll be a bit of a breathing space … but down the track it'll be serious,'' said Graeme Hays from Deakin University, one of the report's authors.
It has been known for decades that reptile reproduction is highly sensitive to temperature, with the ratio of male to female offspring varying. For species of sea-turtles, the pivotal temperature is an oddly uniform 29 degrees for incubation, beyond which more females emerge from the eggs.
At about 30.5 degrees, populations become fully female. As remaining males die off, ''it will be end of story without human intervention'', Professor Hays said. At higher than 33 degrees, embryos do not survive.
The study focused on a globally important loggerhead turtle rookery on the Cape Verde Islands in the Atlantic but its results also apply to species elsewhere, including the Pacific. It found light-coloured sandy beaches already produce 70.1 per cent females, while beaches with darker sands are at 93.5 per cent.
The findings should help steer conservation efforts to make a priority of protecting lighter-coloured sandy beaches or planting more vegetation near dark ones to ameliorate the warming, Professor Hays said. ''If you have to build a hotel, build it behind the dark-coloured beach,'' he said.
Since breeding populations are likely to swell in coming decades, sea turtle adult populations are ''unlikely to be dire in the next 150 years'', the paper said.
Professor Hays said any near-term increase in turtles would be modest compared with past populations. Green turtles in the Caribbean, for instance, are ''a fraction of 1 per cent'' of their original numbers.
Other changes linked to global warming, including effects on food sources, will also likely offset some of the benefits of having more breeding females, he said.
''Rising sea levels resulting in the loss of nesting beaches [through erosion] could push local turtle populations over the brink unless new suitable nesting beaches are found,'' the paper said.
It remains to be seen whether sea turtles, which have survived hundreds of millions of years, can adjust quickly enough to a changing climate, Professor Hays said.
Possible adjustments could include females laying their eggs at milder times of the year or shifting to cooler regions.