While warming temperatures will produce more female than male sea turtle hatchings, sea turtle populations will not crash — at least for the next few decades, a new study suggests.
In fact, sea turtle populations will increase because males breed more frequently than females, report researchers today in Nature Climate Change.
Sex in many reptile species is determined by temperature during incubation. For sea turtles incubation temperatures below 29°C produce male hatchlings; above that temperature females are produced.
This has caused worldwide concern that increasing temperatures will result in all-female populations, ultimately leading to their extinction.
To develop a more accurate picture of the future, an international team of researchers led by Jacques-Olivier Laloë from Swansea University studied a loggerhead turtle rookery in the Cape Verde Islands.
The team combined sand temperature measurements with hatchling sex ratios and operational sex ratios (actual numbers of males and females breeding at any one time).
These were then compared with more than 150 years of air temperature records and predicted warming scenarios from the Intergovernmental Panel on Climate Change (IPCC) to develop a long-term outlook for the population.
“What we’ve done for the first time is to include data on breeding periodicity collected by satellite tagging to show that the tendency of males to breed more frequently than females will help offset female skewed hatchling sex ratios,” says research team member Professor Graeme Hays, Chair of Marine Science at Deakin University.
“This shows that males are more likely than females to breed in successive years, because females invest so much to produce eggs and lose so much body condition that it takes several years for them to recover,” he adds.
“It means that even though the hatchling sex ratios are skewed in favour of females, the operational sex ratio is much more balanced with more females ready to breed in a given year.
“This is good news for the future of this species – at least for the next 150 years or so. We can’t say what will happen further down the track.”
The study also found a strong relationship between the colour of a beach, incubation temperature and hatchling sex ratios. Because light coloured sandy beaches reflect more heat (a phenomenon known as sand albedo) they produce an estimated 70 per cent female hatchlings. Dark-coloured beaches produce more than 93 per cent females.
Using IPCC predictions, these were expected to rise to 97.8 per cent and 99.5 per cent respectively.
“Temperatures are not projected to increase uniformly around the world, and the characteristics of beaches in a given area can vary markedly, whether it is the colour of the sand or the presence of vegetation that shades nesting sites,” Hays explains. Rainfall and wind can play a role, while temperature can also vary within a nest depending on the location of an egg.
“By using the same approach we have taken and applying it to other marine turtle species and other sites around the world, we will be able to identify key areas of concern, such as where the entire population might become female in the next few decades.
“Then we can take steps to manage that, by artificially cooling nests, or even moving eggs to hatcheries.”
Hays says Australia’s predominantly light-coloured beaches bode well for the huge populations of sea turtles that nest here.
However, he believes that accurate measurements of sand temperature on beaches used by green, loggerhead and hawksbill turtles in the Great Barrier Reef, and of the nesting beaches used by flatback turtle in the Pilbara will provide a clearer picture of the future.
“Even though lighter beaches are relatively cooler than dark beaches, they still need to be cool enough to produce some males.”