Sea turtle populations are stable for now, though rising temperatures are producing more female hatchlings. Having more breeding females later on could increase the rate of population growth -- naturally bolstering the population size.
For species with temperature-dependent sex determination, such as turtles and some other reptiles, sex chromosomes don’t decide the hatchlings’ sex. Warmer incubation temps (typically above 29 degrees Celsius) produce female hatchlings. This “threat of feminization” has been known for years, though few studies offer predictions on how sex ratio populations may change.
An international team led by Jacques-Olivier Laloë from Swansea University, UK, studied one of the world’s largest loggerhead sea turtle rookeries: Cape Verde Islands off the western coast of Africa in the Atlantic. About 10,000 to 15,000 nests are laid annually across the archipelago. They focused on one of the 10 islands, Sal, in the northeast.
The team combined a variety of data from separate sources: sand temperature they collected using small data loggers during nesting season (August to October) on Sal from 2009 to 2012, air temperature data from National Center for Atmospheric Research (NCAR) records dating back to 1850, and warming scenarios from the Intergovernmental Panel on Climate Change (IPCC). Using these, they were able to calculate a 250-year time series of incubation temperatures, hatchling sex ratios, and operational sex ratios (or adult breeding sex ratios).
From 1845 to 2013, hatchling sex ratios were 55.60 percent female for light-colored beaches and 87.90 percent female for dark-colored beaches. They estimate that light beaches currently produce 70.10 percent females, and dark beaches produce 93.46 percent females. Both of those numbers will continue to rise from now through 2100: One particularly dramatic scenario, at a pivotal temperature of 28.8 degrees, predicts the percentage of female hatchlings will increase up to 99.99 percent.
The implications for the size of the adult population is unlikely to be dire in the next 150 years. Despite increasingly female skewed sex ratios, entire feminization of this population is not imminent. “In fact warm, incubation temperatures may have an unexpected conservation benefit of increasing the number of breeding females and hence the total size of the population,” study coauthor Graeme Hays of Deakin University, Australia, explains in a press release. Male loggerheads also breed twice as often as females.
However, their results suggest that by the middle of the next century, operational sex ratios at the Cape Verde rookery will include only 3.95 to 16.74 percent adult males on the breeding ground. At about 30.5 degrees, populations become fully female, and as remaining males die off, ''it will be end of story without human intervention,” Hays tells the Sydney Morning Herald. Also, embryos don’t survive when temperatures are higher than 33 degrees.
The work was published in Nature Climate Change this week.
[Via Sydney Morning Herald]