The sex of hatchlings in sea turtles — and in a few other species such as alligators and crocodiles —depends on the temperature of the sand in which the eggs incubate, with warmer temperatures resulting in female hatchlings and cooler temperatures in males. The US National Oceanic and Atmospheric Administration (NOAA) website specifies the temperature ranges that lead to offspring of one sex or the other: male when it is 27.7°C or cooler, female when 31°C or warmer, and a mix of male and female baby turtles when the temperature fluctuates between these two limits.

In a study led by NOAA research biologist Michael Jensen, an international team of scientists used a new research method to assess sex ratios in two nesting populations of green sea turtles in Australia’s Great Barrier Reef. Turtles of the northern Great Barrier Reef (GBR) are genetically different from those of the southern GBR. The results of the study, published in Current Biology, showed an alarming female bias in turtles from the northern Great Barrier Reef, which is warmer than the southern GBR: 86.8% female among adults, 99.8% female among subadults and 99.1% female among juveniles. A female bias showed again in turtles from the cooler, southern GBR, but this was less stark: between 65% and 69% (see chart).

The fact that the ratio exceeds 99% among the younger turtles from the warmer region, while being 86.8% among adults of the region, indicates that the proportion of females has increased in recent decades.

“Yes, the rising temperatures are shifting the ratio of turtles from male to female. All sea turtles affected by temperatures this way are subject to this change as beaches where they nest become warmer,” NOAA public relations officer Michael Milstein told The Indian Express, by email. Although researchers have known for decades that warming temperatures alter the sex of sea turtle offspring, this is the first time they have directly documented the trend in a major wild population.

In an article on the research, NOAA’s Southwest Fisheries Science Centre briefly describes the way in which the sex ratios were determined. Researchers previously determined the sex of individual hatchlings through anatomical exams at nesting beaches, providing only a snapshot in time from only a few nests.

The new study used an innovative combination of endocrinology and genetics to assess the sex of hundreds of turtles across a large foraging ground, called the Howick Group of Islands, where turtles of both the northern and southern GBR stock aggregate. The assessment revealed sex ratios from different nesting beaches over many years, the article says.

While Milstein, the NOAA public relations officer, told The Indian Express that no threshold has been identified yet to determine when the number of females becomes so great that it interferes with reproduction, the study authors stress that the lack of male turtles will eventually impact the overall fertility of females in the population. Sea turtles being long-lived species, they will also have little room to adapt to a rapidly changing climate, which is predicted to raise temperatures by several degrees in a few turtle generations.

“We know that species evolve in response to climate and other environmental changes, but they need time for that. Unfortunately in this case, that may be one thing they do not have,” the NOAA article quotes research biologist Camryn Allen, one of the study authors.

Besides Jensen and Allen, both of NOAA’s Southwest Fisheries Science Centre, the research team included scientists from Queensland’s Department of Environment and Heritage Protection and WWF-Australia. Apart from raising new concerns over the immediate threats of climate change to sea turtle populations, the authors stress the need for effective conservation efforts in the long term.

“But more importantly,” they write, “our study highlights the need for immediate management strategies aimed at lowering incubation temperatures at key rookeries to boost the ability of local turtle populations to adapt to the changing environment and avoid a population collapse—or even extinction.” WWF-Australia CEO Dermot O’Gorman has suggested one possible management strategy.

“Scientists and wildlife managers now know what they are facing and can come up with practical ways to help the turtles,” a WWF-Australia news release quotes O’Gorman. “One possibility is shade cloth erected over key nesting beaches, like at Raine Island, to lower nest temperatures to produce more males.”