The northern Great Barrier Reef is a part of the world’s largest coral reef system and home to one of the world’s largest green sea turtle populations. Over the course of a nesting season, thousands of female sea turtles lay hundreds of eggs that, if all goes to plan, hatch, enter the sea, and when they reach sexual maturity, return to their natal beach to begin the process all over again.
That’s the way it’s been for hundreds of years. However, in a study published Monday in Current Biology, scientists reveal that something new is happening: The sea turtles being born are overwhelmingly female. A team of American and Australian researchers explain in their new paper that this high proportion of female hatchlings is due to rising incubation temperatures caused by climate change. Because the average ocean temperatures will continue to trudge upwards the researchers fear sea turtle populations are in danger of high egg mortality well before the end of the century.
“With warming global temperatures and most sea turtle populations naturally producing offspring above the pivotal temperature, it is clear that climate change poses a serious threat to the persistence of these populations,” the scientists write.
This study found that in the northern Great Barrier Reef nesting beaches, which are warmer than the southern Great Barrier Reef nesting beaches, females accounted for 99 percent of juvenile turtles, 99.8 percent of adolescent turtles, and 86.9 percent of adult turtles. In the cooler southern beaches, the females account for a lower (though no less worrisome) proportion of 65 to 69 percent of the entire turtle population.
Like other reptiles such as crocodiles and freshwater turtles, sea turtle sex is determined by temperatures during embryonic development, versus sex chromosomes in mammals. Cooler temperatures create more males and warmer temperatures result in more females, while an ideal temperature of 85 degrees Fahrenheit results in an equal distribution of the sexes. Even minor shifts of just a few degrees can result in very lopsided demographics.
To come to this conclusion, the research team combined temperature data with an analysis of plasma samples collected from turtles ranging from juveniles to adults. This focus on the plasma of the turtles is a new technique: Because turtles don’t have sex chromosomes, the scientists examined the hormones within their plasma to identify whether the turtle was male or female. An analysis of historical sea and air temperatures of the Great Barrier Reef breeding grounds collected between 1960 and 2016 suggested that sand temperatures had become consistently higher than the pivotal temperature by 1990.
“Combining our results with temperature data shows that the northern GBR [Great Barrier Reef] green turtle rookeries have been producing primarily females for more than two decades,” the scientists write, “and that the complete feminization of this population is possible in the near future.”
This “complete feminization” won’t happen overnight. Turtles can live up to 70 years old and they don’t really need that many males to keep up the overall turtle population. But hatchling survival rates are already lower because of increasing water levels and altered weather patterns. The warming of the habitat, and consequent loss in a balance of breeding partners, will solidify the precarious position of the species’ survival.
“Our study raises new concerns over the immediate threats of climate change to sea turtle populations,” the scientists write. “But more importantly, 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.”
Check out this video where Bill Nye predicts the future of animals and the environment.