North of Australia, between the Arafura and Coral Seas, lies Raine Island, one of the most important marine sanctuaries on the planet for the reproduction of green turtles (Chelonia mydas). Annually, at least 200,000 females lay their eggs annually on the sands of island beaches in the region and thousands of baby turtles are born and raised there. However, as a result of climate change, 99% of newborns green turtles are females, which can drastically impact the future for this endangered species.
American and Australian researchers, most of them from the United States National Oceanic and Atmospheric Administration (NOAA), revealed that, the overwhelming majority of green turtles born in the region are female. This is a direct result of climate change. The article was published in the latest issue of the scientific journal Current Biology.
Why are only female turtles born on the Australian island?
For mammals, like us, the definition of each individual’s biological gender occurs during the genetic constitution of the embryo and develops along with the fetus’ growth, still in the mother's womb. Some species of reptiles, however, develop their gender differently, as is the case with green turtles.
The development of the turtles’ genitalia is defined by an external factor to the fetus: temperature. When green turtle mothers lay eggs, the temperature of the soil and water is the determining factor. If it is below 29.3 °C, more males are born; above 29.3 °C, more females are born.
Global warming puts the future of green turtles at risk
For scientists and authors of this study, there is no doubt about the direct correlation between global warming and the imbalance between female and male green turtles. Historical temperatures of sand, sea, and air of Raine Island have been analyzed from 1960 until 2016. The study found that, since the 1990s, there has been a consistent increase in heat measurement.
The region is especially affected because it is located in an area with naturally warmer water. As a result, global warming raises temperatures even more. In 2016, there was a peak of heat there, which strongly affected Australia's Great Barrier Reef.
“With the average global temperature predicted to increase 2.6°C by 2100, many sea turtle populations are in danger of high egg mortality and female-only offspring production,” the article affirms. “Our results show that green turtle rookeries have been producing primarily females for more than two decades and that the complete feminization of this population is possible in the near future.”
The disproportionality between green turtles’ genders is not a danger in the short term, say researchers, because each specimen lives up to 70 years and there are still enough males for breeding. However, this may be an irreversible problem in the future.
A census in the Marine Sanctuary of Raine Island found that 87% of adult turtles are already female, and that the overall proportion is 1 male for every 116 females.
Finally, scientists conclude, “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 to avoid a population collapse or even extinction.”