Rising temperatures are leading to an increase in the proportion of female Green Sea Turtles (Chelonia mydas), recent studies show. Studies carried out on beaches of the Great Barrier Reef in Australia show that a striking 99% of a total 200,000 infant turtles on northern beaches are female. This is compared to females making up 87% of the total Green Sea Turtle population in Australia, which indicates that temperatures are having a generational impact. A similar study concluded that 97% of infant turtles in Palm Beach Country, Florida, are also female. The Green Sea Turtle species is already highly endangered, with continuously decreasing populations threatening the survival of the species.
Green Sea Turtles are one of few reptiles that display temperature-dependent sex determination. Hence, these trends are due to influences of temperature and moisture on turtle eggs during the incubation period. Research shows that increasing temperatures and decreasing moisture availability, ultimately linked back to climate change, are causing a greater proportion of embryos to develop as female. Identification of Green Sea Turtles is not as simple as you might think, with no morphologically defining features between the sexes. Scientists therefore used the presence of a male-specific protein, Sox9, to distinguish infant male turtles from the females.
Northern Australia has a warmer tropical climate than the temperate south, and the absence of the polar air experienced in the Northern hemisphere means north Australian winter temperatures are also particularly mild. As a result, the female bias is not balanced by cooler temperatures in winter months, leading to further increased proportions of females in the population.
A 50:50 ratio of males to females is expected at 29°C. Increased greenhouse gases in the atmosphere are causing a global warming effect, which has led to a temperature increase of 0.7°C in Australia in the last 100 years. Although this may seem quite low, the current trend of exponential increase predicts a further rise of 2.5°C by 2100. Australia is also very dry in comparison to other countries; with an average rainfall of 419 mm per year (compared to a 1,100 mm per year average in Britain). Together, these factors are detrimental to the survival of the male Green Sea Turtle population.
Populations are already threatened by human activity, including fishing and habitat destruction from an abundance of tourism around the Great Barrier Reef. Now the added threat of global warming is endangering the population even further, which could have devastating consequences for the species and associated ecology in the future. So, what can we do to protect this fragile species? Some solutions have been proposed which utilise tents or artificial rain to cool the sands so that male embryos can develop under cooler temperatures. However, if trends continue as they are, Green Sea Turtles will soon be a beauty of the past.
Images: Brett Monroe Garner/National Geographic (header), Miguel Roberts/The Brownswille Herald