Thanks to rising temperatures and warming seas, nearly all of the green sea turtles in the Pacific Ocean’s largest and most important sea turtle population are turning female.
Scientists from the National Oceanic and Atmospheric Administration, California State University and Worldwide Fund for Nature Australia examined two genetically distinct populations of sea turtles on the reef for the study and found the northernmost group of 200,000 turtles was overwhelmingly female.
A sea turtle’s sex is determined by the heat of the sand incubating their eggs, according to National Geographic. As the climate continues to change, air and sea temperatures rise, a phenomenon that favors female offspring.
"This is extreme—like capital letters extreme, exclamation point extreme," NOAA scientist Camryn Allen told National Geographic. "We're talking a handful of males to hundreds and hundreds of females. We were shocked."
Female sea turtles in the rookery, who typically lay eggs on Raine Island and on nearby cays, now outnumber juvenile males by at least 116 to 1. More than 99 percent of the young turtles are female, the scientists found, and 87 percent of mature turtles are female.
In the 1970s and 1980s, turtles that hatched in the same area were also mainly female. But that ratio was 6 to 1.
So, what’s happening along that stretch of the reef?
“Temperatures are changing incredibly fast," NOAA scientist and lead author Michael Jensen said. "Evolution requires many generations for animals to adapt. But these are animals that live for 50 years or more, and things are changing dramatically just in their lifetimes."
The increase in heat has also led to significant coral bleaching and rising seas, destroying nesting sites and drowning eggs.
“The reptiles are so temperature-sensitive that a rise of just a few degrees Celsius could in many places eventually produce entirely female offspring. That could wipe out whole populations. If temperatures climb too high, things actually get worse; eggs literally cook in their nests.”
Recent studies of 75 sea turtle rookeries around the world found the average ratio of females to males is roughly 3 to 1.
“Combining our results with temperature data show that the northern GBR green turtle rookeries have been producing primarily females for more than two decades and that the complete feminisation of this population is possible in the near future,” the scientists wrote.
But scientists worry that in 20 years or so, when the largest sea turtle population near Raine Island is entirely female, will there be enough to sustain the population?
The turtles don’t require an equal ratio of males to females to survive. Because males mate more frequently, a few can go a long way.
“It’s hard to say whether it's good or bad but it’s big and it could have a lot of cascading consequences,” Rory Telemeco, a biologist at California State University Fresno, who was not affiliated with the research, told the Washington Post. “Though it does seem a little scary.”
In the south, near Brisbane, researchers found the corals were much healthier and temperatures had not increased as significantly. Female sea turtles in that population outnumber males by only 2 to 1.
"This combined with some neat modeling shows that cooler beaches in the south are still producing males, but that in the more tropical north, it's almost entirely females hatching," Brendan Godley, a conservation science professor at the University of Exeter who wasn’t involved in the study, told National Geographic. "These findings clearly point to the fact that climate change is changing many aspects of wildlife biology."
Study researchers hope to continue applying their knowledge and techniques to other testing places around the globe.