The sex of green sea turtles is determined by the egg's temperature as it's being incubated.
Warming temperatures are having a profound and potentially devastating impact on one of the most important green sea turtle populations in the world.
Scientists were surprised to find that "virtually no male turtles" are being hatched in a key breeding ground in the northern Great Barrier Reef.
Like many reptiles, the sex of a turtle is determined by how warm the egg is as it's being incubated. And small temperature differences can cause dramatic changes in the male-to-female ratio.
"Within a few degrees Celsius you go from 100 percent males to 100 percent females," says marine biologist Michael Jensen. "A really narrow range, that transition." The team's research was published this week in Current Biology.
The scientists studied turtles from two distinct breeding grounds – a larger population from the warmer northern Great Barrier Reef, and a smaller population from a cooler area about 1,200 miles to the south.
What they found was dramatic: the population that hatched in cooler areas is about 69 percent female. But in the group from the warmer north, scientists found more than 99 percent of juvenile and young adult turtles are female.
The scientists were able to compare ratios across turtles of different ages. Study coauthor Camryn Allen, a scientist focusing on turtle endocrinology, says the results show a change over time. The older turtles from the northern breeding grounds have a less extreme ratio. "In the past 20 years since these turtles were hatched, there's been some sort of a drastic change, going from one male to seven females to now — one male for one hundred females."
The difference in turtles from the two breeding grounds is more dramatic than the scientists expected. After hatching, the turtles from each place travel to the same foraging ground, eventually returning to the places where they hatched to breed.
Because the two populations were all in the same area, Jensen said previous researchers hadn't noticed the stark differences in their sex ratios. This team's genetic testing, which identified the turtles as from the north or south, made the difference clear. "We saw that one of the populations was slightly female biased, one to two roughly, while the other was extremely female biased."
It's not clear what proportion a population needs to remain healthy. Allen says that even with the skewed sex ratio, current adults from the north "seem to be doing quite well." The researchers say that males can breed with multiple females in a single season, and also breed more frequently than females.
Allen is more worried about what happens in a couple of decades: "What we are concerned about is when the cohort of young female turtles become adults. Will there be a male for them to mate with?"
The researchers say a "complete feminization of this population is possible in the near future."
Historically, turtles have dealt with swings in temperature and continued to flourish. But, "what's happening now is the climate is likely changing faster than ever," says Jensen. And the fact that turtles take some 25 years to reach sexual maturity means that the species could take centuries to adapt.
Now, the researchers are exploring strategies to cool the nests, like using shade cloth or putting water on top of them.
And aspects of male turtle behavior are still mysterious, the scientists add. Questions like whether male turtles ever breed out of their population, and how frequently they can mate in a single season, could have huge consequences for the future of these green sea turtles.