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Pacific green sea turtles - 99% Are Turning Female

a crisis is unfolding, likely thanks to warming temperatures

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robert99 robert99 Sweden Posts: 1360
1 11 Jan 2018

Pacific green sea turtles spend years cruising this northern Australia feeding ground, fattening up on sea grasses before heading to nesting areas to mate and lay eggs. The scientists simply wanted to know: which of these reptiles were male and which were female?

You can't always tell a sea turtle's sex by looking, so researchers kicked off a "turtle rodeo." They stood atop skiffs and raced toward swimming turtles and launched themselves like bull wrestlers onto the animals' carapaces. After gently steering each turtle to shore, they took DNA and blood samples, and made tiny incisions to inspect turtle gonads.

Since the sex of a sea turtle is determined by the heat of sand incubating their eggs, scientists had suspected they might see slightly more females. Climate change, after all, has driven air and sea temperatures higher, which, in these creatures, favors female offspring. But instead, they found female sea turtles from the Pacific Ocean's largest and most important green sea turtle rookery now outnumber males by at least 116 to 1.

"This is extreme—like capital letters extreme, exclamation point extreme," says turtle scientist Camryn Allen, with the National Oceanic and Atmospheric Administration in Hawaii. "We're talking a handful of males to hundreds and hundreds of females. We were shocked."

New research published in Current Biology Monday by Allen and her colleagues is just the latest to suggest that rising temperatures around the world can turn sea turtle populations female. But it is the most detailed look to date at just how significant this problem is already, and raises new questions about the risks globally for marine turtles, as well as for other temperature-dependent species—from alligators and iguanas to inland silversides, an important fish in many streams and estuaries.

At a turtle conference in Mexico, he bumped into Allen, a former koala researcher. Allen had used testosterone levels to track pregnancies in the tree-loving marsupials. She went on to perfect ways of deciphering the sex of marine species based on hormone levels. All she needed was a little blood.

The pair teamed up with others, including Australian turtle expert Ian Bell, and drew blood from Great Barrier Reef turtles. They performed a few laparoscopy exams to confirm the accuracy of Allen's methods. They compared their results with temperature data for nesting beaches. And they examined turtles of varying age. The results caught them by surprise.

"We immediately said, 'Holy Smokes!'" Allen says. "It was way worse than we thought."

It appears that Raine Island has been producing almost exclusively female turtles for at least 20 years. This is no small thing. Eighty-acre Raine and its associated coral cays host one of the largest green sea turtle rookeries on Earth, where more than 200,000 turtles come to nest. During high season, 18,000 turtles may settle in at once. And those are just the females.

Since scientists also were able to determine rough ages for the turtles they sampled, they also made another discovery. Along that stretch of the northern Great Barrier Reef, where increasing heat had led to significant coral bleaching in recent years, the ratio of females to males had grown more severe with time. Turtles that hatched there around the 1970s and 1980s were also mostly female, but only by a ratio of 6 to 1.

"This is groundbreaking work," says Brendan Godley, a sea turtle expert and professor of conservation science at the University of Exeter. He was not affiliated with the study. The scope—encompassing the length of the Great Barrier Reef—and the multidisciplinary approach make the research highly valuable, he says.

Equally important is what Jensen and Allen found down south. There, turtles hatching from the southern reef near Brisbane—where temperatures have not increased as significantly, and where corals remain quite healthy—fare far better. There, female turtles today 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," Godley says. "These findings clearly point to the fact that climate change is changing many aspects of wildlife biology."