Almost all baby green sea turtles hatching as females
Green sea turtles do not develop into males or females due to sex chromosomes, like humans and most other mammals do. Instead, the temperature outside a turtle egg influences the sex of the growing embryo. And this unusual biological quirk, scientists say, endangers their future in a warmer world.
Already, some sea turtle populations are so skewed by heat that the young reptiles are almost entirely female, according to a new report in the journal Current Biology.
“This is one of the most important conservation papers of the decade,” said biologist David Owens, a professor emeritus at the College of Charleston who was not a part of this research. It will not be long, perhaps within a few decades to a century, until “there will not be enough males in sea turtle populations,” he warned.
“They have temperature-dependent sex determination,” said Camyrn Allen, a National Oceanic and Atmospheric Administration endocrinology researcher and co-author of the new study. “It’s not genetics. It’s actually the temperature.”
At what biologists call the pivot temperature, turtles hatch as a mixture of males and females. For green sea turtles, this temperature is 29.3 C. A few degrees below 29.3 C, all the sea turtles are born male. Heat up the eggs and only females are born.
“That transitional range, from100-per-cent males to 100-per-cent females, spans a very narrow band of only a couple of degrees,” said NOAA marine biologist and study co-author Michael Jensen.
For several weeks, the scientists collected turtles, took plasma samples and released the animals.
It is difficult to distinguish young male turtles from females. Their external features are unhelpful — you cannot simply flip a subadult turtle over and inspect the undercarriage. In the past, researchers cut open juvenile turtles to inspect their gonads. But the researchers want to minimize their impact on the population; the International Union for Conservation of Nature lists these animals as endangered. And laparoscopic turtle surgery is an untenable proposition at scale.
Allen worked on a new technique to reveal the turtles’ sex through their hormones. The scientists examined the plasma samples in a California lab.
Why turtle sex is linked to temperature remains unclear, though some biologists have a hypothesis: Turtles that develop in colder conditions grow larger and it might benefit a turtle species if the larger ones are males.
Previous studies had predicted that green sea turtle sand other temperaturedependent reptiles might be changing in response to a warmer climate. Turtles around the world “are absolutely being affected right now,” said Owens, who has collaborated with some of the study authors in the past. “Many of the other species and populations my colleagues are studying are already showing 90-percent or more female populations.”
But no one had seen anything quite to this extent.
Doom will not come for these turtles tomorrow. In fact, the overall turtle population might briefly increase, as long as the more numerous female turtles can find males to fertilize their eggs. Turtles do not need a 50:50 ratio of males to females. “A few males can go a really long way,” Jensen said. “Male turtles mate more frequently” than female turtles do.
“It’s hard to say whether it’s good or bad but it’s big and it could have a lot of cascading consequences,” said Rory Telemeco, a biologist at California State University — Fresno, not affiliated with this research, who studies temperature and reptile development. “Though it does seem a little scary.”