Scientists find that rising temperatures are dramatically skewing sex ratios of a 200,000-strong Australian colony of endangered green sea turtles, threatening the animals’ survival.

The beaches and crystalline waters of the northern Great Barrier Reef are swarming with female green sea turtles. And that’s a big problem in paradise.

In one of the world’s largest sea turtle colonies, home to some 200,000 endangered animals, there are almost no young males. More than 99 percent of the turtles born on the reef’s northern beaches since the late 1990s have been female – the alarming outcome of rising global temperatures, according to a groundbreaking study published Monday, January 8, in the journal Current Biology.

Ambient temperatures influence the gender of a sea turtle during the incubation period in the egg. Cooler environments produce more males, while warmer conditions result in more females. Such temperature-dependent sex determination occurs in many reptile species and can benefit a population with more breeding females. Too many females and too few males, however, could spell disaster.

In the case of green sea turtles, never before has the phenomenon of temperature-dependent sex determination been documented to such an extent in so large a population of endangered animals. The authors of the new study – led by Camryn Allen and Michael Jensen, both researchers with the United States National Oceanic and Atmospheric Administration (NOAA) – warn that rising global temperatures could cause the “feminization” of most sea turtle populations, rendering them unable to reproduce and possibly threatening this population with extinction.

“We also know that higher incubation temperatures cause higher mortality in the eggs,” Allen said in an interview. In the study, she and her colleagues concluded that “it is clear that climate change poses a serious threat to the persistence of these populations.”

The scientists studied two genetically distinct populations of green sea turtles on Australia’s Great Barrier Reef. Taking blood and skin samples from 411 individual animals at offshore foraging grounds, they analyzed each turtle’s genetics and endocrine data. This novel approach, largely innovated by Allen, allowed them to precisely identify each animal’s birth beach and determine its sex without killing and dissecting them – the conventional means of identifying the gender of a turtle. Moreover, by collecting the data at offshore foraging areas, rather than using traditional methods of sampling newborn turtles emerging from their nests, the researchers were afforded a broad view of the population and its different age classes.

What they saw was startling. About two-thirds of the turtles from the southern population were female – roughly the average natural sex ratio for the species, Allen and Jensen said.

But of the turtles linked to warmer northern nesting beaches – especially Raine Island, where nearly 200,000 females return to lay their eggs – few to none were male. Of the adult animals, 86 percent were females. Among juvenile and subadult turtles ranging from 4 to 23 years of age, more than 99 percent were female. The scientists noted that the increasing ratio of females to males has tracked a rise in temperatures in the region, linking climate change to the phenomenon.

“Virtually no male turtles are now being produced from these nesting beaches,” the scientists wrote.

The breeding population remains reproductively functional for now, since males make up 14 percent of the mature animals. However, their viability as a population may diminish as the younger green sea turtles reach sexual maturity at about 25 years of age.

But it isn’t clear to the scientists when the tipping point toward extinction would be reached.

“A few males can go a long way in a sea turtle population,” Jensen said. “What we need to know is how far you can push the female bias before it becomes a problem.”

In biological terms, the incubation temperature that produces an equal percentage of males and females of a species is called the pivotal temperature. In the green sea turtles of the northern Great Barrier Reef, that temperature is 85F (29.3C) – a level the scientists report has been exceeded consistently since at least the early 1990s.

Global temperatures are projected to rise above the pivotal temperature for many of the world’s sea turtles, a clear existential threat to the animals, the researchers warned in the paper. NOAA classifies all six of the seven sea turtle species found in U.S. waters as threatened or endangered.

But sea turtles can adapt and through natural selection modify their pivotal temperatures. Already, pivotal temperatures in any given turtle species vary from region to region, according to Nicolas Pilcher, executive director of the nonprofit Marine Research Foundation in Sabah, Malaysia. This has allowed populations in warmer and cooler areas to maintain adequate ratios of males to females.

Pilcher, who was not involved in the Great Barrier Reef research, has studied sea turtles in the Persian Gulf, where he says sea surface temperatures are on a par with what is projected for northeast Australia in 50 years. In spite of this warm environment, which Pilcher has described as “a living laboratory for understanding climate change effects on marine species,” there are many male turtles here. The local hawksbill sea turtle population, according to Pilcher’s research, is about 80 percent male, and the gulf’s green sea turtles are about 40 percent male.

This doesn’t surprise Pilcher.

“The planet has warmed and cooled many times in sea turtles’ existence, and we know they can adapt – geographically that proof is already there,” he said. “What we don’t know is if they can adapt to the current rate of temperature change.”

The additional pressures of turtle poaching, fishery bycatch and other human impacts will make adaptation to changing environments still more difficult, if not impossible, Pilcher added.

The green sea turtle is listed as endangered by the International Union for Conservation of Nature. The species lives in tropical and subtropical waters around the world. In theory, any male from any population could breed with any female – but they don’t necessarily cross paths.

Along the Great Barrier Reef, the northern and southern groups have become genetically distinct through reproductive isolation. Turtles from the two groups may forage together but they do not often intermingle in courtship areas, according to Jensen and Allen. So even though plenty of males – about one for every two females – remain in the southern Great Barrier Reef population, they are not likely to provide reproductive services for the northern group. That means the northern population will likely become reproductively stranded, and possibly doomed to vanish, if it is completely feminized.

The authors say they plan to study more green sea turtle populations around the Pacific Ocean.

“What we need to know is how many males is enough to sustain a population,” said Jensen.

Efforts to shade beaches or otherwise keep them cool during key incubation periods could potentially help the animals by producing more male turtles, Allen said.

She says she is optimistic for the population, since the highly feminized generations are still years from their reproductive age.

“We still have 5–20 years on our hands to figure out what we can do to help them,” Allen said.