Rising temperatures turn turtles all female: study

One of the world’s largest green sea turtle colonies is shifting to an all-female population as the planet heats up, in a signal that may be repeated in other reptiles whose offspring gender is determined by temperature, a new study of the Great Barrier Reef has found.

The research, published on Tuesday in Current Biology, found more than 99 per cent of juvenile and sub-adult turtles studied at a site in the northern Great Barrier Reef are now female.

A slightly lower 86.8 per cent of adult turtles are female, indicating the gender tilt has been occurring for more than two decades.

By comparison, rates among the same green sea turtle species in the southern Great Barrier Reef are roughly two-to-one female, or close to the natural ratio.

Turtles, such as this leatherback, adjust the gender of their offspring according to temperatures. Photo: Supplied

“This research is so important because it provides us with a new and a better understanding of what these populations are dealing with,” said Michael Jensen, a marine biologist with the US’s National Oceanic Atmospheric Administration and lead author of the report.

For a species where males mate with several females, it is beneficial to be slightly female biased to increase reproductive potential, but Dr Jensen said the problem is scientists do not know how many male sea turtles are needed to sustain a population.

“Sea turtles have been around for more than 100 million years and they’ve been adapting to changing climates,” Dr Jensen said.

“However, the climate right now is probably changing faster than ever. How are these populations going to respond and will they be able to respond fast enough?” Pivotal temperature

As with some other reptiles, temperatures can determine the sex of offspring.

For green sea turtles nesting in the northern Great Barrier Reef, the so-called pivotal temperature for whether the hatchlings lean towards more males or females is about 29.3 degrees.

Just half a degree to a degree warmer than the pivotal point produces all females, and any hotter can be lethal for offspring, Dr Jensen said.

While a similar gender bias takes effect at cooler temperatures – potentially producing all-male offspring – that shift is becoming less likely as climate change heats up background conditions.

The report stated the average global temperature is predicted to increase by 2.6 degrees by 2100 and as a result, many sea turtle populations may see high egg mortality rates and female-only hatchlings.

Researchers tracked the sex ratios of green sea turtles as they returned to feeding grounds on the Great Barrier Reef. Here, multiple generations of turtles could be studied, providing a window into the past and the future.

Dermot O’Gorman, chief executive of WWF-, said the results are more bad news for a region that has been hard hit by an unprecedented bout of coral bleaching over the two previous summers.

“First back-to-back mass coral bleaching and now we find that virtually no male northern green turtles are being born [in the north],” Mr O’Gorman said.

“These impacts show that the Great Barrier Reef really is at the frontline of climate change.” ‘Alarm bells are ringing’

Dr Jensen, however, remains optimistic that green sea turtle populations can prevail.

“These results on the north Great Barrier Reef are concerning, alarm bells are ringing, but there’s still time [to save the turtles],” he said.

“Knowing the sex ratio in that adult breeding population today and what that may look like five or 10 or 20 years from now … [is] going to be incredibly valuable for conservation managers.”

Mr O’Gorman agreed, saying that all was “not lost for this important population”.

In terms of what can be done, Mr O’Gorman said “one possibility is shade cloth erected over key nesting beaches, like at Raine Island, to lower nest temperatures to produce more males”.

The research is expected to have other benefits, such as providing ways to track changing sex ratios of species other than turtles.

“We’ve now proven that this is a really good way to look at that broad spectrum of sex ratios,” Dr Jensen said.

“These are methods that can be applied to other populations of turtles around the world and other species.”

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