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Pollution Threatens Endangered Turtles With Imbalanced Sex Ratio, Warns Study

Heavy metals mimic female sex hormones, increasing the risk of extinction for green sea turtles.
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An endangered species of turtle faces a new threat from pollution – as it may lead to an excess of females being born, warns new research.

Heavy metals appear to mimic female sex hormones – feminizing broods of green sea turtles, say scientists.

The species Chelonia mydas is listed as “endangered” on the IUCN Red List of Threatened Species.

Poaching, collisions with boats, habitat destruction, and accidental capture in fishing gear are all a threat to the turtles, say conservationists.

But now another “more insidious” threat, linked to climate change, has emerged, warns a study published in the journal Frontiers in Marine Science.

Heavy metals appear to mimic female sex hormones – feminizing broods of green sea turtles, say scientists. PHOTO BY ARTHUR D.BARRAZA/SWNS 

Scientists explained that sea turtles have temperature-dependent sex determination, which means that more and more embryos develop into females as temperatures keep rising.

Already, in the northern part of the Great Barrier Reef off Australia, hundreds of females are born for every male.

Now, researchers have shown that the resulting risk of extinction due to a lack of male green sea turtles may be compounded by pollution.

Heavy metals appear to mimic female sex hormones – feminizing broods of green sea turtles, say scientists. PHOTO BY ARTHUR D.BARRAZA/SWNS 

Study first author Dr. Arthur Barraza said: “Here we show that contaminants from human activities may also influence the sex ratio of developing green sea turtles, increasing the already extant bias towards females.”

Dr. Barraza and his colleagues at the Australian Rivers Institute, Griffith University, studied the effects of pollution on the development of green sea turtles on Heron Island, a small coral sand cay in the southern Great Barrier Reef.

The island, where between 200 and 1,800 females visit to nest every year, is a long-term monitoring site for the species.

The sex ratio on the island is currently more balanced than nearer the equator, with around two to three females hatching for every male.

The study was part of WWF-Australia’s Turtle Cooling Project, which looks into ways to counter anthropogenic influences on turtle sex ratios.

The researchers collected 17 entire clutches within two hours after the eggs were laid, and reburied them nearby next to automatic temperature probes which registered the temperature inside the nest and at the beach surface every hour.

When the hatchlings emerged, they were euthanized and their sex was determined by examining the sexual organs.

Their livers were also removed and contaminants in it were measured.

The research team focused on 18 metals including chromium, antimony, and barium, as well as organic contaminants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs).

The researchers said that, in model organisms, the contaminants are all known or suspected to function as ‘xenoestrogens’ – molecules that bind to the receptors for female sex hormones.

Dr. Barraza said: “Accumulation of these contaminants by a female turtle happens at the site where she forages.

“As eggs develop within her, they absorb the contaminants that she accumulated.

“These then are sequestered in the liver of the embryos, where they can stay for years after hatching.”

He said the final sex ratio varied from 100 percent males to 100 percent females between clutches, although the majority of nests produced mainly female hatchlings.

The greater the average amount of the heavy metals antimony and cadmium in the hatchlings’ liver, the greater the bias towards females within the nest.

The researchers concluded that the contaminants mimic the function of the hormone estrogen, and tend to redirect developmental pathways towards females.

Dr. Barraza said: “As the sex ratio gets closer to 100 percent females, it will get harder and harder for adult female turtles to find a mate.

“This becomes especially important as climate change will continue to make nesting beaches warmer and more female-biased.”

Senior author Dr. Jason van de Merwe added: “Determining which specific compounds could change the hatchling sex ratios is important for developing strategies to prevent pollutants from further feminizing sea turtle populations.

“Since most heavy metals come from human activity such as mining, runoff, and pollution from general urban center waste, the best way forward is to use science-based long-term strategies to reduce the input of pollutants into our oceans.”

Produced in association with SWNS Talker

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