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Pollution fueling a sex imbalance among endangered green sea turtles

Female green sea turtles crawl onto beaches to lay their eggs. Eggs nesting in warmer sand produce more females, and those in cooler sand produce more males. (iStock)
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Green sea turtles are producing more females in response to a warming climate — and human-caused pollution is helping fuel the surge, a recent analysis suggests.

Writing in Frontiers in Marine Science, researchers say ocean contaminants are contributing to a surge of female green sea turtles.

Like many other reptiles, sea turtles’ sex development is influenced by the temperature of their nests. Green sea turtles incubate in large clutches of eggs their migratory mothers bury in the sand on nesting beaches. Over the course of about two months, they develop from embryos into tiny turtles, with warmer sands producing more females and cooler sands producing more males.

On one hand, this form of sex determination is a brilliant evolutionary strategy, tipping the scales toward more offspring-producing females and possibly helping sea turtles adapt to climate change. But there’s a downside: As global warming increases temperatures on land and at sea, green sea turtles are producing so many females that both their genetic diversity and species survival are at risk.

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Too many females means fewer males to mate with, and the female green sea turtle ranks are already booming, leading to groups that are up to 99 percent female. One of the largest populations in the world has been producing primarily females for at least 20 years, another study found, threatening the “complete feminization” of the already endangered animals.

In the current study, researchers took liver samples from hatchlings, then analyzed them for substances that might have influenced sex development. They found evidence of contaminants thought to mimic the female sex hormone estrogen inside the developing embryo, pushing the odds toward female sex development.

Turtles born with higher concentrations of these substances — including metals such as chromium, lead and cadmium and industrial byproducts like polychlorinated biphenyls (PCBs) — were likelier to be female.

The study points to the compounded effects of human activities on ocean life. But it could also trigger solutions, the researchers say.

“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,” Jason van de Merwe, a marine ecologist at Griffith University and the study’s senior author, said in a news release.