The Green Sea Turtle, Chelonia mydas, is listed on the IUCN Red List of Threatened Species. Their endangerment is attributed to factors such as poaching, boat collisions, habitat destruction, and accidental capture in fishing gear.

However, a more insidious threat linked to climate change exacerbates the situation: green sea turtles have temperature-dependent sex determination, meaning that as temperatures rise, more embryos become female. In the northern part of the Great Barrier Reef, off the coast of Australia, hundreds of females are born for every male.

Now, researchers have demonstrated that the subsequent risk of extinction due to the lack of male green turtles can be intensified by pollution.

Dr. Arthur Barraza, a researcher at the Australian Rivers Institute at Griffith University and the lead author of a new study published in Frontiers in Marine Science, states: Here, we show that pollutants from human activities can also influence the sex ratio of developing green sea turtles, further increasing the existing bias towards females.

Barraza and his colleagues studied the effects of pollution on the development of green sea turtles on Heron Island, a small coral sand cay south of the Great Barrier Reef. This island, where 200 to 1,800 females nest every year, is a long-term monitoring site for this species. Currently, the sex ratio here is more balanced than closer to the equator, with about two to three females born for every male.

Green turtle hatchlings, Chelonia mydas, on Heron Island, Australia | photo Arthur D. Barraza

This study is part of WWF-Australia’s Turtle Cooling Project, which explores ways to counteract anthropogenic influences on turtle sex ratios. Every effort was made to minimize animal suffering and maximize the data obtained from each sacrificed turtle hatchling. The study was approved by the animal ethics committee of the University of Queensland and the Queensland Parks and Wildlife Services.

The researchers collected 17 entire nests within two hours of egg-laying and reburied them nearby with automatic temperature probes. These probes recorded the temperature inside the nest and on the beach surface every hour.

When the hatchlings emerged, they were euthanized, and their sex was determined by dissecting and examining their reproductive organs. Their livers were also extracted, and the contaminants were measured using inductively coupled plasma mass spectrometry (ICP-MS) and bioassays on cultured sea turtle cells.

The study focused on 18 metals such as chromium, antimony, and barium, as well as organic pollutants like polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs). It is known or suspected that all of these function as ‘xenoestrogens’ in model organisms: molecules that bind to female sex hormone receptors.

Accumulation of these contaminants by a female turtle occurs where she searches for food. As the eggs develop inside her, they absorb the contaminants she has accumulated. These pollutants are sequestered in the embryos’ livers, where they can remain for years after hatching, explains Barraza.

The final sex ratio varied from 100% males to 100% females among the nests, although most nests predominantly produced female hatchlings. The higher the average amount of heavy metals like antimony and cadmium in the hatchlings’ livers, the greater the bias towards females within the nest.

Green turtle hatchlings, Chelonia mydas, on Heron Island, Australia | photo Arthur D. Barraza

The researchers concluded that these pollutants mimic the function of the estrogen hormone and tend to redirect development pathways toward females.

As the sex ratio approaches 100% females, it will become increasingly challenging for adult female turtles to find mates. This is particularly significant as climate change continues to make nesting beaches warmer and more skewed towards females, says Barraza.

Dr. Jason van de Merwe, the senior author from the same institute, added: Identifying specific compounds that could shift the sex ratio of hatchlings is crucial to developing strategies to prevent pollutants from further feminizing sea turtle populations.

Since most heavy metals result from human activities like mining, runoff, and general urban waste pollution, the best way forward is to implement science-based, long-term strategies to reduce the entry of contaminants into our oceans.


Frontiers Science News | Arthur D. Barraza, Larissa Young, et al., Exploring contaminants as a disruptor of temperature-dependent sex determination in sea turtle hatchlings. Frontiers in Marine Science, 13 November 2023. Sec. Marine Megafauna, Volume 10 – 2023.

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