Threatened green sea turtles at one beach in the northern Great Barrier Reef are almost entirely producing female hatchlings, with exposure to heavy metals identified as the cause.
The sex of green sea turtle (Chelonia mydas) hatchlings is temperature-determined. In 2018, researchers began connecting warming sands, due to global warming, with high female birth rates in some turtle clutches in the northern Great Barrier Reef.
In addition to established threats like poaching, vehicle collisions, fishing tackle entanglement and habitat loss, the influence of warming sands is a major threat to the long-term survival of green sea turtles along the reef.
But now, a team from three Queensland universities has identified exposure to chemical contaminants as another cause influencing high female birth rates. These chemicals, including cadmium, cobalt, lead, antimony and barium, have been shown to mimic female sex hormones within the species.
The findings, published in Frontiers in Marine Science, suggest that these metals accumulate inside mother turtles at their foraging sites, which can be thousands of kilometres offshore from their egg-laying sites.
Environmental pollutants have been shown to act as endocrine disruptors in other turtles and reptile species like alligators and caimans. To determine the effect of these substances on turtles, ethical clearance was granted to relocate and euthanise 17 clutches from Heron Island near Gladstone, with cooling applied to some groups to determine the influence of temperature control on sex determination.
Once euthanised, the hatchlings were dissected to determine sex and chemical samples were taken. About two-thirds of clutches produced more female than male turtles, and increasing metal concentrations within specimens were found to be a reliable indicator of female clutch ratios.
Uneven data drives metal research
“One of the things I noticed was that even when you try, even if you try really hard to predict what kind of sex ratio you’ll get at the end, a lot of these studies had a lot of error or variation in their results.”
Dr Arthur Barraza, a toxicologist at Griffith University, made this observation at a sea turtle conference when reading research into temperature as a sex determinant for these animals.
His work in toxicology had already exposed him to similar studies, so he decided to explore whether female hatchlings showed traces of chemicals at birth.
This study specifically looked for trace element samples within turtle tissue, rather than ‘organic contaminants’ – carbon-based chemicals that form pesticides, plastics and oil byproducts. It found cadmium was most associated with high female sex ratios in turtle clutches.
Other trace elements were associated with a female trend, but not significantly so.
“One of the things about toxicology is that sometimes it’s not one thing that causes a problem, it’s a mixture of things that make up to cause a problem,” Barraza says.
Some metals are known to have estrogenic behaviours. As well as checking for concentrations of cadmium and antimony, he bundled these estrogenic elements together to see whether they influenced sex as well, finding a trend – though not significantly – in favour of female hatchlings.
“Cadmium is well known to be an endocrine disruptor, it is well known to cause issues such as skewing organisms to be more feminine,” says Barraza.
Keeping eggs cool
The effects of temperature on turtle sex determination have been the subject of several studies in the years since researchers from the US National Oceanic and Atmospheric Administration made their first findings.
Much research has been conducted by WWF-Australia’s Turtle Cooling Project, which researches ways to limit incubation temperatures and prevent them from exceeding 29.1°C, which results in predominantly female hatchings.
Among potential remedies are the use of shade structures and irrigation methods to reduce egg temperatures. In 2021, University of Queensland researchers found treating nests with fresh or seawater would decrease temperatures by 1.3°C, with 4 in 5 turtles successfully hatching.
These studies into shade and irrigation ran alongside Barraza’s work. He hopes that his study will prompt further investigation into the role of chemical pollutants in influencing birth sex among marine species.
“We don’t know if this is a factor with some of these nests and if it is, it should be something that is looked at more closely,” he says.
“Oftentimes in science, something doesn’t really get investigated until it’s shown that it’s possible. That’s the point of this paper is to show ‘Yes, this is a possibility, this is another factor that is there’.”
While his research doesn’t definitively indicate metals like cadmium are driving high levels of female sea turtle births, he flags the risk of ignoring the findings. In particular, he said it shows an example of how pollutants can act in less lethal ways, contrary to popular assumptions.
“In a weird way, the main highlight of my paper is to show that sometimes our effect on the environment can be a lot more subtle, but still important,” he says.
“As these nests get warmer and warmer from climate change, you’re going to be pushing these nests more and more female because of those warmer temperatures, but if contaminants are further feminising them, then that might mean that this is a problem we need to address a lot sooner.
“It’s one of those things where it’s going to be a problem 20-25 years from now, and in a lot of ways sea turtle conservation is like a big ship – if you don’t steer it early, you’re not going to be able to steer it at the last second.”
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