In fact, one might think it is advantageous for the preservation of a species if more females are born. After all, a male can fertilize multiple females, but in the case of green sea turtles, the gender ratio is sometimes so imbalanced that females simply cannot find a male. In some locations around the Great Barrier Reef in Australia, there are hundreds of females for every male. Reproduction becomes highly improbable.
According to a recent study, exposure to heavy metals leads to an even higher proportion of female sea turtles being born, as reported by a research team in the journal Frontiers in Marine Science. The pollutants may mimic female sex hormones, resulting in the hatching of even more females—a threat to the species, which is already classified as endangered on the International Union for Conservation of Nature’s (IUCN) Red List.
The Warmer the Sand, the More Females
The gender development of turtle embryos in the egg has long been known to depend on the incubation temperature. The warmer the sand in which the nests are located, the higher the proportion of females. Crucial are temperature fluctuations of a few degrees Celsius. Due to climate change and the associated global temperature rise, in many places, only a few male sea turtles are currently being born. Therefore, in pilot projects, conservationists are attempting to cool nests.
However, it seems that not only rising temperatures contribute to the increase in female turtles but also environmental pollution. “We have found that pollutants from human activities can also influence the gender ratio of green sea turtles and intensify the existing tendency towards females,” says Arthur Barraza from the Australian Rivers Institute at Griffith University. Heavy metals, for example, enter the environment through steel and coal power plants.
For their study, Barraza and his team examined Green Sea Turtles on Heron Island, a small coral island in the southern Great Barrier Reef. Every year, between 200 and 1800 turtle females nest here.
Heavy Metals Remain in the Animals’ Bodies for Years
The research group collected 17 clutches within two hours after egg-laying and reburied them near temperature probes. These probes recorded the temperature in the nest and on the beach surface every hour. Based on the measured temperature, the team made predictions about which clutches should produce more females and which should produce more males.
However, when the researchers examined the gender of the turtles after hatching, it turned out that more females were born than expected. The likely cause was found in the liver of the sea turtles: the organ had accumulated heavy metals such as chromium, antimony, and cadmium, as well as organic pollutants suspected of acting as so-called xenoestrogens. These are molecules that bind to receptors for female sex hormones.
The mother turtle likely absorbed these pollutants through its diet. “These pollutants are then deposited in the embryos’ livers, where they can remain for years after hatching,” explains Barraza.
The tendency toward females became more pronounced as the liver was more burdened with antimony and cadmium. These substances likely act similarly to the hormone estrogen, steering the development toward females, the researchers speculate.
While it is not conclusively proven that these specific pollutants lead to more females, scientists are concerned. “The closer the gender ratio gets to one hundred percent females,” says Barraza, “the more challenging it becomes for adult female turtles to find a mate.”