In a nest of green sea turtle eggs, the sex of the hatchlings depends on the surrounding sand’s temperature—warmer sand produces clutches with more females, while cooler sand leads to more males. Because of this, green sea turtles are in trouble.
As the world warms because of human-caused climate change, a larger amount of hatchlings are being born female. At some beaches in the northern Great Barrier Reef in Australia, that proportion has reached 99 percent. This puts the species at risk of extinction, because without males, the endangered reptiles can’t reproduce.
Now, researchers have discovered another factor that may be adding to the sex bias: metal pollution.
Adult female green sea turtles foraging for sea grass and algae might accidentally ingest contaminants, which they can then pass on to their offspring. Some of these heavy metals could mimic female sex hormones and contribute to the rising proportion of female embryos, according to a study published Monday in the journal Frontiers in Marine Science.
“These data provide a disturbing insight to the more subtle effects of human-induced environmental contamination in a species with populations already heavily skewed in favor of females and climatic influence,” says Arthur Georges, an ecologist and herpetologist at the University of Canberra in Australia who was not involved in the new research, to New Scientist’s Chen Ly.
In the experiment, researchers gathered 17 clutches of green sea turtle eggs just after they were laid on Heron Island on the southern Great Barrier Reef. They reburied the eggs nearby, where sensors recorded the sand temperature every hour.
Once the eggs hatched, the scientists euthanized hatchlings from 16 of the clutches so they could dissect their livers and examine their sex organs. They found that 11 of the clutches had more females than would have been expected from the sand temperature alone. Two of the clutches aligned with the predicted sex ratio, while three had more males than anticipated.
Turtles from the 11 female-heavy clutches had higher amounts of antimony, cadmium, lead and cobalt in their systems, while those from the male-skewed clutches had lower levels of these metals.
Notably, the findings show correlation only—not that the heavy metals caused the feminization of the embryos. However, the scientists have some theories about what may be going on. They suspect that the metals are binding with female sex hormone receptors, just like naturally occurring estrogen would. This may “redirect developmental pathways towards females,” says study co-author Arthur Barraza, a toxicologist at Griffith University in Australia, in a statement.
Though the findings are grim, they also point to possible solutions. Researchers say the heavy metal contaminants identified in this study come from urban waste, mining operations and other runoff, which means reducing the amount of pollutants going into the ocean from these sources could help protect the turtles.
Pollution and global warming aside, green sea turtles are also facing extinction because of habitat destruction, poaching, entanglement with fishing gear and boat strikes. Rising sea levels and storms are eroding the beaches where they lay their eggs.
In the short term, the feminization of green sea turtles may help the species’ numbers rebound. Once they reach sexual maturity between age 25 and 35, females can lay up to nine clutches each season, with each clutch containing roughly 110 eggs. More females means more hatchlings—and, indeed, this may be boosting populations in places like Florida and Texas, which both had record sea turtle nesting seasons this year.
But longer term, the lack of males will make it harder for females to find mates. Over time, this may also lead to more inbreeding and a lack of genetic variation.