Endangered Sea Turtles Face More Risk

Green sea turtles may be running out of males, as the effects of rising temperatures and pollutants are making them hatch mostly female.

This endangered species has seen some beaches where over 99 percent of hatchlings emerge as female, with others seeing only one male per 200 females, putting the species at even greater risk of extinction, a new paper in the journal Frontiers in Marine Science has found.

The researchers, hailing from Griffith University in Australia, found that heavy metal pollutants including cadmium and antimony, as well as organic contaminants, may be responsible for some of the feminization of the hatchlings. Previous research has found that the warming climate is also pushing turtles and other reptiles towards hatching more females, as reptiles are often determined as male or female in their eggs based on their local temperatures.

"Sea turtles do not have a genetic sex, the sex of sea turtle hatchlings is determined by the temperature of the during development. Because of climate change, many beaches in the Northern Great Barrier Reef where sea turtles nest have warmer sand temperatures, which results in over 99 percent of the hatchlings being female," Arthur Barraza, a researcher at the Australian Rivers Institute at Griffith University and study co-author, told Newsweek. "I had the idea that perhaps there could be other factors, such as human pollution, that could further feminize sea turtle hatchlings."

Green sea turtles are the largest hard-shelled turtle in the world at 3 to 4 feet long, second in size only to the leatherback turtle. They are listed as "endangered" on the IUCN Red List of Threatened Species, and face a large number of threats that are causing their populations to decline, ranging from bycatch in the fishing industry, destruction of their nesting habitats due to coastal development, and plastic and chemical pollution.

The researchers investigated green sea turtle nests on Heron Island, a small sandy island in the southern Great Barrier Reef off Australia's east coast. Between 200 and 1,800 females nest on this island every year. The scientists collected 17 clutches of eggs that had been laid within 2 hours and reburied them next to probes that recorded the internal temperature of the nest and the outside beach surface temperature. After the turtles hatched, they determined their sex, and measured the levels of 18 metals, and several organic contaminants, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polybrominated diphenyl ethers (PBDEs).

They found that the presence of heavy metals in the hatchlings' livers was correlated with more females being hatched.

"The female sea turtles tend to live in one area for long periods of time only leaving for nesting. Some female turtles will live near cities or areas of human industries which can then contaminate their food sources with human pollutants. These pollutants are absorbed by the female turtles and when eggs develop inside of her, some of those pollutants transfer from the mother to the eggs," Barraza said. "What our study found was that some heavy metals, such as cadmium and antimony, were correlated with more females than expected in the clutches that we investigated."

The sex ratios that hatched at the Heron Island study site were around two to three females to every male, which is much less extreme than the 200-to-1 seen on some other beaches around the world. This indicates that heavy metal pollution can exacerbate the already strong effect that temperature is having on the sex ratio of turtles.

The reason that these pollutants cause more females to be born is because they sometimes function as "xenoestrogens", which are molecules that bind to the receptors of female sex hormones in the body.

"In freshwater turtles and crocodiles/alligators, we already have a lot of research showing that pollutants can change the sex of these reptiles whose sex is also determined by temperatures. Our study shows that it could possibly happen with sea turtles as well," Barraza said. "To be clear, our study is not definitive. Often in science, a topic isn't explored until it is shown to be a possibility, and that's what our study did. It showed that some metals correlated with more females than expected with just temperature and warrants further study."

If factors including temperature and pollutants like these heavy metals continue to shift the sex ratio of these turtles towards producing more and more females, it could be very bad news for the species as a whole, as both males and females are needed to reproduce.

"If the sea turtle hatchlings feminize to an extreme extent, eventually there will be too few males to mate with all the females," Barraza said. "Luckily sea turtles live for a long time and this is something that could be a problem in a few decades."

The researchers hope that by determining which compounds have this effect on the turtles, they can locate their source and act to prevent the turtles being exposed to the contaminants.

"Doing something about this problem now is much easier than waiting 20 years for the problem to progress," Barraza said. "It is therefore important that there are comprehensive laws and policies in place that reduce the number and extent of pollutants that humans put into the environment and our ocean ecosystems."

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