Maybe for every Jack, there is a Jill, but for every Tyron Turtle, there may soon be thousands of Tillie Turtles.
The balance between male and female sea turtles has gone askew, threatening the future existence of the animal, according to some marine researchers.
Scientists began seeing a shift in the gender balance a few decades ago, and they blamed it on rising seawater temperatures caused by climate change. As with many reptiles, nest temperature plays a key role in sex determination for turtles. Simply put, the warmer the sand where a turtle nest is buried, the more embryos in it are likely to develop into females, and the cooler the sand, the more males.
Ecologically, the warming shift was bad enough.
Now, a new study suggests the trend is being exacerbated by human pollutants with an estrogen-like chemical component. Ingested by a female and passed on to her eggs, the chemicals appear to enhance the chances an embryo will morph into a female hatchling.
The research was led by the Australian Rivers Institute of Griffith University, Australia. It was published earlier this month by the periodical “Frontiers in Marine Science.”
The researchers collected 17 clutches of Green sea turtle eggs from Heron Island, a small coral sand cay in the Southern Great Barrier Reef, which is a prime turtle nesting site.
They took liver samples from the hatchlings and examined them for chemical components. They found high concentrations of numerous contaminants, including lead, chromium, and polychlorinated biphenyls (PCBs) and various industrial byproducts thought to mimic the female hormone estrogen.
Results varied among the clutches, but most of them produced significantly more females than males. And, the greater the amount of estrogenic trace elements there were (particularly antimony and cadmium), the higher the female bias was.
While the pollution study was confined to green sea turtles, research on the effects of warming waters has included many species in various parts of the world, with results indicating a widespread and growing gender imbalance.
For instance, as the Australian study notes, “Loggerhead sea turtles nesting within Cabo Verde Islands are predicted to reach over 99 percent female hatchling production, with 90 percent of nests reaching upper thermal limits under current climate change predictions.” And there are other sites that already are 99 percent female.
In fact, in the Virgin Islands, at the territory’s primary nesting sites on St. Croix, hatchlings now are “almost all female,” according to Paul Jobsis, professor and director of the Center for Marine and Environmental Studies at the University of the Virgin Islands.
Three types of turtles nest locally: green sea turtles, hawksbills, and leatherbacks. Scientists and volunteers have been monitoring nesting sites at Sandy Point on St. Croix since the late 1970s.
Jobsis was unaware of the Australian study on pollutants but said the trend toward female-dominated nestlings has been ongoing in territory waters since at least the early 2000s; that’s when it was first documented.
“It might be a long-term problem,” Jobsis said.
Then again, “Turtles are really tough. They’ve been around since the dinosaurs,” he said. They are notoriously adaptable.
Historically, the breeding ratio of most sea turtles is thought to have been one-to-one, he said, which doesn’t necessarily mean they are monogamous. It’s difficult to establish breeding patterns since they don’t lay eggs annually. Some species make nests every two years, some every three years. Most research has been conducted on females when they come ashore to nest. Males stay in the ocean and are consequently more of a mystery.
“What we don’t know is what’s going to happen in another 20 years,” Jobsis said. The natural lifespan of most sea turtles that make it through the hatchling stage is 60 or 70 years. It takes roughly 20 to 25 years for them to mature and be ready to reproduce, which means that it will take that long for the current generation of hatchlings to have a noticeable influence on the breeding process.
The future may be grim, but some fairly recent research projects undertaken by some of his UVI students, in conjunction with others, have bolstered Jobsis’ optimism for sea turtles.
There is data from Sandy Point, compiled and analyzed in a study led by Makayla Kelso, that points to the success of conservation efforts established in the second half of the 20th century. In the 15-year period from 1993 to 2008, the average number of hawksbill nests at Sandy Point was 44. In the 13-year span from 2009 to 2022, the average number had grown to 241.
And, at least regionally, on the immediate question of gender imbalance because of warming waters, there may be a glimmer of good news.
An abstract of a study led by UVI student Katie Ayres suggests that turtle nests on St. Thomas may produce more males than females and could potentially help balance the turtle population in the immediate area, at least in the near term.
Turtle nesting on St. Thomas pales in comparison with that on St. Croix or even on St. John, Jobsis noted. There are many times more regular nesting sites at Sandy Point than on St. Thomas and most of the research has been done on St. Croix.
However, Ayres and her colleagues identified four St. Thomas sites: Abi Beach, Caret Bay, Hendriks Bay and Neltjeberg Beach. They found one green sea turtle and 15 hawksbill nests in 2021 and two hawksbill and six leatherback nests in 2022.
The researchers investigated the temperature of six of the nests and monitored the eggs and hatchlings. They also monitored 22 of the nests on St. Croix. The St. Thomas sites tended to be cooler than those on St. Croix.
One factor for the difference may be that the St. Thomas nests were on the Atlantic rather than the Caribbean side of the island. They also tended to be more shaded, Jobsis said.
The tipping point for nests is around 29 degrees Celsius (84.2 degrees Fahrenheit), according to Jobsis. Get above that, and chances of male hatchlings are nil.
Each of the 22 St. Croix nests produced an estimated all-female set of hatchlings. On St. Thomas, two of the six monitored nests also produced hatchlings estimated to be all-female. One, however, was estimated as all-male. The other three sites did not produce hatchlings.
The study was small and far from conclusive, but it did provide some hope.