A population of Green Sea Turtles, like this one, has hatched almost no males during the last hatching cycle. Photo: Brocken Inaglory via Wikimedia, image licensed under the Creative Commons Attribution-Share Alike 4.0 International, 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license. A population of Green Sea Turtles, like this one, has hatched almost no males during the last hatching cycle. Photo: Brocken Inaglory via Wikimedia, image licensed under the Creative Commons Attribution-Share Alike 4.0 International, 3.0 Unported, 2.5 Generic, 2.0 Generic and 1.0 Generic license.

A strange phenomenon is taking place amongst sea turtles living in a region of the Great Barrier Reef. According to a new study done on the turtles, almost every turtle born in the northern part of the region is female.

The fact that 99% of all the turtles born in this region are female poses a threat to the entire colony, as it may lack the sufficient number of males required to sustain itself in the coming decades.

ADVERTISEMENT

Temperature Determines Sex

The colony of approximately 200,000 sea turtles that exists off the east coast of Queensland, Australia is thought to be in danger due to climate change. Unlike many other animals, Green Sea Turtles don’t attain their sex due to sex chromosomes.

According to Camryn Allen, a scientist at the National Oceanic and Atmospheric Administration and co-author of the new study, while most mammals have sex chromosomes that will turn an embryo into either a male or female, the sex of Green Sea Turtles is influenced by the temperature of the world outside the embryo.

They have temperature-dependent sex determination, said Camryn Allen. It’s not genetics. It’s actually the temperature.

Global climate change may have only raised the normal temperature by a few degrees in some areas, but these few degrees are enough to devastate some ecosystems and animal populations. Sea turtle nests are dug into beaches, and a change of just a few degrees might be enough to warm the sand they are nested in, causing the vast majority of the eggs in the nest to hatch as females.

ADVERTISEMENT

The researchers involved in the study examined two groups of Green Sea Turtles. The sea turtles who live along the Great Battier Reef are split into two separate populations. These populations move around different areas and utilize different breeding grounds. One population returns to the northern part of the reef to breed, while the other population returns to the southern part of the reef to breed. Sea turtles are known to lay their eggs on the same beach where they themselves hatched, but this doesn’t entirely isolate the two groups genetically, as members of the two groups may occasionally breed with one another.

The Pivot Temperature

The two groups of sea turtles laid their eggs almost 3200 kilometers (2000 miles) apart, at the north and south ends of the reef. 3200 kilometers is quite a distance, and as a result, the climate of the two breeding grounds is somewhat different. It was the northern colony which saw the disproportionate birth of females, which suggests that the temperature in that region may have passed a “pivot point”, above which eggs will almost always hatch as females.

To discover that the phenomenon was taking place, the researchers had to take blood samples from turtles around the colony’s foraging grounds. Levels of hormones in the blood can be used to identify the sex of a turtle, and the DNA provided clues as to whether the turtle was from the northern or southern group.

When the researchers examined the data they collected, they found that on the northern beaches almost 87% of the oldest turtles were female, and around 99% of the juveniles were female, which suggests that the northern population has been producing almost exclusively females for more than twenty years. By contrast, males accounted for 30-35% of the southern population of sea turtles, regardless of their age.

The researchers then dig some digging through climate data gathered over the past few decades. They discovered that sometime in the early 1990s the temperature of the northern nesting grounds likely passed 29.3 degrees Celsius, or 84.7 degrees Fahrenheit, which is likely to be the pivot temperature.

ADVERTISEMENT

Implications For Sea Turtle Populations

The results of the study hold worrying implications for Green Sea Turtles. They are already endangered, according to the International Union for the Conservation of Nature’s Red List, and protected under the Endangered Species Act. Substantial progress has been made in the past few decades bringing the species back from the brink, but climate change could undo much of that progress.

Green Sea Turtles are important parts of undersea ecosystems. They feed on plants like algae and seaweed, effectively keeping populations of these plants under control. Without the grazing of sea turtles, seabeds can become overrun with seaweed. Sea turtles have an important role in maintaining coral reefs and transporting nutrients, which means that other species will no doubt be harmed if their numbers dwindle. Entire undersea ecosystems could become less diverse due to the shifting makeup of sea turtle populations.

It’s difficult to say just how badly sea turtles will be impacted by the changing climate. On the one hand, sea turtles are an ancient species that have lived through many fluctuations of temperature. On the other hand, the climate may be changing too rapidly for them to adapt.

There are many ways the changing climate and changing sex ratios of sea turtle populations could play out. On average, male turtles mate more frequently than female turtles do, and male turtles can live between 60 to 70 years. This means that sea turtle populations can theoretically get by without all that many males (the average sex ratio for sea turtles is 3-to-1 females-to-males), meaning that sea turtle populations could potentially bounce back from the change. Evolution takes time though, at least decades for a species like the Green Sea Turtle, and the climate is changing at an unprecedented rate. The rate of global warming has increased over the past 30 years, with eight of the top 10 hottest years having occurred since 1998.

Allen and her co-author David Owens say that action is still possible to preserve sea turtle populations in the Great Barrier Reef area. Management strategies like cooling nests by pouring water on them or shading beaches could help reverse the trend a little, but Allen and Owens are most worried about all the sea turtle populations that haven’t been studied yet. The extent of the problem could go much farther than is currently known, so the changing climate could pose real problems for sea turtles, as hardy as their species is.