Humans figure out whether to develop as males or females based on the presence of a single gene on the Y chromosome. But that's just one of a dizzying number of ways that plants and animals determine their sex. A large group of reptiles, including crocodilians and many species of turtles, use the ambient temperature. If the eggs are above a certain temperature during a critical period in their incubation, the animal will be likely to develop as a female; below that temperature, you're more likely to get a male.

And that, in a world where temperatures are rising, is a problem.

A new study of sea turtles that live near the Great Barrier Reef has found that populations closest to the equator, where the temperatures are warmest, have been producing over 99 percent females for two decades. While turtles have obviously weathered changing climates in the past, the current rate of change, coupled with sea turtles' long life span, raise concerns about how well they'll cope with our current warming.

Reef turtles

The turtles in question are the green sea turtles. Along Australia's Great Barrier Reef, these are split into two populations. While both feed widely along the Reef, they return to distinct north and south breeding areas, and females lay their eggs on the same beach where they hatched. The two populations aren't genetically distinct, though, which suggests at least some males aren't entirely faithful to the region in which they were born (or some matings take place outside the normal breeding areas).

The two areas where eggs are laid, at the north and south ends of the Great Barrier Reef, are nearly 2,000 miles apart. They experience notably different climates, which can affect the turtles' sex determination. As mentioned above, there's a "pivot temperature" above which eggs develop as females. Thus, a warming climate runs the risk of pushing the northern population, where conditions are warmer, toward producing nothing but females.

Figuring out whether that's happening, however, is not a simple thing. At hatching, the turtles have no external indications of their sex; the only way we have of figuring it out involves dissecting them. And, later in life, only females return to the beaches, making it hard to know how many males are present in the population.

To get around this issue, an Australian-US collaboration obtained blood samples from turtles in the foraging area. The hormone levels in the samples were used to identify the sex of the turtle. The samples also provided DNA that allowed researchers to identify whether a turtle was from the northern or southern population. The size of the turtle also provided an indication of its age.

All girls

Among the southern population, there was a slight but consistent bias towards females. Males accounted for 30-35 percent of the population, whether the turtles were juveniles, subadults, or mature.

But things were very different with the turtles that originated on northern beaches. Here, the oldest turtles were 87 percent female. Among juveniles and subadults, females accounted for over 99 percent of the turtles sampled. "The northern Great Barrier Reef green turtle rookeries have been producing primarily females for more than two decades," the authors conclude.

They then went back and checked historic temperature records for the area near the nesting grounds. The pivotal temperature for these turtles is 29.3°C, and records indicate that eggs in the area would have been above that temperature since the early 1990s.

As noted above, genetic data suggest at least some of the northern females are mating with males from the southern population. But it's not clear whether this is occurring at a high enough frequency to keep the northern population viable. And, as temperatures continue to rise over this century, it's possible that the southern population will eventually reach a problematic incubation temperature, too.

The researchers note that there are plenty of opportunities for the turtles to adapt. The pivotal temperature could shift, or the turtles could breed at a different time of year (eggs currently incubate during the warmest months). The entire population could also shift to using beaches further south over time, as rare strays could establish new breeding areas. But the turtles take decades to mature and can live to 80 years old, which means any genetic adaptations will be slow. And lots of evidence indicates that the climate is currently changing faster than it has at any point we've identified.

Current Biology, 2017. DOI: 10.1016/j.cub.2017.11.057 (About DOIs).