People determine whether they should develop as male or female based on the presence of a single gene on the Y chromosome. But that’s just one of a staggering number of ways plants and animals determine their sex. A large group of reptiles, including crocodilians and many species of turtles, take advantage of the ambient temperature. If the eggs rise above a certain temperature during a critical incubation period, the animal will likely develop as a female; below that temperature you are more likely to get a male.
And that is a problem in a world where the temperature is rising.
A new study of sea turtles living near the Great Barrier Reef has found that populations closest to the equator, where temperatures are warmest, have produced more than 99 percent females for 20 years. While turtles have endured markedly changing climates in the past, the current rate of change, combined with the longevity of sea turtles, raises concerns about how well they will cope with our current warming.
The turtles in question are the green sea turtles. Along the Great Barrier Reef of Australia, these are split into two populations. While both feed widely along the reef, they return to different breeding grounds to the north and south, and females lay their eggs on the same beach where they hatched. However, the two populations are not genetically different, suggesting that at least some males are not completely faithful to the region they were born in (or that some matings take place outside of normal breeding areas).
The two egg-laying areas, on the north and south sides of the Great Barrier Reef, are nearly 2,000 miles apart. In particular, they experience different climates, which can affect the sexing of the turtles. As mentioned above, there is a “turning temperature” above which eggs develop as females. A warming climate thus risks pushing northern populations, where conditions are warmer, toward producing nothing but females.
However, finding out whether that happens is not easy. When hatching, the tortoises have no outward indications of their sex; the only way we have to figure it out is to parse them. And later in life, only females return to the beaches, making it difficult to know how many males are present in the population.
To get around this problem, an Australian-US partnership 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 yielded DNA that allowed researchers to identify whether a turtle came from the northern or southern population. The turtle’s size also gave an indication of its age.
Among the southern population, there was a slight but consistent preference for women. Males accounted for 30-35 percent of the population, whether the tortoises were juveniles, minors or adults.
But things were very different with the turtles that emerged on the northern beaches. Here, the oldest tortoises were 87 percent female. Among juveniles and subadults, females accounted for more than 99 percent of the turtles sampled. “Green turtle colonies in the northern Great Barrier Reef have been producing mostly females for more than two decades,” the authors conclude.
Then they went back and checked historical temperature records for the area near the breeding grounds. The central temperature for these turtles is 29.3°C, and records indicate that eggs in the area have been said to have been above that temperature since the early 1990s.
As mentioned above, genetic data suggests that at least some of the northern females mate with males from the southern population. But it’s not clear whether this is happening at a high enough frequency to keep the northern population viable. And as temperatures continue to rise this century, it’s possible that southern populations will eventually reach a problematic incubation temperature as well.
The researchers note that there are plenty of opportunities for the turtles to adapt. The central temperature could shift, or the turtles could breed at a different time of year (eggs currently breed during the hottest months). The entire population could also shift over time to using beaches further south, as rare strays could establish new breeding grounds. But the tortoises take decades to mature and can live up to 80 years, meaning genetic adaptations will be slow. And plenty of evidence indicates that the climate is currently changing faster than at any time we’ve established.
Current Biology2017. DOI: 10.116/j.cub.2017.11.057 (About DOIs).