Great Barrier Reef: rising temperatures are skewing the sex ratios of sea turtles

In one of the world’s largest sea turtle colonies, home to some 200,000 endangered animals, the population of young males is almost nonexistent.

More than 99 percent of the turtles born on the reef’s northern beaches since the late 90s have been female – which a landmark study published in Current Biology explains is a result of rising global temperatures.

Ambient temperatures influence a sea turtle’s sex during the incubation period in the egg. Cooler environments give rise to more males, while warmer conditions result in more females. Such temperature-dependent sex determination occurs in a number of reptile species, and can provide benefit to a population with more breeding females. Too few males, however, could lead to a serious population cut, or worse.

The researchers found a “moderate female sex bias” in 411 turtles from beaches in the cooler, southern Great Barrier Reef, where about 65-69 percent were female.

But those in the warmer, northern Great Barrier Reef were “extremely female-biased,” at 99.1 percent female among juveniles and 99.8 percent for those between juveniles and adults. A total of 86.8 percent of adult-sized turtles from the area were female.

In the case of green sea turtles, there has never been such a documentation of temperature-dependent sex determination in such a large a population of endangered animals.

The sex of a sea turtle is determined by the heat of sand incubating their eggs. With climate change driving air and sea temperatures higher, scientists found that female sea turtles from the Pacific's largest green sea turtle rookery now outnumber males by at least 116 to 1. https://t.co/Wu4xUog6jy

— Shah Selbe (@shahselbe) January 8, 2018

The authors of the new study —led by Camryn Allen and Michael Jensen, who are researchers with the United States National Oceanic and Atmospheric Administration (NOAA) — warn that rising global temperatures could skew the sex ratios of most sea turtle populations, making them unable to reproduce, and vulnerable to possible extinction.

“We also know that higher incubation temperatures cause higher mortality in the eggs,” Allen said in an interview. In the study, she and her colleagues concluded that “it is clear that climate change poses a serious threat to the persistence of these populations.”

“With average global temperature predicted to increase 4.7 Fahrenheit (2.6 Celsius) by 2100, many sea turtle populations are in danger of high egg mortality and female-only offspring production.”