An out-sized proportion of sea turtles are likely to be born female as the climate warms, a skewed result that may increase turtle populations in the short term but could ultimately lead to their extinction, according to a new study.
The study, published in Nature Climate Change, looked at one of the largest breeding colonies of loggerhead sea turtles in the world to try to predict how rising temperatures would affect the female-to-male ratio in their hatchlings. Sea turtles are dependent on temperature for sex determination, which means the temperature of the nest plays a large role in the sexes of the baby turtles. At a nest temperature of about 84.2°F, sea turtle hatchlings will be born approximately 50 percent male and 50 percent female. Temperatures warmer than that yield more females, up to the point of about 87.8° F, wherein almost all hatchlings will be born female.
Graeme Hays, one of the lead authors of the study, told the Guardian that this skewed sex ratio could harm sea turtle populations as the earth’s temperature rises.
“Over the next 20 to 30 years, it’s not going to create problems,” Hays said. “In fact, there’s going to be a benefit to the turtles, because there’s going to be more females produced, which means more females laying eggs. More females will lead to a population expansion. But ultimately, if you extrapolate long enough into the future … once you get 100 years or more into the future, then things start to look serious. You have so few males left that it’s likely to be a problem. There will be heaps of female but not enough males to fertilize all those eggs.”
The study, not unpredictably, found a strong correlation between average monthly air temperature and average sand temperature, and also noted that nests laid in dark-colored sand beaches produce a larger percentage of females than those in light-colored beaches. As the temperature warms, it states, female-to-male ratios in sea turtles may hit a threshold where human intervention could be necessary to prevent extinction.
“Ultimately if males are so scarce that egg fertility is compromised, management intervention will be needed,” the study reads. “Translocating nests from dark beaches to light beaches could be a strategy to produce more male hatchlings. Another strategy would be to shade nests (for example, with beach vegetation) to lower incubation temperatures.”
Sea turtles, however, aren’t the only group of species whose sex depends on temperature — they’re part of a group of reptiles (and certain species of a family of birds known as megapodes, though their temperature sensitivity differs slightly from reptiles’) that share this trait. Other turtles, such as the box turtle, are also temperature-dependent, with higher nest temperatures yielding more females. Crocodiles and some lizards, too, depend on temperature for sex determination, as do alligators — though in the alligators’ case, warmer temperatures produce more males.
Scientists aren’t sure yet why some animals evolved to have temperature-dependent sex determination, rather than a reproductive process that determines sex genotypically. Many think that the ancestors of these creatures were hermaphroditic and later evolved to have their sex determination dependent on temperature. There are several hypotheses of what advantage this evolution could have given them, though: some think that temperature-dependency allows sex to best match the environment the baby is born into, while others think the dependency allows a mother to choose the sex of her babies by controlling the temperature of her nest (though there hasn’t been evidence to show that mothers do manipulate temperatures like this). However, despite the fact that rising temperatures may effect the sex ratios of temperature-dependent animals, some may still be able to adapt in other ways, such as moving their range to higher altitudes or other cooler regions.