Now, scientists have revealed another unexpected climate effect: It may be disrupting the sex ratio among baby sea turtles.
In a new paper published last week in the journal Endangered Species Research, researchers explored how changes in rainfall and temperature could affect the sex of baby loggerhead sea turtles, and found that, in southeast Florida at least, climatic changes seem to be producing more female babies.
Loggerheads, like other species of sea turtles, develop into males or females while they’re still incubating in their eggs depending on a variety of environmental influences. Temperature is one of the most important of these. Specifically, the temperature of a turtle’s nest during the middle part of the incubation period has an important influence on the sex of the developing babies.
In general, scientists agree that that temperatures lower than about 29 degrees Celsius (or about 84 degrees Fahrenheit) are likely to produce males, and higher temperatures are likely to produce females. It’s not totally clear why this is, but it means that as global temperatures rise, there’s a risk that more and more sea turtles could be born female — leaving the population unbalanced.
Jeanette Wyneken, a professor of biological sciences at Florida Atlantic University, and her Ph.D student Alexandra Lolavar, were interested in how changes in both rainfall and temperature could affect sea turtle sex ratios, as global climate change is likely to strongly affect these two variables in many places around the world in the future. Understanding the factors that affect loggerhead sex ratios is important because the turtles are already listed by the federal government as endangered or threatened throughout their habitats, Wyneken said.
Previous laboratory experiments on the effects of environmental conditions on turtle sex ratios had produced some intriguing results that seemed worth following up with a field study, Wyneken said. “When I went back and looked at 10 years of data, we had years where we effectively had 95 to 100 percent female samples…and it turns out all of those years were really hot years,” Wyneken said. “And then the years where we had a high percentage of males were wet years.” These results begged the question of whether rainfall was having a cooling effect on the nests, which was allowing males to develop, or whether it was the moisture itself that was having an effect on baby turtle development.
Wyneken and Lolavar investigated the question by documenting rainfall, sand temperatures and the resulting sex ratios of baby loggerheads on a nesting beach in Boca Raton, Fla. during the 2010 to 2013 nesting seasons (which typically take place between April and October). They found that rainfall had a slight tendency to cool the sand, particularly at shallow depths — but, in general, “there isn’t very much temperature change at nest level” as a result of rainfall, said lead author Wyneken, unless there’s an especially heavy rainfall event.
At the same time, they found that while hot, dry years seemed likely to produce more females, wetter years were more likely to produce both females and males — although in general more females were produced than males.
The years 2010 and 2011 had shorter-than-average rainy seasons and high temperatures — and during these years, all the baby sea turtles in the observed nests were born female. But in 2012 and 2013, nesting seasons were wetter than normal, and in these years, the scientists documented both male and female babies hatching. The results suggest that rainfall may play a role, in addition to temperature, in determining the sex of developing sea turtles. Particularly heavy rainfalls may lower sand temperature below the “turnover” point that determines a sea turtle’s sex, but Wyneken says the results also suggests that there’s something about the moisture itself that could affect the development of the babies.
“We showed that if you get really, really strong rains, it’s producing a drop in the temperature of the nest, and we can’t say whether that drop is itself sufficient to cause the production of males,” Wyneken said. “No one at this time understands what the actual trigger is to send an egg to become either male or female.” However, it’s clear that temperature is still the major player, and sex ratios are being skewed more and more heavily toward females as nesting seasons become warmer and warmer.
It’s important to note that there is a complicated relationship between climate change and temperature and rainfall. In many locations, climate change is expected to increase both variables, meaning that even though it will get hotter it will also get wetter. But in many places, increasing temperatures are associated with both drought and extreme precipitation events at different times of the year. These disparate projections make it unclear just how much rainfall could mitigate the effects of rising temperatures on sea turtle sex ratios.
For loggerheads, and other sea turtle species as a whole the implications could be big. A skewed sex ratio could severely diminish turtles’ ability to reproduce effectively.
“If you have, for example, an environment that’s going to promote too many boys, then that’s gonna slow production,” Wykenen said. “And if there are more females than there are males that can service them, then you lose production as well.” In recent years, scientists have noticed that there’s already a strong bias toward females, she added, so it’s concerning that rising temperatures may continue to promote this skew, even if increases in rainfall may slightly temper this effect.
And rising temperatures are just one of the ways climate change is threatening sea turtles, Wyneken added. There’s already concern among conservationists about how sea-level rise could affect sea turtle populations, as the beaches on which they lay their eggs could be washed away in the future. “If you look at a map of what is expected for sea-level rise, the biggest areas of vulnerability are also hotspots for sea turtle nesting,” she said. And Wyneken noted that rising ocean temperatures also seem to be causing sea turtles to nest earlier and earlier in the season.
So Wyneken and her colleagues have unearthed just another way that global warming could be threatening sea turtles. In doing so, they’ve underscored the complex balance of environmental influences these animals depend on for survival — and demonstrated just how devastating tiny changes to a big system can be.