A loggerhead sea turtle hatchling emerges from its shell.
Photo provided
Related story: Green sea turtle nesting sets record
By Cheryl Blackerby
Scientists have long thought sand temperatures surrounding turtle nests determined the gender of sea turtles. Because sea turtles don’t have an X or Y chromosome, their sex is defined during development by the incubation environment.
The prevailing theory was “hot chicks, cool dudes” — at sand temperatures above 84 degrees clutches of hatchlings would be predominantly females, below that temperature, hatchlings tended to be males. At 84 degrees there is about 1-1 sex ratio.
But a recently published four-year study of loggerhead hatchlings by FAU biologists disputes this theory and has scientists stunned.
“Our understanding (of sand temperature) came from research done mostly in labs and we had every reason to expect that to be the case in the real world,” said Dr. Jeanette Wyneken, FAU biologist who along with Alexandra Lolavar directed the study, which appeared in the journal Endangered Species Research.
But the study showed that probably wasn’t the case in nature, she said.
The study was conducted in Boca Raton at Red Reef Park, South Beach Park and a small part of Spanish River Park, where there was still some original sand on the beach. The biologists wanted a baseline that excluded different types of replacement sand.
But if sand temperature does not cause the embryonic sex of turtles, the researchers needed to find out what does. They knew that gender was determined by the environment but was it rainfall, sun, shade, sand type?
The need for answers was urgent because FAU biologists had discovered that in some years Florida beaches were producing no male loggerhead hatchlings.
“It’s a pretty big deal,” Wyneken said.
Florida produces about 85 percent of the loggerhead turtles in the Atlantic north of the equator. About 11 percent of that number comes from Palm Beach County, she said.
The county has historically produced a majority of female turtles, which was considered ideal and good for the recovery of an imperiled species.
“It can be good that most are females, but it’s a little alarming when we have years with no males,” Wyneken said. “In the last 10 years, there have been three years with no males. We’re sampling a fraction of the nests so we may be missing some males, but not many.”
Wyneken and her researchers found that the numbers of females and male hatchlings wildly fluctuated, she said. “We had a crazy range of 65 percent females one year, 95 percent the next, and 100 percent the third year. That means that we needed several years of data to start to identify normal sex ratios.”
After four years of research, the FAU biologists found that moisture seems to be the key factor in gender determination, not sand temperature alone, she said. And it’s not because rainfall lowers sand temperature, as many scientists had assumed.
“We found out that by the time the water percolates down to the nests buried 18 inches down in the sand, it’s pretty much the same temperature as the nest,” she said. “The rain is not cooling it down unless it’s four or five days of hard and steady rain. Those events do cool things down, but only for a short time.”
The study has prompted more questions about how the moisture determines gender.
“There are a lot of different hypotheses about how moisture and temperature direct the sex of embryonic turtles to be male or female. Maybe moisture facilitates turning genes on and off. Another is that the sexes might be directed by factors associated with developmental rates. When there’s more moisture the rate of incubation may slow down,” Wyneken says. “If you have more water in the sand you have less oxygen. If there’s less oxygen, development slows down.”
FAU biologists including Lolavar, lead author of the study which is the basis of her honors thesis program, will continue to study nest environments.
“The next step is figuring out what the moisture is doing to the rate of development,” said Wyneken. “An observation is that nests that produce males tend to incubate longer. But maybe we’re missing something. Maybe they’re incubating longer for other reasons.”
These turtles are already fighting an uphill battle since roughly 1 in 2,500 to 7,000 sea turtles make it to adulthood.
Meanwhile, loggerhead hatchlings didn’t fare well this year.
“There was a record number of nests in the state, but the success of the nests was really poor. We had a super hot June and July on the East Coast, and hot and dry is pretty bad for hatchlings,” Wyneken said.
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