Loggerhead Turtle Sex Ratio Raises Concerns
By Cat Lazaroff
BEAUFORT, North Carolina, December 18, 2002 (ENS) - Up to 85 percent of loggerhead sea turtle hatchlings found on beaches in the southern United States are female, a finding that researchers say has implications for the recovery of the threatened species. The data is part of a large scale project to explore the impacts of a variety of hazards faced by migratory loggerheads.
Researchers from three institutions have collected and raised about 1,200 loggerhead hatchlings though their first months. The ongoing study has revealed an unexpectedly small percentage of males among baby turtles collected from Carolina and Georgia beaches, which could affect the future of the entire Southeastern loggerhead population, the investigators report.
Scientists, conservationists, students and volunteer nest monitors have collected a total of about 1,200 baby loggerheads from 10 beaches as far south as Miami and brought them to the Duke Marine Laboratory, an FAU facility at Boca Raton and the Mote Marine Laboratory in Sarasota, Florida.
The scientists said this is the first time so many loggerhead hatchlings have been raised and studied so intensely. The research is intended to provide information critical to boosting the numbers of the threatened species.
As a bonus, the selected animals are able to avoid the predators that hatchlings normally face when they crawl from nest to surf to begin their lives at sea. After the animals reach at age at which their sex can be determined, they are released directly into the sea off the beaches where they were born.
"Just as they emerged from the nest, we captured them, packed them in wet sand from their beach and transported them," said Larry Crowder, the Stephen Toth Professor of Marine Biology at Duke University's Nicholas School of the Environment and Earth Sciences.
"The first time they hit the water, it was here," Crowder added, referring to the water filled tanks at the Duke Marine Laboratory, where the baby loggerheads are kept in baskets for almost three months.
"These turtles have very small gonads at this age and are difficult to identify," said Wyneken, who performed the surgeries. "By relying on several different criteria we were able to get the information."
While the gender analyses are just beginning, Wyneken's first results have shown that hatchlings from the southern subpopulation are 85 percent female and 15 percent male.
Such gender ratios for loggerheads from the warm southern beaches have been documented before. The skewed sex ratios can arise because the temperature of the sand surrounding a turtle nest plays a strong role in determining the sex of turtles, with warmer temperatures favoring females.
Cooler temperatures were thought to favor males. But Wyneken's initial results for the northern subpopulation - 60 percent female and 40 percent male - were unexpected.
"What we're seeing is very few males being produced in the north," said Crowder. "So the situation is we have a large and recovering adult loggerhead population in the south that's increasing at four percent a year but is producing almost 90 percent females. And we have a northern population that is still in decline and isn't producing nearly the percentage of males as we thought it was."
With most males now hatching in the north, the scientists are concerned that when these two separated subpopulations reach reproductive age at sea, the result could be a gender imbalance.
In areas south of Jacksonville, Florida, female loggerhead counts that were declining by two percent each year are now increasing at a rate of four percent a year, Crowder said. From Jacksonville to the limits of their northern range in North Carolina, however, female loggerhead inventories are still decreasing by up to three percent a year.
"If we lose this northern subpopulation, which is still in decline despite all we've done, it has potential ramifications for the entire regional population," said Crowder.
"There may simply not be enough males," added Wyneken. "Additionally, the genetic diversity that this northern group contributes to both the northern and southern subpopulations should not be lost."
If additional gender results continue to contradict expectations, the scientists will be searching for explanations. Previous researchers have suggested that rising global temperatures could lead to rising numbers of female turtles, reducing chances for successful reproduction.
"We'll have to start thinking about global warming and climate change," Crowder said. But "there are a whole string of other possibilities that we are going to consider," he cautioned.
The research project may have a side effect of increasing turtle populations, at least in the short term. Following a two week recovery period after the laproscopic examination, the turtles are ferried out from shore to begin their lives at sea in the warm waters of the Gulf Stream.
Wyneken and Crowder both noted that safely releasing the animals as winter approaches is almost as much of a challenge as the study itself.
"We're basically taking them to where they would have been if they hadn't gotten waylaid," Crowder said. "In the process, they probably survive that interval way better than they would have on their own."
In planning their project, the researchers tried to address concerns that they might be interfering with the animals' abilities to migrate. But studies at FAU and the University of North Carolina at Chapel Hill show that even hatchling loggerhead turtles can sense and use the global magnetic field, Crowder noted.
"So, based on that work, when we plop them into the Gulf Stream we think they'll be able to access the latitude and longitude," Crowder said.
As part of the research, funded by a $350,000 grant from the U.S. Environmental Protection Agency, the scientists are also conducting growth studies that require repeated measurements of both turtles and their food. Since no one had tried rearing newly hatched loggerheads in such numbers before, the scientists had to answer such questions as how much food, and what kind of food, a baby loggerhead requires, recalled Crowder.
"We decided to use 10 percent of their body weight per day as a ration," Crowder said, adding that the turtles are growing well on a menu of mostly shrimp, laced with extra vitamins and minerals.
In response to research by Crowder and other scientists, the National Marine Fisheries Service now requires commercial fishermen to install turtle excluder devices (TEDs) on their nets. These TEDs appear to have slashed the number of fishing related injuries and deaths among juvenile and young adult loggerheads, Crowder said.
The excluders and other conservation measures also appear to have helped reverse declines in the numbers of adult female loggerheads counted at monitored nest sites, at least in their farthest south U.S. range. Only nesting females can be counted, since the males remain at sea.