As measures to protect sea turtles ebb and flow, the turtle’s “lost years” spent out at sea after they hatch on beaches along Florida and around the tropics have remained a tricky factor in conservation efforts.
Now, a new study has found a surprising place where their secret lives can be archived: the shells on their backs.
”The reason we're interested in how fast the scutes grow is because if you take a biopsy, you essentially can obtain a record of diet and movement through time,” said Bethan Linscott, one of the study’s lead authors and an assistant professor at the University of Miami Rosenstiel School. ”The missing element for a long time has been knowing what that timescale represents.”
READ MORE: 'There Was Just Turtle After Turtle After Turtle': Could Sea Turtles Be Surging In Biscayne Bay?
Now, using the same technology to date dinosaurs and early humans, the team confirmed radio carbon dating can more precisely tell them the timing of changes in scutes. Importantly, knowing that rate of change can shed light on impacts from hazards, including oil spills, red tides and sargassum invasions, which can take a lasting toll on the ocean’s long-lived turtles.
”The more we know about foraging locations and movement patterns,” Linscott said, “the better we can conserve sea turtles and assess how well they respond to these environmental disasters.”
The study is rooted in the disastrous 2010 BP Horizon oil spill in the Gulf of Mexico. For 87 days as workers struggled to cap the spill, more than three million barrels of oil oozed into waters. Surely, that major event impacted sea turtles, so habitual in their foraging and nesting grounds that they can return to the same beaches for decades, scientists thought.
”Even though there was this huge environmental nightmare, the loggerheads had stayed put,” Linscott said, referring to 2016 findings by co-author Hannah Vander Zanden, a University of Florida turtle researcher. “So that set off alarm bells for us because it means they're not capable of avoiding these kinds of disasters.”
It also raised the question of whether the turtles suffered lasting harm. So Vander Zanden recruited Linscott, who had never before worked on sea turtles.
A trained geochemical archeologist and expert on carbon dating, Linscott previously focused on critters from thousands of years ago, including shedding light on the mystery of the “Lapedo child” last year. The child skeleton discovered in 1998 had been found in a grave dating back 30,000 years. It showed a mix of both Neanderthal and modern human traits, leading scientists to theorize the two species mated that triggered years of debate. Linscott finally confirmed the skeleton’s age, adding evidence to the theory.
Just like human fingernails and hair, scutes are made of keratin. So Linscott said she felt confident that carbon dating used on humans and dinosaurs would work on turtles. It also helped that the turtles being examined were far younger. For carbon dating anything from the last 75 years or so, the so-called “bomb pulse” can be applied for changes in radiation in the earth’s atmosphere.
”After the Nuclear [Test] Ban treaty in 1963, that huge spike that we saw in the anthropogenic radiocarbon started to slowly, slowly fall. So what that ends up looking like on the calibration curve is this big spike. And that's what we use for anything that we want to date beyond or more recent than 1950,” she said.
To see how the forensic dating worked and what kind of light it shed on turtles, the team used biopsies collected by co-author Amy Wallace from the shells of 24 dead loggerhead and green turtles found off Florida. The turtles stranded — a term for dying — along the state’s coast between 2019 and 2022. Surprisingly, the samples showed scute growth rates for all 24 turtles slowed between 2015 and 2018.
Linscott said the researchers at first suspected their computer modelling misfired.
”We tried calibrating those days against a whole range of different calibration curves, and we still had these synchronized declines,” she said. “And then we started looking through the literature and realized, hang on a minute. These declines coincide with huge red tide events.”
The slowdown also overlapped with two massive invasions of sargassum
The turtles' scute growth rates stabilized after the harmful events passed, but never returned to their previous pace. Linscott said that suggests those hazardous conditions could cause ongoing damage t the turtles, with significant impacts. Smaller turtles have fewer babies.
”There's still a lot we don't understand about their behavior: their movement patterns, their habitat connectivity,” she said. “But if we can use forensic techniques to reconstruct those…,it gives us a window into their life histories that we wouldn't have had before.”
Since the work was completed, Linscott has switched her focus to sea turtles. Her campus office on Virigina Key is filled with recovered turtle shells along with a loggerhead skull taken from a smuggler at Miami International Airport — and she's started working on two additional projects using carbon dating to help diagnose turtle problems.
In the U.S. Virgin Islands where hawksbill sea turtles are critically endangered, conservation efforts since the 1980s have helped revive the population. But nesting female turtles now grow much smaller than in the past, meaning they lay fewer eggs. The National Park Service in St. Croix, which has been collecting scutes for the last three decades sent samples to Linscott for answers.
”Because one of the leading theories is that with climate change and so much anthropogenic influence, it might be that their food source has dwindled and therefore they're not getting the nutrition that they need to grow to full size,” she said.
Another study closer to home will look at how young green sea turtles use Biscayne Bay. The bay and its once verdant seagrass meadows were ideal territory for the plant-eating green turtles.
”Most of the strandings in Miami-Dade are greens. And so by sampling those individuals, we're hoping that we can get an idea of how long they were residing in the bay before they stranded,” she said. “Are they hanging out here for years or are they moving in from further afield just before they strand? And also what is their foraging ecology like, because we have amazing seagrass beds around this region, all of which are in decline.”
Not unlike crime scenes, forensic ecology is turning out to provide a window into what’s ailing the turtles.
“ I was born in Derby, which is a town in England, which is about as far away from the water as you can possibly get,” Linscott joked. “I knew about sea turtles, but I didn't realize how beloved they are to people, especially in Florida. There's a lot of energy in people to try to conserve and help them. We just have to tap into it.”
