Dr. John Maerz: Helping turtles find their balance

November 17, 2021

Written by Kristen Morales, Warnell School of Forestry & Natural Resources

The idea began to take shape in CICR faculty affiliate John Maerz’s Animal Behavior class.

Lizzy Ashley, an undergraduate Honors student majoring in ecology and biology, was looking for topics for her Honors program thesis. She also wanted to get some research experience and approached Maerz about potential projects.

“And he was like, ‘Well, I have the opportunity to get these hatchlings,” said Ashley, who graduated from the University of Georgia in 2019 and is now pursuing her Doctor of Veterinary Medicine and Ph.D. through a dual-degree program at the University of California-Davis. She and Maerz, a professor of wildlife at the Warnell School of Forestry and Natural Resources, devised a project that would look at the physiological and behavioral responses of juvenile terrapins, which are turtles, to water salinity levels. 

While adult terrapins prefer to live in brackish water, terrapins lay their nests out of the reach of the tide—salt water can kill them before they’ve even hatched. Also, little is known about why terrapins live where they do—they populate marshes up and down the East Coast but aren’t everywhere. Studying hatchlings’ tolerance for salty water, they thought, could shed some light on how and why terrapins live where they do.

And so, they began the process: Maerz worked with a community on Skidaway Island to obtain the hatchlings, thanks to a vigorous turtle rescue program there. They put together the different treatment groups, how to take measurements and observations, and ironed out the overall plan.

Ashley realized she needed some help. Luckily, it was an easy ask.

“Baby terrapins—everyone wants a piece,” she said with a laugh. “They’re pretty cute, and it was a lot of work. But I think the kind of people Dr. Maerz’ lab attracts—they love herps, obviously, but also animal behavior and husbandry. And I loved working with the team.”

As it turned out, Ashley’s team was all undergraduate students. For many it was their first research project, and they took the opportunity and ran with it, Maerz said.

“They were like this army—they’d be changing out water and weighing food and doing behavior observations, and these five undergraduates killed it,” he said. “They were so careful about data collection and doing it right. (Lab assistant) Vanessa (Terrell) and I would check in and see how it was going, but they had everything in check.”

In total, the study was conducted by Ashley with assistance from Lauren Head, Connor Lake, Cady Carden and Rebacca Choe.

Head, who graduated from Warnell with her wildlife sciences degree in 2020, had gotten to know about Maerz’s lab through her work with a reptile-focused summer camp. After making the initial connection through the outreach animals housed in the lab, she began to volunteer there. She’d assisted with other graduate students’ projects, but knew she wanted to take on a larger role in the terrapin study.

“I’d heard about Lizzy and her project, and I saw the cute baby turtles and said, ‘I’m definitely hopping on this,’” said Head. “I was doing behavioral observations, and it was really interesting to watch the way the turtles in different salinities behaved.”

The goal was straightforward: house turtles in different salinities ranging from near-fresh water to seawater. Periodically, freshwater pools would be placed on the elevated, dry portion of the tanks and taken away, and the turtles’ reactions to the water source would be noted.

Not long after the project started, it became clear that the turtles with access to low-salinity water were content—they showed little interest in the pools of fresh water placed in their enclosures. But for the turtles with access to high-salinity water, their stress levels increased the longer they went without access to fresh water, as indicated by an altered makeup of white blood cells that suggests a stress response.

Then, the team began to notice something unexpected: The stressed turtles would go to the empty pools where they’d found fresh water. They had learned where to find the freshwater source—even though it was empty—and opted to wait for more water rather than sit in the salty solution.

“It was really interesting to watch the way the turtles in different salinities behaved,” added Head. “Because we started to notice the ones in higher salinities were spending more time out of the water and were eating less and growing at a slower rate. There was a noticeable size difference between the turtles, and it was obvious which ones were in the higher salinity because they were hanging out in those kiddie pools.”

Ashley recalled being able to watch the turtles gulping fresh water once it was added.

“When we would fill the water dishes they would go up and dunk their heads in and you could see them physically drinking—that was kind of crazy,” she said. “We ended up truncating the experiment a little bit—we intended it to last around 90 days, but we decided to prioritize terrapin well-being and stop on day 75.”

At that point, with behavior and growth documented, all the turtles received the spa treatment—shrimp, pellets and ideal water conditions. Turtles that had been stressed and showed slower growth due to higher salinity exposure were allowed to catch up to the others. Once they recovered, all were released back to Skidaway Island.

Not only did the project become Ashley’s Honors thesis—supported with a fellowship through the Center for Undergraduate Research Opportunities—but Maerz then realized the data could be published. Ashley wrote the manuscript and Maerz helped her navigate the process of submitting the paper, making revisions and communicating with editors.

Understanding the terrapins’ responses to low and high salinity was the main goal of the study, said Maerz. But watching the turtles learn where to find freshwater was a bonus. Knowing they can learn where to find freshwater opens the door to potential solutions for land managers looking to mitigate the effects of sea-level rise and low freshwater levels.

“We don’t know much about the upland habitats of these tiny turtles, but little pockets of areas that get fresh water from time to time are going to be important habitats. These turtles show the capacity to learn where they have access to these little pockets of water—even if it’s ephemeral. They learn where these things are if they encounter them,” said Maerz.

While much is known about salinity tolerances of adult terrapins, we know very little about juveniles. The data collected by the students is some of the first of its kind and offers insights into how juvenile turtles can navigate the marshy areas where they live. 

In the future, Maerz said, land managers could help plan for inconsistent salinity in these areas by protecting areas where freshwater can pool, whether it’s from an inland water source or a rainfall. This could help stabilizing future populations as the climate changes.

And that was a major goal for Ashley, too.

“I was thinking about hydrologic land use changes around marshes and how climate change will affect the composition of the habitats that terrapins use,” she said. “So, knowing that they can use ephemeral water sources—pools of rain or areas where there’s more river inundation—was somewhat of a relief. We can identify important sources for terrapins, in addition to providing safe places for them to nest, and we can try to find the perfect formula for terrapin persistence in a changing world.”