Exploring the Nature of Stress
It’s only getting more stressful in our stressful world, particularly if you are an animal trying to deal with a pesky invasive species or the impacts of humans in your environment.
Associate Professor of Biology Tracy Langkilde’s career thrives on that stress. Langkilde conducts research in evolutionary ecology by studying the stress caused by global environmental change, and how animals are able to deal with these stressors.
Get Twitchy With It
One of the animals Langkilde studies is the eastern fence lizard, a species commonly found in the southeastern United States. For thousands of years, when fence lizards faced adversity, they had a pretty simple approach to survival: laying very still and blending into their surroundings. This was quite a successful approach for them—until fire ants invaded their territory.
Fire ants were not deterred by frozen-in-place eastern fence lizards, and proceeded to bite and sting the lizards anyway. Sometimes, the lizards died from these attacks. The survival behavior that had worked for their species for years was no longer protecting them from predators.
Some lizards broke from the standard approach and started to twitch to flick off attacking ants off or run away, which was a much more successful way to survive. Stressing out about survival fueled this adaptive behavior.
“Fire ants have a venomous sting and act as both a novel predator and a novel toxic prey to many animals, including fence lizards,” says Langkilde. “They are predicted to eventually occupy over 50 percent of the terrestrial surface of the earth, so many animals are going to have to deal with these threats.”
Langkilde noticed that in areas where fire ants had lived for years, a higher percentage of lizards exhibited the twitching and flicking behaviors than in areas where fire ants had not yet invaded.
“Getting constantly attacked by fire ants seems like it should be stressful,” Langkilde said. “We tested this by capturing lizards from fire ant–invaded and uninvaded sites and taking blood samples.” They discovered that the populations of lizards who live among fire ants on a regular basis were, in fact, more stressed.
Langkilde then tested whether there were any benefits to these higher stress levels. By applying drops of the stress hormone corticosterone mixed with oil to the lizards’ backs, Langkilde could mimic the stress caused by fire ant attacks.
“We can make unresponsive lizards from uninvaded sites very responsive to fire ants simply by stressing them out. By being stressed out, lizards from populations dealing with invasive fire ants are primed for future encounters with predators,” Langkilde said.
Her team began to monitor the health of the lizards exposed to varying levels of stress. Their findings showed that the duration, intensity, and frequency of stress were key factors in determining whether it was beneficial for the lizard or not.
“Chronic stress, usually defined by stress occurring over a long period, is generally thought to be bad,” said Langkilde. But Langkilde’s team found that this really depends on how much stress is experienced. “A small amount of stress every three days for nine days total was good for the lizards,” Langkilde said, “in that it enhanced the immune system.”
But more stress than that can be bad for the lizards, she said. When the lizards were given higher amounts of the stress hormone, at the same frequency and over the same duration, they experienced negative effects. “This higher-intensity stress caused the immune system to crash,” Langkilde said.
“We generally expect long-term stress to produce negative consequences, but our results demonstrate that really intense stress can produce a similar result," said Gail McCormick, a graduate student in the Langkilde lab who is leading this project. “This matches up with what we know about post-traumatic stress disorder in humans—that a short but intense stressor can produce lasting consequences.”
Langkilde’s team was curious about how the high levels of stress experienced in fire ant–invaded areas might affect the next generation. They are currently testing the effects of stress on both pregnant lizards and the babies they give birth to. They want to know whether stress during pregnancy could make the offspring better prepared for life’s stresses.
“We are putting the offspring of lizards that experienced high and low stress during pregnancy into high- and low-stress field enclosures that vary in the presence of fire ants,” Langkilde said. “We predict that babies of stressed moms will do better in high-stress environments.”
While Langkilde isn’t sure yet whether it’s a behavioral reaction of the mothers to stress or something the offspring experience during development, pregnant lizards exposed to high stress gave birth to bigger babies that survived better than the offspring of pregnant lizards exposed to lower-stress environments.
“Hormones can be important regulators of fetal growth. High levels of stress hormones circulating in stressed mothers could be passed directly to their offspring. Or it may be that high-stress moms are eating more and allocating more nutrients to their young,” said McCormick.
Research Langkilde has conducted on “bearded lady” fence lizards, or female lizards who display a more masculine, colorful appearance, could also be worth examining in the context of stress response. “Males really don’t like these bearded ladies. So they breed later and invest less energy in reproduction, but their babies are tougher, having a higher survival rate. Bearded ladies also sprint much faster than more feminine lizards,” said Langkilde.
Could these adaptations make bearded ladies or their offspring better able to deal with stress? “In high-stress lizard populations, 90 percent of the females are bearded, so it’s possible,” she said.
Who Needs Noise-Canceling Headphones?
Langkilde took these questions about stress and applied them to a different species: wood frogs native to the northeastern United States. Sound is very important to this species, because they use calls to find mates and detect incoming predators. For wood frogs that live nearer to sound pollution like road noise from high-traffic roads, this can complicate things and cause stress.
“We exposed wood frogs from quiet Pennsylvania woods to road noise,” said Langkilde, “and found that this dramatically increased their stress levels.” Road noise also has effects on the immune function of these frogs, making them less likely to produce antimicrobial peptides, compounds in the frog’s skin that defend against pathogen invasion. Langkilde found that road noise also had effects on female wood frog reproductive behavior. Road noise impaired their ability to locate calling males. Wood frogs have a very short reproductive window, breeding for only a few days each spring.
“This could have important implications for their mating success,” she said. Langkilde wanted to know whether wood frogs could adapt to a noisy environment. “People living near train tracks or highways often tune this noise out over time,” Langkilde said. “We wanted to know if frogs could do the same.”
Her team took wood frog eggs from both noisy environments such as forests adjacent to New York interstate highways, and quiet countryside environments. They raised the tadpoles under common conditions in the lab until they became frogs. Then they compared the stress responses of each group to prolonged exposure to high levels of road noise.
As with their previous study, the “country frogs” were really stressed by the road noise, and weren’t able to tune it out over the course of the eight-day experiment. The “city frogs,” however, were not at all stressed by the noise.
This is very interesting because these frogs were raised in the same environment, and the only change was that one group of eggs was collected in a more stressful environment than the other, says Langkilde. “It is possible that frogs from roadside ponds have evolved to be desensitized to road noise, in order to avoid the costs of being stressed.”
Langkilde’s research will continue to examine the consequences of stress caused by environmental change. “What is the effect of stress within a lifetime and across generations? Can animals adapt to high-stress environments, and how do they do so? How can we predict when stress will become bad? Those are questions I’m working to investigate further,” she said. Her findings are all the more important because they could inform topics of stress and human health.
Enthusiasm for Mentoring
Mentoring students is a much-loved part of Langkilde’s job as principal investigator of her lab.
“She's very easy to talk to and enthusiastic. I like the freedom to ask interesting questions and explore them in a variety of different ways,” said Christopher Thawley, a graduate student in the Langkilde lab.
“She’s done a great job of creating a lab where everyone can work to their strengths and help each other out,” added graduate student Caty Tylan.
Her graduate students like her commitment to a low-stress environment.
“You never have to worry about coming to her for help, even if it’s something you messed up on,” said Dustin Owen, also a graduate student in her lab. “She’s really good about helping you and providing feedback without making you feel stupid or uncomfortable.”
“She is conscious of the needs of students and researchers to have a good balance between productive work and a healthy social life,” added graduate student Braulio Assis.
Since coming to Penn State, Langkilde has mentored more than 50 undergraduate researchers in addition to the graduate students she advises.
“Their involvement is critical to the success of my projects, and they benefit from the experience,” Langkilde said.
“Dr. Langkilde has been by far my greatest mentor at Penn State,” said undergraduate researcher Mark Herr. “She doesn’t merely allow students to act as lab technicians for the various graduate students in the lab. From the beginning, she’s encouraged me to conduct my own projects.”
“She’s taught me how to write scientific papers. She’s edited grant proposals of mine and assisted me with funding at every step. It’s truly incredible, especially when you consider that she has lots of other undergraduates in the lab and does the same for them,” he added.
In addition to field and lab work, the Langkilde Lab works to hone their writing skills, which postdoctoral researcher Chris Howey finds extremely helpful.
“The members of the Langkilde Lab are very supportive and helpful with regard to writing manuscripts and other job-related documents. Meeting every week to discuss these manuscripts continually pushes each of us to make progress on these assignments until they are published,” Howey said.
Langkilde Lab members also use their writing skills to write the lab blog, The Lizard Log (thelizardlog.wordpress.com). “We wanted to share our love of research and the thrill of discovering new things with the general community,” said Langkilde. “My graduate students started the blog back in 2011 as a way to show people what fieldwork was like. It’s taken hold and is read by 900 people per month.”
The blog showcases posts and pictures of the lab members with updates on their research, travels, and even updates about past members of the lab. “I hope that the blog gives readers a glimpse into how science is performed. Most people don’t have a good understanding of the scientific process, so hopefully readers can get a sense of how we’re doing our research so that they can better understand the results,” McCormick said. “And see just how cool our research is!”
As the blog updates indicate, Langkilde’s connection to her lab members doesn’t stop when they graduate. A recent trip to a conference with a few of the lab members was planned to include a stop to visit Brad Carlson, Langkilde lab alumnus and current assistant professor of biology at Wabash College. Langkilde is also proud to announce lab alumna Lindsey Swierk was recently awarded a Gaylord Donnelly Postdoctoral Fellowship at Yale—the same fellowship that Langkilde herself received as a postdoc.
“I love seeing what they achieve when they leave,” she said. “We all still stay in touch.”