There’s a species of poison frog called the “strawberry frog” or the “blue jeans frog,” depending on who you ask. These frogs are smaller than a quarter, with bright red bodies and navy blue limbs, and they live in shady Costa Rican forests. Or, they did, until humans began cutting the forests to create farmland. These sunny fields and pastures are hotter and drier than the forests, and scientists wanted to know how the strawberry frogs were adapting to their new environment. To figure it out, the researchers built mini temperature-controlled frog habitats to see what temperatures the frogs gravitated towards. They discovered that frogs from sunny pastures tend to seek out higher temperatures than their forest friends – but the ceiling for temperatures they can survive hasn’t changed.
The project was led by two students working on NSF-funded Research Experiences for Undergraduates, first author Juana Rivera-Ordonez (University of Washington) and Adrian Manansala (University of Guam), and their mentors, Justin Nowakowski (University of California, Davis) and Michelle Thompson (Field Museum). The research overseen by senior author Brian Todd of UC Davis.
“We’re trying to understand what happens to species when we transform forests for human land uses,” Nowakowski, one of the corresponding authors of a paper on the project in the journal Biotropica.
“For this study, we were trying to understand how strawberry poison frogs that live in these converted pastures handle warmer temperatures in these land uses compared to individuals that live in the forest.”
“We found that if there’s an increase in temperature, there may be some ability to acclimate or adapt, but in situations where temperatures change drastically, it may be bad news for them,” says Thompson, a conservation ecologist and herpetologist at the Field Museum in Chicago and one of the co-authors of the study.
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“Amphibians are really dependent on the environment for their body temperature – therefore, they’re really impacted by changes to the environment,” explains Thompson, who began work on the study at Florida International University. Unlike birds and mammals that can heat their own bodies to a constant temperature (like how humans stay around 98.6° F whether we’re in a blizzard or a sauna), frogs are “cold-blooded.” To survive in forests converted into fields and pastures, the strawberry poison frogs (aka Oophaga pumilio) camp out in the few remaining shady places like isolated trees and fallen logs.
“To do this study, we went out into pastures and the forest and measured the air temperature, and we took the temperature of frogs using an infrared thermometer – it’s like a gun that takes the temperature of the frog and the substrate where the frog was,” says Nowakowski.
“When you go into the forest, it’s really much cooler, more humid. In the pasture, you get a taste of what these frogs experience, because it’s incredibly hot, and you really want to spend all your time under a tree.”
“The forest has more shade, so sometimes the frogs can be found out in the open, on trees, or by digging through leaf litter. It was harder to find them in open pastures because the frogs have to really find ways to avoid prolonged sunlight exposure,” says Adrian Manansala, a recent graduate of the University of Guam and one of the undergraduate authors of the paper. “The fact that our study species is a vibrant red color made it easier to spot compared to other species that might blend in to the environment. Our mentor also showed us the sound our study species makes, so after some practice we were able to distinguish the sound from others and follow these sounds to help us find them.”
After recording the temperatures of 111 frogs in the wild, the researchers caught 32 of the frogs by hand and brought them back to the laboratory for further research. (While some species of poison frogs are dangerous to touch, strawberry poison frogs are toxic if eaten, but safe to handle.) In the laboratory, the researchers delved into a trickier-to-measure metric: the frogs’ preferred temperature.
Since they couldn’t just ask the frogs what temperature they like the thermostat set to, the researchers built temperature-controlled experimental habitats using aluminum catering pans, sand, ice packs, and heating pads.
“We were using whatever supplies are available in this rural area and MacGyvering it together,” says Nowakowski.
The resulting frog enclosures were long rectangles with a temperature gradient, with one cold end and one hot end. The researchers put each frog into the enclosure for two and a half hours and observed the temperature where the frogs preferred to hang out. They found that the frogs taken from warm pastures chose to spend their time in a warmer part of the enclosure, while the forest frogs preferred a slightly cooler environment.
But while the frogs changed their preferred temperature, the maximum temperature they could withstand didn’t change. To test the frogs’ heat tolerance, they placed the frogs in water baths and gradually raised the water’s temperature. The researchers stopped at the first sign that the frogs were under stress: no longer righting themselves when put on their backs. (The frogs weren’t harmed and were released back into the wild after the study ended.)
No matter where they were from, the frogs got too hot at about the same temperature. That means that while the frogs can adapt to have different preferred temperatures based on what they’re used to, the upper limit of what their bodies can tolerate is a hard line. “It appears that for the time being, this species can eke out a living in these converted habitats using different behaviors to avoid extreme temperatures. But this is all subject to change,” says Nowakowski. “On top of habitat change, temperatures are rising due to climate change. We don’t expect things to stay at the status quo – these frogs are bumping right up against their thermal tolerances, and it’s unclear whether they’ll persist or not.”
The frogs’ maximum temperature limit being seemingly fixed makes long-term climate changes more dangerous for them, but Thompson notes that there is hope in the way the frogs can adapt on the short term to habitat change.
“In the current biodiversity crisis that we’re a part of, we have to think of ways to make changes for positive impact,” says Thompson.
“This is something we can act on. Where habitat conversion cannot be avoided, leaving small pockets of vegetation will let small populations of frogs persist. It shows that people can make a change.”
And in addition to the potential of the study to help frogs and other animals, the researchers emphasize the importance of the REU program the students took part in. “You can learn a lot about science and research in the classroom, but it’s really all abstract until you go into the tropical forest or the laboratory,” says Nowakowski. “You learn how to develop questions and collect data to answer those questions. For students, it can be really transformative and eye-opening.”
“The REU was an unforgettable experience,” says Manansala. “Before this research program, I had never caught frogs or even held one before, so doing this research was a very new experience for me. This program was my first time conducting an independent study and before this, I had never thought of using creative ways to conduct research such as using a tea kettle and buckets of ice cubes to manually control the temperature of a lab trial. This program taught me research skills and knowledge that I still use today and will continue to apply in a future career in science.”