There are over 7,000 described species of frogs, and they live in a wide variety of habitats. Some spend nearly their entire lives in the trees, and among those tree frogs, a few have evolved specialized features for gliding or parachuting between branches.
For scientists like 2024 Grass Fellow Andrea “Dre” Gaede, gliding tree frogs present a rare opportunity to understand how neural systems evolved for the unique challenge of “flying” up to 50 feet between dense trees, even at night. How do their neural systems compare to tree frogs that don’t glide, and how different are they from more sedentary, earth-bound frogs?
Gaede, a lecturer in locomotor biomechanics at the University of London’s Royal Veterinary College, did her postdoctoral research on the visual control of flight in hummingbirds. She applied her experience and skills to ask similar questions about gliding tree frogs: What are the mechanisms and limitations of their visual motion processing? How do they make the split-second decisions required to successfully get from here to there without crashing? What can this tell us about the evolution of neural systems?
Frogs were the model system for some early, fundamental research on vertebrate vision systems, so with this solid foundation of knowledge, comparative studies between frog species can uncover visual motion processing lessons that can be relevant to a wide range of animals. “This enables us to explore why visual motion circuits evolved as they did,” said Gaede.
In the Grass Lab at the Marine Biological Institute in Woods Hole, Massachusetts, Gaede worked with six species of frogs (three species of gliding tree frogs, two species of non-gliding tree frogs, and one species of terrestrial frogs), observing some as they jumped, taking recordings from the visual processing center of their brain, and investigating the distribution of mechanoreceptors in their feet.
First, Gaede built a chamber lined with LCD panels attached to three of its walls that displayed scenery for the frogs to react to. While filming the space from the top and side, she could record the frogs’ movements as they jumped between a platform and perch. The preliminary data she collected showed that she could indeed track the frog moving through the chamber while manipulating their visual environment. She plans to continue this work in an expanded space with a 20-camera array to allow her to take more precise measurements, including limb positioning during glides.
She also recorded signals from the frogs’ optic tectum, a part of the brain that receives visual input from the retina and processes it. With these recordings, Gaede created a map of the frogs’ receptive fields and tracked the response and sensitivity to visual motion, considering variables like the direction of the motion, as well as spatial frequency, temporal frequency, and contrast. A testament to the value of being at the Marine Biological Laboratory, Gaede had the opportunity to collaborate with former Grass Fellow Duncan Leitch to perform the same recordings in American alligators, allowing for even further comparison.
Curious about the unique anatomical features of gliding tree frogs, like their toe webbing and skin flaps, Gaede looked closely at the receptors in their feet and compared the distribution to that in their non-gliding tree frog cousins. She used a dye that stains mechanoreceptors to visualize their density and distribution in the limbs, toes, and webbing. She plans to supplement this preliminary data with genetic studies to look at differences in expression of mechanoreceptors across species.
With summer 2024 behind her, Gaede describes the Grass Fellowship as, “a really great opportunity to be creative and to try something new.” She went on, “Often when you’re applying for funding, you have to show such a track record. And it was nice to say, I have this idea, and I want to pursue it, and somebody is letting me just run with it. I was able to try out a lot of new tools, which was great.”
Like Grass Fellows before her, Gaede remarked that the experience at Woods Hole, the community of researchers, and the surrounding environment propped her up and gave her fresh insight into her career aspirations.
“It made me feel much more excited and rejuvenated,” she said. “Being in science, you’re often met with rejection and doubting yourself. There’s always a lot of emotions, and you learn to deal with that, but having those moments of real intellectual dialogue with people and excitement become very important because it injects you with enthusiasm.”
Gaede continues to ask questions about flying animals’ vision and how these creatures have evolved. Having worked with birds and now frogs, she’s also interested in flying insects and other animals with this remarkable ability. It’s all part of the effort to understand the mechanisms underlying how different animals perceive their surroundings and respond, especially while flying through the air (in some cases at considerable speed). The answers to these questions have broad implications for our understanding of vision as well as neural systems writ large. By seeing what visual systems animals have in common, and what makes them unique, we can better understand the function and evolution of the brain, while also possibly informing the design of efficient visual systems for future technologies.