A poison frog about the size and color of a mint chocolate bonbon is quietly upending what we thought we knew about how frogs think. Native to tropical rainforests of central and South America, the land-adapted species Dendrobates auratus eschews rivers and lakes entirely, instead laying its eggs on the forest floor. When the eggs hatch, the frog piggybacks its tadpoles up into trees, placing them in water accumulated within tree holes and bromeliads.
To find, remember, and navigate among egg nests and tadpole nurseries in such a complex, changeable landscape requires a brain that can make and revise a mental map of its surroundings. Many mammals and birds form such maps. And now, new research in the Journal of Experimental Biology provides the first evidence of the same skill in amphibians.
“We hypothesize that because of their natural history, poison frogs evolved a more advanced cognitive ability—to flexibly use environmental cues to find locations,” says Sabrina Burmeister, associate professor at the University of North Carolina and senior author of the study, published earlier this year. “You’re not likely to find these abilities in all frog species.”
Frog brains are still poorly understood. Scientists have long favored studying the more obviously complex brains of animals such as primates or corvids. And it’s challenging to modify lab tests commonly used for rats or homing pigeons—bred for the ability to find their way home over many miles—to suit a frog’s natural inclinations.
A classic lab test of navigation involves placing the study subject in a circular swimming pool, called a Morris water maze, which contains a hidden escape platform to clamber onto and distant landmarks at compass points. The more adept the animal is at visual assimilation and recall, the easier it’s able to find the platform in subsequent trials. (Related: Why don't poison frogs poison themselves?)
But D. auratus, also known as the green-and-black poison frog, gets spooked in such a maze, trying to cling to the edge for safety instead of exploring and taking in its surroundings, explains Yuxiang Liu, a postdoctoral researcher at the University of Texas Southwestern Medical Center, who conducted this work as part of his Ph.D.
He converted the Morris water maze into a wading pool with a deep moat to discourage the frogs away from the edge. The new setup encouraged the animals to explore the shallows and eventually find the escape platform. Once the frogs learned what to do, the scientists shifted the visual cues and removed the platform. They were able to demonstrate that the frogs did, in fact, search for their escape in a manner consistent with using a mental map of their surroundings.
Field biologist Andrius Pašukonis, who researches poison frog homing behavior in the wild and was not involved in this study, is excited about the results and hopes to see more work done on these and other species. A postdoctoral researcher at Stanford University, he notes that there’s plenty more to learn. “At the moment we have truly no idea of what potential ‘frog maps’ might look like,” he says.
While he and fellow field researchers follow poison frogs in the rainforest using twine and GIS loggers or tiny frog pants with radio transmitters, Burmeister’s lab is doing comparative work on D. auratus and another species of frog, the túngara, which apparently lacks similar navigational flexibility and learning in tests done so far.
Meanwhile, the frogs themselves are doing their best to survive. They’ve evolved diverse and ingenious parenting styles, habitat preferences, and cognitive capacities. Now, all are put to the test against human-mediated forces of habitat loss, disease spread, wildlife trade, and climate change. It will take more than putting ourselves in the mind of a frog to figure out how to escape such a maze.