Frogs make mental maps
Researchers find the first evidence of cognitive mapping outside birds and mammals. Tanya Loos reports.
A cognitive map is a mental representation of the external world, and our place in it, allowing us to plot the most efficient route to any destination. Humans, other mammals and many bird species use them – and now for the first time, amphibians have been shown to do so, too
Poison arrow frogs, members of the Dendrobatidae family and native to South and Central America, have a parenting style that requires sophisticated navigation and spatial recall in their complex rainforest habitat.
After their eggs hatch in damp leaf litter on the forest floor, the adults transport the tadpoles, one or two at a time, to tiny ephemeral pools of water in tree holes and epiphytes such as bromeliads. The parents spend considerable time locating and monitoring the pools to ensure their tadpoles remain in water.
Field based studies on the frogs have suggested that parents most probably use a cognitive map to locate their tadpoles, but laboratory experiments are necessary to conclusively establish its existence.
A team of US researchers, led by Sabrina Burmeister from the University of North Carolina, Chapel Hill, trained five adult green-and-black poison dart frogs (Dendrobates auratus) to use a modified version of the Morris water maze, which they dubbed the moat maze.
The Morris water maze has been used with some success with rats in cognitive map research, with trained animals learning visual cues to locate a hidden platform under opaque water, and thus be able to stop swimming.
In previous tests, frogs simply swam to the sides of the Morris water maze and remained attached to the walls.
To adapt the equipment for the amphibians, the team created a shallow area in the middle of the maze, with a deep area around the edges. They also increased the temperature of the water to 35 degrees Celsius, providing an impetus to get out of it. The frogs were able to stand and walk in the shallow area, and thus follow visual cues to find the platform to escape the warmth.
The maze was divided into four quadrants, each with a visual cue provided just above the water level, such as a blue artificial flower, or a red flashing light.
After 10 days of training, four of the five frogs reached 80% success; and the last frog located the platform correctly on the thirteenth day. The pathways of each animal were recorded visually and analysed, revealing that the frogs chose ever more direct pathways to the platform as the training continued.
“In our moat maze,” the researchers write, “the poison frogs were able to use a configuration of visual cues to find the hidden platform”. The frogs chose direct paths to the goal from multiple random initial positions, which is “a hallmark of a cognitive map”.
The team noted that the performance of the frogs was on par with rodents in a classic Morris water maze, and represents the first conclusive evidence of a cognitive map in an amphibian.
The findings, published in the Journal of Experimental Biology, provide researchers with further clues into the evolution of spatial cognition in vertebrates.