Light and shadow: visual recognition of the stationary environment by leopard frogs.

We determined how leopard frogs respond to non-moving aspects of the environment. We have discovered that these frogs are attracted to dark, stationary, opaque objects. This attraction depends on the relative reflectance of the object, i.e., the darker the block, the more attractive it is, and the attraction is found under both bright and dim ambient light levels. Larger blocks are more attractive than smaller blocks, but frogs are still attracted to blocks much smaller than themselves. Previous studies have shown that frogs are also attracted to sources of light. Using a choice experiment, we show that the probability a frog will choose a dark object versus a light source depends on the intensity of the light source relative to the intensity of the ambient light. The frog only moves toward a light source when it is at least 20 times brighter than the brightest object in the environment. These findings help to clarify the frog's "phototactic" nature.