They look cuddly, but don't be fooled: red-eyed treefrogs (Agalychnis callidryas) have a secret dark side. When Michael Caldwell, of the Smithsonian Tropical Research Institute, filmed the frogs under infrared light he saw a behavior had hadn't seen before -- the frogs started vigorously shaking the branches they were sitting on. Caldwell and colleagues, including Karen Warkentin of Boston University, decode the meaning of the shakes in Current Biology this week.