Discrimination and categorization of actions by pigeons.

Recognizing and categorizing behavior is essential for animals (e.g., during mate selection, courtship, and avoidance of predators). In a study examining if and how animals classify different actions, a go/no-go procedure was used to train 4 pigeons to discriminate among "walking" and "running" digital animal models (each portrayed from 12 different viewpoints). Action discrimination acquired for two models significantly transferred to six novel animal models moving in novel and biomechanically characteristic ways. Randomization of frame order in the animated sequences, stimulus inversion, and static presentation all disrupted this discrimination, whereas changes in the direction and speed (both increases and decreases) of the actions did not. These results suggest that the pigeons discriminated the behaviors on the basis of generalized recognition of the models' sequence of poses across time and provide the best evidence yet that animals use action categories to identify contrasting behavioral units.