The reason this question is dead is because traditional Skinnerianism, which viewed rats and pigeons as furry and feathered black boxes, guided by simple principles of reinforcement and punishment, is theoretically caput. It can no longer account for the extraordinary things that animals do, spontaneously.
Thus, we now know that animals form cognitive maps of their environment, compute numerosities, represent the relationships among individuals in their social group, and most recently, have some understanding of what others know.
The questions for the future, then, are not "Do animals think?", but "What precisely do they think about, and to what extent do their thoughts differ from our own?"
MARC D. HAUSER is an evolutionary psychologist, and a professor at Harvard University where he is a fellow of the Mind, Brain, and Behavior Program. He is a professor in the departments of Anthropology and Psychology, as well as the Program in Neurosciences. He is the author of The Evolution of Communication, and Wild Minds: What AnimalsThink.