• evolution;
  • animal cognition;
  • empathy;
  • facial expressions;
  • instinct

Emotions suffuse much of the language employed by students of animal behavior—from “social bonding” to “alarm calls”— yet are carefully avoided as an explicit topic in scientific discourse. Given the increasing interest in human emotional intelligence and the explicit attention in neuroscience to the emotions, both human and nonhuman, the taboo that has reigned for so long in animal behavior research seems outdated. The present review seeks to recall the history of our field in which emotions and instincts were mentioned in the same breath and in which neither psychologists nor biologists felt that animal emotions were off limits. One of the tenets supporting a renewed interest in this topic is to avoid unanswerable questions and to view emotions as mental and bodily states that potentiate behavior appropriate to environmental challenges. Understanding the emotionally deep structure of behavior will be the next frontier in the study of animal behavior.