A fine day out with the family might itself be justification enough for the existence of zoos if the zoo animals are all happy to be there. Alas, there’s plenty of heartbreaking evidence that many are not. In many modern zoos, animals are well cared for, healthy and probably, for many species, content. Zookeepers are not mustache-twirling villains. They are kind people, bonded to their charges and immersed in the culture of the zoo, in which they are the good guys. But many animals clearly show us that they do not enjoy captivity. When confined they rock, pull their hair and engage in other tics. Captive tigers pace back and forth, and in a 2014 study, researchers found that “the time devoted to pacing by a species in captivity is best predicted by the daily distances traveled in nature by the wild specimens.” It is almost as if they feel driven to patrol their territory, to hunt, to move, to walk a certain number of steps, as if they have a Fitbit in their brains. The researchers divided the odd behaviors of captive animals into two categories: “impulsive/compulsive behaviors,” including coprophagy (eating feces), regurgitation, self-biting and mutilation, exaggerated aggressiveness and infanticide, and “stereotypies,” which are endlessly repeated movements.