Who knows what Arturo the polar bear was thinking as he paced back and forth in the dark, air-conditioned chamber behind his artificial grotto? Just down the pathway Cecilia sat quietly in her cage, contemplating whatever chimpanzees contemplate. In recent years, both creatures, inhabitants of the Mendoza Zoological Park in Argentina, have been targets of an international campaign challenging the morality of holding animals captive as living museum exhibits. The issue is not so much physical abuse as mental abuse — the effect confinement has on the inhabitants' minds. Last July, a few months after I visited the zoo, Arturo, promoted by animal rights activists as "the world's saddest polar bear," died of what his keepers said were complications of old age. (His mantle has now been bestowed on Pizza, a polar bear on display at a Chinese shopping mall.) But Cecilia (the "loneliest chimp," some sympathizers have called her) has been luckier, if luck is a concept a chimpanzee can understand