Humans—who enslave, castrate, experiment on, and fillet other animals—have had an understandable penchant for pretending that animals do not feel pain. On whether we should grant some modicum of rights to other animals, the philosopher Jeremy Bentham stressed that the question was not how smart they are, but how much torment they can feel. […] From all criteria available to us—the recognizable agony in the cries of wounded animals, for example, including those who usually utter hardly a sound—this question seems moot. The limbic system in the human brain, known to be responsible for much of the richness of our emotional life, is prominent throughout the mammals. The same drugs that alleviate suffering in humans mitigate the cries and other signs of pain in many other animals. It is unseemly of us, who often behave so unfeelingly toward other animals, to contend that only humans can suffer.
Carl Sagan & Ann Druyan, Shadows of Forgotten Ancestors, New York, 1992, pp. 371-372