Humans often describe pain as sharp, dull, pricking, burning, or itching. Animals cannot relate such descriptions, so pain has to be assessed by observing their behavioral or physiologic reactions. Although many similarities between humans and animals can be used for pain detection (Vierck, 1976; Vierck et al., 1983; Zimmermann, 1984; Kitchell and Johnson, 1985), marked differences in pain tolerance must be kept in mind. The anthropomorphizing of pain perception should be tempered by the recognition of the many differences between humans and animals.
Pain thresholds are remarkably similar among all species and breeds of animals, but the perceived intensity and tolerance of pain vary among individual animals and in the same animal under different circumstances. Many factors—including strain, species, experience, age, health, and stress—affect pain tolerance (Wright et al., 1985; Breazile, 1987). Young animals might have a lower tolerance of acute pain than do older ones. A systemically ill animal might be less tolerant of pain than a healthy one, but a moribund or severely ill animal might be nonresponsive albeit in distress.
Some marked differences in pain responses between humans and animals are related to the site of pain. Abdominal surgery is thought to be less painful in four-legged animals than in humans, because humans use their abdominal muscles to a much greater extent in maintaining posture and for walking. A median sternotomy could be classified as producing low to moderate pain in humans and much pain in animals, probably because animals use their front limbs in walking and movement of sternal edges after the sternum has been surgically separated would be slight but painful during walking. A lateral thoracotomy is likely to be accompanied by less pain in animals than in humans, because respiration is more abdominal in animals and more thoracic in humans. Such differences account at least in part for the wide variety of signs seen in response to surgery (Soma, 1987).
Nonhuman primates show remarkably little reaction to surgical procedures or to injury, especially in the presence of humans, and might look well until they are gravely ill or in severe pain. Viewing an animal from a distance or by video could aid in detecting subtle clinical changes. Loud and persistent vocalization is an occasional but unreliable expression of pain; it is more likely to signify alarm or anger. Therefore, it should be recognized that a nonhuman primate that appears sick is likely to be critically ill and might require rapid attention. A nonhuman primate in pain has a general appearance of misery and dejection. It might huddle in a crouched posture with its arms across its chest and its head forward with a "sad" facial expression or a grimace and glassy eyes. It might moan or scream, avoid its companions, and stop grooming. A monkey in pain can also attract