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 can aid in detecting subtle clinical changes. Loud and persistent vocalization is an occasional but unreliable expression of pain as 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 altered attention from its cagemates varying from a lack of social grooming to attack. The animal may show acute abdominal pain through facial contortions, clenching of teeth, restlessness, and shaking accompanied by grunts and moans. Head pain may be manifest by head pressing against the enclosure surface. Self-directed injurious behavior may be a sign of more intense pain. Primates in pain usually refuse food and water. If an animal is well socialized (e.g., trained to perform tasks as part of a research protocol), changes in response to familiar personnel or in willingness to cooperate may indicate pain.