Rats and Mice
Rats and mice are the two rodent species most widely used in research generally and in pain-related research specifically, so it is important that researchers and IACUCs recognize when these species are in pain. Rats and mice in acute pain may vocalize and become unusually aggressive when handled. Because rodents also vocalize at ultrasonic frequencies inaudible to humans, the absence of audible vocalization does not necessarily signify the absence of acute pain. Inappetence or a change in feeding activity can be noted; for example,they may eat bedding or their offspring. If they are housed with others, the normal group behavior or grooming might change. Rodents in pain may separate from the rest of the animals in the cage and attempt to hide, or they may no longer exhibit nest-building behavior. In rats, porphyrin secretion (“red tears”) may appear around the eyes and nose, although this is a general response to stress of any kind.
Normal guinea pigs stampede and squeal when startled, when attempts are made to handle them, or when strangers are in the room, but sick guinea pigs and those in pain usually remain quiet. However, since the initial response to the presence of an observer by a normal guinea pig is to remain immobile, assessing signs of pain can be extremely difficult. Guinea pigs in pain reduce their food and water consumption, or become anorexic.
Hamsters and Gerbils
There is virtually no information about signs of pain in hamsters and gerbils, although it is assumed that they, like rats and mice, will show decreased activity, hair “standing on end,” and an ungroomed appearance when in pain. As with other species they may adopt an abnormal posture, which may be particularly obvious when moving. Respiration may change.