As is the case with pain, physiological parameters and behavioral responses provide important cues to distress in animals, although distress is more difficult to define and identify than pain. Physiological parameters include hormonal responses (e.g., changes in the levels of adrenal hormones), increased susceptibility to disease (which may indicate an impaired immune system) or weight changes. Any unusual behavior in an animal which shows physiological signs of stress may be such a cue. Some behavioral changes, however, may be normal adaptive responses which help the animal cope with a new environment or moderate stress, so behavioral changes should not automatically be considered pathological. Behavioral changes which occur in the absence of physiological signs of stress may also indicate suffering. For example, conflict behavior, in which an animal exhibits conflicting tendencies to perform different or incompatible behaviors, may indicate fear or frustration. Conflict behavior can, however, result in positive stimuli also, such as conflict between the desires to eat and mate. Finally, the appearance of abnormal behavior, such as the pacing of big cats in zoo cages or the self-biting of monkeys housed alone in small cages, can signal stress. Even clearly abnormal behaviors may help animals cope with the conditions of captivity, however. For instance, chimpanzees which throw feces at zoo visitors may be enhancing the psychological condition of their captivity; thus, this might be considered an adaptive behavior.