of physical contact appears to be the most important cause of abnormal behavior, both in infants and in adult animals (Bayne and Novak, 1998).
Animals that are isolated to disrupt the infant-parent bond often display acute responses to indicate stress. Distress vocalization, changes in general activity and heart rate, as well as elevated cortisol/corticosterone concentrations can occur and are adaptive under normal circumstances. However, if the separation is prolonged, as during experiments where the effects of infant-parent bond disruption are being studied, it becomes distressful and can lead to maladaptive behaviors as the infant animal matures. Self-injurious behaviors, stereotypic behaviors, extreme timidity or aggressiveness, and inability to mate or provide adequate care to offspring are maladaptive behaviors that might result from the social disruption (NRC, 1992).
Kittens separated from their mothers at an early age tend to be more aggressive and nervous as adults (Seitz, 1959), and social play is critical for a kitten’s development (O’Farrell and Neville, 1994). Puppies that are not adequately socialized to other dogs or people may be excessively fearful or aggressive (O’Farrell, 1996). Wolfle (1990) has described a puppy-socialization program and behavioral scoring method specifically for use in the research environment. Monkeys reared in partial or total social isolation develop a syndrome of behavioral abnormalities that includes rocking, huddling, self-clasping, and excessive self-orality (Cross and Harlow, 1965; Harlow and Harlow, 1965). As the animals age, stereotypic patterns emerge, such as repetitive locomotor patterns, floating limbs, and eye poke or salute. The isolation syndrome is also manifested in the development of abnormal social relationships (Mason, 1968).
A restricted social environment can also affect adult animals. For example, long-term (2-year) individual housing of adult nonhuman primates has been shown to alter social behavior (Taylor et al., 1998). Unless the research focuses on social restriction or veterinary concerns develop, infant animals should be reared in a social environment with mother and peers, with mother only, or with peers only to reduce or prevent psychopathologic conditions (Bayne and Novak, 1998). Similarly, when the research, health, and safety of the animals allow it, adult social animals should be maintained in a social environment (for example, pair- or group-housed).
The primary animal care and use concern associated with social disruption is the distress that leads to the display of maladaptive behaviors. When studies involve the use of social disruption, the animal-use protocol should include humane endpoints for removal of the animal from the study. Determining endpoints that are predictive of severe distress is a matter of professional judgment and should evolve through discussions between the IACUC, veterinarian, and PI.