In the rat isolation has both short- and long-term influences upon behavior. Rats isolated at any age will show increases in timidity and aggression, but both effects can be reversed by periods of social housing. However, isolation before 50 days of age has permanent effects upon behavior. We have previously found that rats between 25 and 45 days of age may be protected from the deleterious effects of isolation by short daily periods of social contact if, during these daily contact periods, the rats engage in intense bouts of rough-and-tumble play. In this study we examined the permanence of the effects of isolation on the rat, mouse, guinea pig and gerbil. As predicted by the play hypothesis, species which do not engage in extensive social play do not show permanent deficits if isolated prior to 50 days. Only rats which engage in long bouts of rough-and-tumble play between 20 and 50 days show any permanent behavioral effects of isolation during this period.

Earlier experiments have shown that many of the effects of early social isolation in the rat may be due to deprivation of social play. The isolated ferret, like the isolated rat, is hyperactive; furthermore, its hyperactivity appears to be related to deprivation of social play. However, the relationship between object investigation and social rearing is more complex in the ferret than it is in the rat.