Mesial prefrontal cortical lesions and timidity in rats. III. Behavior in a semi-natural environment

Behavior of adult male rats with and without lesions in mesial prefrontal cortex (MFC) was studied in a large (72 m2) semi-natural environment (SNE) equipped with numerous wooden nest boxes. The experiment was conducted in 2 partial replicates, one in summer, one in mid-winter. Subjects were introduced to this environment 11 days after surgery, in groups. In the first replication, control and lesioned rats were subjected to mild food deprivation, beginning two weeks after introduction into the SNE. In the second replication, rats were first restricted to separate areas of the SNE, then later allowed full run of the environment. This procedure was then repeated. Summarizing the findings of both replicates, MFC rats were more likely to be found inside nest boxes than controls, changed nest boxes less often, and were less likely to obtain or hoard food from the daily feeding sessions, or to hoard and use nesting material. MFC rats were lower in social rank than controls, and lost less weight during the initial two weeks in replicate one, perhaps due to their relative inactivity. These differences waned over time when MFC rats were competing only within their group, but not when the brain damaged rats were forced to compete with intact controls. It is concluded that the timidity previously described in studies of MFC rats in a laboratory setting (Holson, Holson and Walker) has ethological validity, in that it is also evident in an SNE. The implications of these findings for an understanding of the evolution and function of prefrontal cortex are briefly discussed.