Behavioural comparisons between individually- and group-housed male rats: effects of novel environments and diurnal rhythm.

Individually-housed rats, when compared with group-housed controls, have been previously shown to be hyper-reactive when exposed to an openfield at the beginning of the dark-phase; this was not due to elevated spontaneous activity. In the present experiments locomotor and rearing activities were determined immediately after exposing animals to a novel environment, and after 6 and 24 h of habituation to the test-environment. Additionally, after 24 h of adaptation, animals' responses towards a modification of the test-environment were examined. Four parallel experiments were carried out, each starting at a different time in the light/dark cycle. Individually-housed rats were hyper-reactive in novel environments and showed a slower habituation at all time points tested. Part of behavioural parameters show clearcut day/night variations, but differences between individually- and group-housed rats do not seem to be influenced by the time of day.