Development of open-field behavior in mice: Effects of age and experience

Offspring from each of two inbred strains of mice (BALB/cJ and C57BL/6J) and their reciprocal crosses were tested in an open-field apparatus at 10 ages, from 15 to 120 days. Cross-sectional and longitudinal analyses were performed to assess separately the effects of age and prior test experience on open-field activity and defecation. Increased activity was associated with increased age in both strains and their F₁ reciprocal hybrids; however, the magnitude of this effect was a function of the genotype. Defecation also increased initially in all groups as a function of age; but, at later ages, different groups exhibited rather dissimilar defecation patterns. Prior experience was found to decrease activity in the open field in inbred strains and their reciprocal hybrids. The effect of prior experience on defecation was more complex: Within the two inbred strains, prior experience had relatively little effect on the pattern of defecation; for the F₁ hybrids, however, prior experience led to a significant increase in defecation at most ages. It was suggested that the differential effect of test experience on open-field activity and defecation represents a separation of two components of open-field behavior, “exploration” and “emotionality.”