Head dipping on a hole-board is a commonly used task in many types of pharmacological, genetic, or neural lesion experiments as a measure of exploratory and/or anxiety-like behavior (31, 32). In the inbred strain experiment, a strong genotypic correlation between head dipping and locomotion was observed (r = 0.77). If genotype is ignored and individual values for head dipping and locomotion correlated, a similar strong correlation was found (r = 0.63, see Table 4). However, within a given inbred strain the magnitude of the phenotypic correlation varied greatly: some strains showed strong positive correlations, whereas others showed only small positive correlations and two strains had slightly negative correlations between head dipping and locomotion (see Fig. 6). HEB and LEB mice at the fifth generation of selection also exhibited a genetically correlated difference in locomotor activity, and within each line and replicate the phenotypic association was variable (r = 0.22–0.67; data not shown). Collectively, these results indicate a somewhat unpredictable relationship between head dipping and locomotion within a given strain of untreated mice and suggest that the choice of mouse strain used for an experiment with the hole-board can have a large influence on this correlation.
