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Reorganization of behaviour in laboratory mice,
 Mus musculus
 , with varying cost of access to resources

Abstract Gaining access to resources can be measured in terms of the time spent with, or the number of visits to, a resource. For some resources, time spent with the resource might be more important, for others the number of visits might be more meaningful. By using traverses of shallow water, the costs of gaining access to food, shelter, a conspecific, increased space, a running wheel, deep sawdust, or enrichments (e.g. balls, a variety of small objects) were increased for laboratory mice. When 30 cm of water was present, the number of visits to each resource decreased to 39–64% of the number recorded when no water was present, but the proportion of time spent with each resource was defended. Increasing the width of water to 120 cm had no further effect on the number of visits or on the proportion of time spent with each resource: thus both the frequency of visits and the proportion of time with each resource were ultimately defended. Possible reasons for this change in behavioural organization, including the importance of patrolling, are discussed. The data support previous findings that laboratory mice are highly motivated to patrol areas made accessible to them, and suggest that care is needed when interpreting what animals perceive as reinforcement during visits to resources. Furthermore, it was shown that for laboratory mice under conditions that penalize initial access only, the time spent with a wide variety of resources was defended more diligently than the number of visits, but the number of visits was rarely allowed to decrease to zero.