Why do male ICR(CD-1) mice perform bar-related (stereotypic) behaviour?

Prolonged interaction with cage bars by captive mammals (usually classed as stereotypic) may reflect poor welfare. Such behaviour may arise from motivation to investigate the external environment or to escape captivity. However, these hypotheses have not been explicitly tested. We raised mice, Mus musculus, to adulthood in modified laboratory cages with two sets of bars at the top and side of the cage. One set provided a potential escape route, and half of each set was backed by Perspex to reduce cues from the external environment. We predicted where mice should interact with the bars according to their motivational priorities. Body weights were recorded weekly to study the relationship between physical development and bar-related behaviour. Serum corticosterone was measured to monitor the effect of bar-related behaviour on stress physiology. Mice preferred to interact with bars where external cues were detectable. As adults, mice responded more to the bars providing a potential exit, though this was affected by the exit location. Corticosterone titres were higher in mice whose potential exit was situated at the cage top. Response to the bars was apparently restricted by the physical development of mice, particularly among those whose potential exit was situated in the cage top.