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Procedencia: The Association for the Study of Animal BehaviourThe importance of a commodity, as perceived by animals, can be determined by measuring the amount of work animals are prepared to perform to gain access to that commodity. In the present study, this method is extended to establish how animals perceive the importance of increasing amounts of one commodity, that is, additionalspace. The behaviouraldemandfunctions of cagedlaboratorymice,Mus musculusfor additionalspace ranging from 196 cm2to 1600 cm2on fixed-ratio schedules ranging from 5 to 80 switch operations were determined. At all fixed-ratio values, the mice worked economically and gained access to additionalspace on several occasions within each 1-h observation session. The amount of access gained decreased as the work required increased, but the slope of the function (&][8722;0.347) was sufficiently shallow to indicate that additionalspace was regarded by the mice as a highly important commodity. The frequency of visits and the time spent in the additionalspace were significantly different between the sizes of additionalspace offered, but unexpectedly, these differences were small. In addition, the elasticity coefficients were not significantly different between the sizes of additionalspace. The absence of large differences in response to disparate sizes of additionalspace may indicate that the mice may have (1) been motivated to escape their home-cage, (2) been motivated to search for unavailable resources, or (3) perceive the different amounts of additionalspace as nearly equally (non-)rewarding. Time of day had a significant effect on the responses of the mice in that towards the end of the active phase, the additionalspace was visited less frequently and for shorter periods indicating a temporally based change in motivational status or efficiency of behaviour. It is argued that these results support previous evidence that laboratorymice are highly motivated to explore and subsequently monitor areas made accessible to them, regardless of size and, to some extent, content.