Male mice (Mus musculus) were given the choice between two housing conditions. The apparatus comprised two standard cages (Makrolon type III, 37x21x15 cm) connected by a tunnel. One cage was left unstructured. The other cage was environmentally enriched with a wooden scaffolding and a plastic box with several openings. The tunnel incorporated a lever and two "one-way-streets" with two doors each. The mice could push the doors open and pass through in one direction only. The door that led to the enriched cage was locked by an electromagnet that was de-energized when the lever was pressed a number of times according to a fixed-ratio (FR) schedule. The way back to the standard cage was left unlocked. The mice had to work on the lever up to 16 times for one single entry to the enriched cage. During a period of 24 h, observations were conducted with digital imaging techniques at five different fixed ratios (FR1, FR2, FR4, FR8 and FR16). Preference was measured by the total time spent within the cage and the number of entries to the enriched cage. All mice preferred the enriched cage even if they had to work for it, although the number of entries to the enriched cage decreased at higher fixed ratios.

Human economics theory predicts that consumers with fixed incomes purchase less of a commodity as the price increases. A demand function can be calculated by relating the quantity of the commodity gained, Y, to the effort required to gain the commodity, X. The rate of decrease of the demand function is related to the perceived importance of the item: for luxury items such as champagne, the quantity purchased decreases at a rapid rate. For more essential items such as bread, the rate of decrease is relatively small. In analogy to human economics theory, the perceived importance of commodities concerning animal welfare (e.g. environmental enrichment) can be rated by means of operant conditioning techniques when an increase of price is simulated by an increase of the required fixed ratio.

The demand curve for access to the enriched cage was calculated by relating the number of entries per 24 h to five different FRs. As a control, a second demand curve was calculated from data derived from the same choice apparatus comprising two unstructured cages. Thus, the perceived importance of an environmentally enriched cage could be compared to the perceived importance of another standard cage.

The slopes of both demand curves were relatively small, a fact that indicates a high demand for both a second standard cage and an environmentally enriched cage. Whereas the slope of the demand curve for the enriched cage was significantly smaller, indicating a greater demand. This leads to the conclusion that (1) mice are clearly motivated to explore any given amount of additional space and (2) the access to environmental enrichment is perceived as even more important.

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