Food demand functions in mice.

Male mice (Mus musculus) of a mixed B6/129 background were used to establish food demand functions in a closed economy. The mice lived continuously in operant chambers and worked for 20-mg nutritionally complete food pellets. First, a series of incrementing fixed ratio (FR) costs per pellet were imposed, and the results showed that demand declined as unit price increased. The number of meals taken per day was dependent on the temporal criterion used to define a meal, but the number of meals did not change across the FR series. Next, a series of incrementing progressive ratio (PR) schedules were used, and a meal was defined by a programmed schedule reset interval. Total food intake declined slightly, and the mean meal size also decreased, across the series. Lastly, a nose poke response requirement was imposed as the procurement cost to activate a lever press device for food; under these conditions the meal number changed dramatically as the procurement cost was increased, whereas total intake declined only modestly. These data show in mice that large changes in unit price or consummatory cost have relatively small effects on demand and meal patterns, but small amounts of foraging (procurement) cost have very large effects.