Meal patterns of mice under systematically varying approach and unit costs for food in a closed economy

Abstract

Several field and experimental studies have investigated the behavioral economics of food intake. In the laboratory, operant behavior has been used to emulate cost and to generate demand functions that express the relationship between the price of food and amount consumed. There have been few such studies of motivated food seeking and intake in mice, and none has reported demand functions. Using albino (CD1) male mice, the present study compares food intake and meal patterns across a series of ratio cost schedules. The first experiment examined unit price. A closed economy was used in which the mice were in the test chambers for 23 h/day and earned all of their food via either a nose poke or lever press response under fixed (FUP5, FUP10, FUP25, FUP50), variable (VUP10, VUP20, VUP50), and progressive (PUP1.25, PUP1.5, PUP1.75) unit prices. Mice were run for 4 days at each cost. There were no consistent differences between the first and last day indicating that behavioral adjustments to schedule changes occurred rapidly. When averaged across all price schedules, mice in the nose poke group consumed more food than their lever press counterparts but the overall shapes of the demand curves did not differ between the two operant responses, with intake decreasing as price increased. The number of meals taken per day differed between two meal-defining criteria that we applied, and there were some differences between the types of unit price schedule. In the second experiment, approach cost in the form of nose poke responses were required to activate a response device (lever) on which a fixed unit price for food was in force. These approach and unit costs were varied systematically. Meal number decreased, and meal size increased, with increasing approach cost even though nose pokes accounted for only a small fraction of the total response activity. Thus, meal patterns in mice are sensitive to approach cost while total amount consumed is more sensitive to unit price. These data are discussed in terms of the concept of foraging cost as either a unitary or a multidimensional variable.

Effects of meal frequency and snacking on food demand in mice

“…Our previous studies on food demand in mice have shown that they spontaneously eat ~15 meals per day (depending on the meal-defining criterion), mostly at night, with a mean size of ~15 pellets (Atalayer and Rowland, 2009). As unit price increased, meal size declined slightly.…”

“…In previous work, we have reported closed-economy food demand functions for mice under a variety of conditions in which they emit behavioral responses to obtain food pellets (Atalayer & Rowland 2009; Chaney & Rowland 2008). In those studies, we have found that when work is imposed as an inescapable fixed unit price (FUP), total food intake declines at relatively high unit prices (eg 50-100 responses per 20 mg pellet), but that the meal pattern remains relatively constant despite the fact that inter-pellet intervals and meals become extended in time as FUP increases.…”

“…In those studies, we have found that when work is imposed as an inescapable fixed unit price (FUP), total food intake declines at relatively high unit prices (eg 50-100 responses per 20 mg pellet), but that the meal pattern remains relatively constant despite the fact that inter-pellet intervals and meals become extended in time as FUP increases. In contrast, when an access cost followed by the unit price is imposed, mice tend to minimize their effort by taking fewer but larger meals (Atalayer & Rowland, 2009; Chaney & Rowland, 2008; Vaughan & Rowland, 2003). Thus, as was first suggested in rats by Collier, Hirsch & Hamlin (1972), meal patterns are determined primarily by the economy in which food is available.…”

Neural substrates underlying effort, time, and risk-based decision making in motivated behavior

“…During this process, the organism is thought to use information and knowledge of the costs of the current situation and weighs them against the anticipated benefit the effort will ultimately result in. There is a rich history of studies from experimental psychology in which various specific parameters of cost and benefit are manipulated which generally show that animals can make adaptive decisions in the face of changing costs and benefits (Atalayer & Rowland, 2009; Collier & Johnson, 1997). Additionally, there is evidence that animals can process and use information related to different response costs, including: distance, number, time, height, force and vigor.…”

Food demand and meal size in mice with single or combined disruption of melanocortin type 3 and 4 receptors

“…One of the important determinants of intake is the cost or effort associated with obtaining food (3,18). A demand function is the mathematical relationship between food intake and its cost; in fixed unit price (FUP) schedules, each unit of food costs a fixed number of behavioral responses.…”