Responses of farmed silver foxes (Vulpes vulpes) to excessive food availability: implications for using food as a yardstick resource in motivation tests.
Food is frequently used as a ‘yardstick’ in operant experiments. However, previous work on silver foxes had suggested that how food is presented can affect how they respond to it, with excess food leading to variable consumption rates, and cages containing food being avoided over control cages except while eating. Our first experiment therefore examined foxes’ consumption of excess food in more detail by comparing their daily intakes with foxes fed a control diet for 4 weeks. Eighteen silver fox vixens were assigned to one of two groups in which the experimental group (N = 9) was feed 2.5 times that of control (N = 9). Although appetite, measured as the amount of food consumed, tended to fluctuate none of the overly fed foxes stopped eating during the experimental period. On average, the excessive fed group consumed more food (317 ± 18 g/day) compared to the controls (273 ± 12 g/day). In Experiment 2 we aimed to test whether the presence of excess food affected the foxes’ use of the cage. Five silver fox males were housed in separate multi-compartment apparatuses and the position of a food bowl was randomly switched between two cages for 18 days. The subjects had to perform one operant response to enter each cage and time inside the compartments was calculated. The foxes spent significantly less time in the food compartment (4 h 13 min), compared to the empty cage (12 h 12 min) and as expected by random (8 h) (P < 0.05). These results confirm that ad libitum fed silver foxes have a more fluctuating intake rate than animals fed just below maintenance, and that placing food in a cage reduces the time spent there. The day-to-day use of food, and perhaps even its value, can therefore be shaped by how it is presented. Implications for using food as a yardstick resource for silver foxes is that the ration should be held at maintenance. Also, to control for a motivation to leave food positioned in the operant test cage a free control cage distant from food could be provided. # 2006 Elsevier B.V. All rights reserved.