Increasing costs of access to resources cause re-scheduling of behaviour in American mink ( Mustela vison): implications for the assessment of behavioural priorities

Abstract

Consumer demand studies of animal preference commonly involve the imposition of costs on access to resource compartments rather than direct costs on resource consumption. The aim of this study was to analyse how such increasing access costs alter the scheduling of behaviour, in order to investigate some potential problems with using this technique to measure behavioural priorities. We investigated the effect on mink of placing a cost (a weighted door) on the access to resources. Six mink were individually housed in closed economy test arenas. Each consisted of a home cage (containing food, water and a nest box), an empty compartment and six resource compartments with access to a box of hay, a water bath, a raised platform, a wire cylinder, a novel object, and small toys. Door weights were increased from 0 to 1000 g and the minks' behaviour was recorded on video for 8 h per day. As expected, increasing door weight reduced the number of visits to each compartment but lengthened each visit. Furthermore, as costs on access increased, there were changes in how mink behaved during each visit to resource-compartments. They became increasingly likely to interact with each resource during a visit, and they showed shorter latencies to interact with the resources. The time spent in compartments not interacting with the resources also declined. Overall, these changes in within-visit behaviour meant that the greater the cost on access, the greater the proportion of time spent with a resource devoted to actually interacting with it. Thus, the time spent with a resource over-estimated the actual use of that resource, particularly at lower access costs. These results illustrate the dangers of simply using time spent with a resource as a measure of consumption in studies of behavioural priorities. As a consequence of longer visits and more interaction, the total time spent swimming, manipulating the novel objects and occupying the hay box did not decline with increasing door weight. However, unexpectedly, this compensatory re-scheduling did not occur for all activities, because the time spent occupying the platform, manipulating the toys and the cylinder and the time in the empty compartment declined with higher access costs. This suggests that increasing access costs for unlimited periods of time with resources may, although not strictly valid for the production of demand curves, yield a ranking of behavioural priorities if high enough costs are used. Further research is necessary to see whether the minks' failure to defend time spent in the platform, manipulating toys and the wire cylinder and in the empty compartment does represent the low priority of these activities. Or whether some other factors, such as constraints on the ability to re-schedule the activities prevent mink from compensating for higher costs on access.