The type of operant response affects the slope of the demand curve for food in mink

Abstract

The behavioural priorities of farm animals may be quantified by demand functions generated by use of operant conditioning techniques. However, there are several aspects of this method that still need to be investigated in more detail. The aim of this study was to investigate the effect of operant response type on the demand function for food in mink. The responses examined were pressing a lever and pulling a chain. The experiment was conducted with eight mature female mink. During each of two periods, the mink were tested in four replicates of successively increasing fixed ratio (FR)-values (4, 8, 16, 25, 60, 80 and 100). The rewards were available for 24 h per day and each reward consisted of 0.5 g of food. Half of the animals worked on the lever in the first period and on the chain in the second period, the other half of the animals vice versa. The mink were weighed regularly throughout the experiment. The behaviour of the mink was video recorded continuously for 24 h on the days when they were working on FR levels 4, 40 and 100. The number of rewards and thereby the amount of food earned decreased as the FR-values increased for all animals. The demand curve for food obtained by chain pulling was steeper than the demand curve obtained by lever pressing ( P<0.001). The results show that the response type can influence the slope of demand functions. One reason for the difference in slopes of the demand curves could be that the unit price paid on the chain and on the lever at a given FR-value was not the same, even though the minimum force required was 35–40 g for both types of response. The slope of the demand curve for food in mink was much steeper than previously found in mice, rats, pigs and hens. This is most likely due to the combination of high FR-values and small rewards in relation to the foraging strategy in mink. High levels of stereotypies were related to high workloads on the chain. The mink had a more restricted food intake and a higher level of stereotypies when working on the chain than when working on the lever. Frustration caused by a combination of high FR-values, small rewards and hunger may explain the significant increase in stereotypies.