Measuring motivation in swine: the food-metric scale

Understanding how nonhuman animals such as swine respond to their environment and understanding how to provide them with a good quality of life involves using a range of experimental approaches. More and more, ethological researchers are turning to operant methods to answer some of these questions. Employing an operant such as a lever, researchers can assess how hard animals will work to get access to environmental resources: increased space or social contact. It is difficult, however, to determine how the effort made by the animals relates to the degree to which they need the resource and, in particular, how to interpret intermediate levels of responding. One approach to understanding the level of need is to compare it with familiar states of deprivation such as hunger. Food is an environmental resource known to range from low to high value depending on deprivation level. Depriving animals of a fixed proportion of their daily ad libitum intake allows the animals to demonstrate the levels of responding produced at satiation: 23 hr deprivation and a range of intermediate points. The resulting scale has both empirical and intuitive value and can help in understanding the value of various degrees of operant effort. Ultimately, this information will help in deciding which environmental conditions should be provided to swine as part of routine husbandry.