Social isolation affects the motivation to work for food and straw in pigs as measured by operant conditioning techniques

Abstract

The aim of the study was to investigate if the presence or absence of a companion pig (pig in an adjoining pen during testing) systematically influenced the demand function for food or straw in growing pigs. The experiment was carried out in two phases. In one phase, the demand for food was tested ( n=10) and in another phase the demand for straw was tested ( n=10). A different set of test pigs was used in each phase. Pigs were tested daily in an experimental room consisting of two identical twin-pens (one for the test pig and one for the companion pig (if present)). The test pigs worked for a resource by pressing a panel on a fixed ratio (FR) schedule, while the companion pig (if present) was simply given the same amount of the resource each time the test pig earned one. The reward size was either 26 g of food or 500 g of straw. The experimental set-up was a cross-over design with all pigs working on both treatments (isolation/companion). Each period consisted of three identical runs of 4 FR-levels (food experiment: FR 8, 20, 40, 60 and straw experiment: FR 2, 5, 10, 15). The results showed a steeper slope of the demand function when pigs were tested in isolation compared to when they were tested with a companion pig (slope of demand function: −0.11±0.01 versus −0.07±0.01) ( F 1,58=4.2; P=0.04). The slope of the demand function for straw was unaffected by social isolation, whereas the intensity ( y-intercept) of the demand function was higher when pigs were tested with a companion compared to in isolation (intercept of the demand function: 3.2±0.15 versus 3.0±0.15) ( F 1,26=4.42; P=0.04). The use of the straw was affected by FR level in that the time spent with each reward before a new reward was obtained increased with increasing FR ( F 1,177=165; P<0.0001). Similarly, the percentage of time during which the pigs interacted with the straw was higher when they were tested with a companion pig compared to when they were tested in isolation ( F 1,170=5.0; P=0.03). The social context in which the pigs are tested affects the demand functions for resources, and these effects differ for different resources, e.g. food was more valued in the presence of a companion. The results suggests that the social environment needs to be considered when investigating the value of resources to pigs.