Methodology for measuring hunger and food needs using operant conditioning in the pig

Abstract

Two reward schedules were compared for their sensitivity in measuring motivational changes due to food restriction in the pig ( Sus scrofa). Six boars were each restricted (proportionally) to 0.8, 0.6 and 0.4 of their predicted ad libitum food intake (PFI) in a Latin square design. Subsequently, they were offered their PFI (Treatment 1.0). Feeding motivation was measured using a fixed ratio of 10 panel presses for each 6-g reward of food (FR schedule) and operant response rates were measured over 20-min sessions at three times post-feeding on each food level. Another 6 boars were subjected to identical conditions, differing only in their being rewarded on a progressive ratio (PR) where the response contingency was incremented by one on each successive reward. Reward rate per session was strongly affected by food restriction on both FR and PR schedules. On FR, reward rate per session increased with food restriction, but only up to a maximum on Treatment 0.6. On PR, in contrast, it was possible to distinguish between reward rates on Treatments 0.6 and 0.4 up to 5 h post-feeding. Progressive ratio appears to be a more sensitive means that FR of measuring changes in feeding motivation, however the cost was found to be an increase in the variability of the response data. The results suggest that the subjects on Treatment 0.6 were maximally food motivated for at least 19 h of the day. This also applies to sows and boars maintained on similar levels of food restriction under commercial conditions, showing the extreme divergence between food-restricted pigs' motivational need for food and their economically determined food allowances.