Possum Food Preferences under Progressive-Ratio and Concurrent-Schedules of Reinforcement

Three experiments compared various ways of examining food preferences in the common brushtail possum (Trischosurus vulpecula). The first experiment compared the preferences obtained between four foods using paired-stimulus (PS) and multiple-stimulus without-replacement (MSWO) assessments. The ranked orders identified by the two methods produced similar orders in individual possums, but were idiosyncratic across possums. Following this, Experiment 2 involved a progressive-ratio (PR) reinforcer assessment with all four foods used in the preference assessments of Experiment 1. For each possum, each of the four foods functioned as reinforcers when presented in a single-schedule arrangement, including those foods identified as less preferred. Experiment 3 used concurrent-schedules, with a PR schedule on one alternative and fixed-ratio (FR) on the other. It was found that higher break points and response rates, as well as flatter demand functions were found for the same food in both Experiment 2 and 3. It was also found that when the concurrent alternative was on an FR 50 schedule, the response rates, break points and Pmax values of the food on the PR schedule tended to be higher than when the concurrent alternative was on an FR 20 schedule. Overall, the PS and MSWO assessments were equally effective at identifying which foods would function as reinforcers during Experiment 2 and 3