Relative value perception in an insect: positive and negative incentive contrasts in ants

When choosing between two options, a sensible strategy is to choose the highest value option. To do this, both options must be evaluated and compared. The way value is judged and utility is perceived therefore has strong effects on which option is chosen. Traditionally, value was considered absolute. However, research on human decision-making suggests that, for us, utility is relative and based on past experiences or expectations. The study of successive contrast effects suggests that the same might be true for animals. Here we show that ants which had previously experienced a low quality food source showed higher acceptance of medium quality food (e.g. 0.1M then 0.5M; positive contrast) than if they had received the medium food all along (e.g. 0.5M then 0.5M; control). Ants also showed lower acceptance of medium food when previously offered high quality food (e.g. 2M then 0.5M; negative contrast). Further experiments demonstrate that these contrast effects arise from psychological, not physiological or psychophysical, causes. Pheromone deposition also correlates with perceived reward value, and ants also showed successive contrasts in their pheromone deposition. Contrast effects occurred not only when ants collected private information outside the nest, but also when information was received through trophallactic interactions in the nest. Relative value perception can therefore be expected to have strong effects not only on individual behaviour, but also on collective decision-making.