Small differences in learning speed for different food qualities can drive efficient collective foraging in ant colonies

Abstract

Social insects frequently make important collective decisions, such as selecting the best food sources. Many collective decisions are achieved via communication, for example by differential recruitment depending on resource quality. However, even species which only rarely recruit can respond to a changing environment on a collective level by tracking food source quality. We hypothesised that an apparent collective decision to focus on the highest quality food source can be explained by differential learning of food qualities. Overall, ants may learn the location of higher quality food faster, with most ants finally congregating at the best food source. To test the effect of reward quality and motivation on learning in Lasius niger, we trained individual ants to find sucrose molarities of varying concentrations on one arm of a T-maze in spring and in autumn after 1 or 4 days of food deprivation. As predicted, ants learned fastest in spring and lowest in autumn, with reduced food deprivation leading to slower learning. Surprisingly, the effect of food quality and motivation on the learning speed was small. However, persistence rates varied dramatically: All ants in spring made all (6) return visits to all food qualities, in contrast to only 33% of ants in autumn after 1 day of food deprivation. Fitting the empirical findings into an agent-based model revealed that even a weak tendency of ants to memorise routes to high-quality food sources faster can result in ecologically sensible colony-level behaviour. This collective-seeming decision is non-interactive, and thus resembles an annealing process. Collective decisions of insects are often achieved via communication and/or other interactions between individuals. However, animals can also make collective decisions in the absence of communication. We show that foraging motivation and food quality can affect both route memory formation speed and the likelihood to return to the food source and thus mediate selective food exploitation. An agent-based model, implemented with our empirical findings, demonstrates that at the collective level even small differences in learning lead to ecologically sensible behaviour: mildly food-deprived colonies are selective for high-quality food while highly food-deprived colonies exploit all food sources equally. We therefore suggest that non-interactive factors such as individual learning and the foraging motivation of a colony can mediate or even drive group-level behaviour in a process resembling annealing. Instead of accounting collective behaviour exclusively to social interactions, a possible contribution of individual processes should also be considered.