Exploration costs promote conservative collective foraging in the social caterpillar Malacosoma disstria

Abstract

Collective choices of a food source are generally thought to arise from the amplification of individual preferences. However, costs associated with foraging can also influence food choice: animals faced with a poor-quality food source may either continue feeding on it (a conservative strategy), or search for a potentially more profitable one (a more risky strategy). Groups of nomadic, trail-following Malacosoma disstria caterpillars were offered a leaf of two different hosts, either side by side to determine preference, or at a distance to investigate how exploration costs affect food choice. Arenas with and without pheromone trails were used to distinguish between direct travel costs and unknown territory costs. When two leaves were side by side, groups of second-instar caterpillars showed clear preferences between host trees, whereas groups of fourth-instar larvae did not. When the two leaves were distant from each other, groups of second-instar caterpillars usually remained on the known source, irrespective of food preference or group size. These groups were more likely to relocate to a preferred food source when pheromone trails were present, suggesting that trails effectively decrease unknown territory costs. Groups of fourth-instar caterpillars were more likely to leave the known food source even when no trail was present, suggesting a developmental shift in costs associated with exploration. Our results show that collective choices reflect food preference when exploration costs are low (second-instar larvae with pheromone trails), but not when they are high (second-instar larvae without trails). Such a conservative foraging strategy can be adaptive if the exploration costs of leaving a known food source are greater than the potential benefits of discovering a better alternative, for example, in a homogenous environment like that of a tree.