Benefits of aggregation in woodlice: a factor in the terrestrialization process?

In the animal kingdom, living in group is driven by a tradeoff between the costs and the benefits of this way of life. This review focuses especially on the benefits of aggregation and crowding in woodlice (Crustacea: Isopoda: Oniscidea). Indeed, woodlice are well known to live in groups. Their aggregation behavior, as described in the early works of Allee, is regarded as a mechanism to prevent desiccation to which woodlice are extremely sensitive. However, it is now clear that there are additional benefits to aggregation in woodlice. Hence, this review addresses not only the limitation of water loss as the main factor explaining aggregation patterns, but also alternative explanations as reduction of oxygen consumption, increase in body growth, biotic stimuli for reproduction, better access to mates, possible shared defenses against predators, promotion of coprophagy as a secondary food source, sheltering behavior and the acquisition of internal symbionts. In addition, we place woodlice in the context of a terrestrialization process and propose that woodlice-the only suborder of Crustacea almost entirely composed of strictly terrestrial species-are a model taxon for studying the evolution of sociality through the transition from water to land. Further, we discuss other ultimate causes of aggregation preserved in terrestrial isopods in light of those explained in aquatic isopods and under the concept of exaptation. This knowledge could help understand, in this and other taxa, how the spatial closeness between conspecifics may promote the colonization of new environments and nonphysiological responses to climatic constraints.