Frank Azorsa, Mario L Muscedere, James F A Traniello
Age-related behaviour is fundamental to the organization of insect societies, but its neurobiological evolution is not well understood. Contrasting sharply with most species in which workers do not forage immediately after adult eclosion, in the 'social fossil' ant Stigmatomma pallipes newly eclosed workers hunt for and directly provision larvae with centipedes, precociously linking foraging and nursing. We analysed the adaptive development of brain size and structure that facilitates rapid behavioural maturation in S. pallipes. Our results demonstrate that the worker brain and most of its functionally specialized compartments attain maximum size in pupae, unlike the neuroanatomical growth and remodelling underlying adult maturation in derived ant species having altricial workers. At the cellular level, we found less dense microglomeruli in the mushroom body of mature workers, reflecting synaptic remodelling that appears conserved across ants. A phylogenetic analysis of 32 ant species supported a positive relationship between development time and precociality, as described in birds and mammals. Our findings indicate life history, dietary specialization and task coupling are selective factors shaping brain architecture and the adaptive patterns of age-related behaviour that are a hallmark of the ants' evolutionary and ecological success.