Small workers are more persistent fighters than soldiers in the highly polymorphic Atta leaf-cutting ants

Abstract

Social animals often coordinate behavioural activities for the gathering, maintenance and protection of resources. In eusocial insects, morphological and/or behavioural heterogeneity among worker caste allows division of labour and, in some instances, the specialization of individuals in specific tasks. In leaf-cutting ants (genus Atta ), the division of labour is based on body size differences (alloethism), meaning that differently sized workers will likely perform different tasks. Although alloethism is now well understood during foraging and processing of food resources in Atta , the relationship between body size and aggressive response has been poorly investigated, and, so far, it is not well understood how different morphological subcastes are integrated into the defensive strategy. Here, we quantified conspecifc and heterospecific aggression, in a within-colony social context (inside the nest), aiming to test which worker size class (subcaste) exhibits the most aggressive response against non-nestmates (NNMs). Workers were divided into three distinct subcastes, minors, majors and soldiers, and reared in subcolonies containing two individuals of each subcaste, as well as brood and the symbiotic fungus. We found a low probability of aggression towards conspecific NNMs and a high probability towards heterospecific NNMs. Quantification of aggression across subcastes showed that minors directed the most overt physical attacks towards heterospecific NNMs. We suggest that the nest, where minors perform most of their tasks, might constitute a strong social contextual stimulus for them, lowering their response threshold and increasing their aggression towards foreign ants. We also suggest that possible signals (i.e. chemical or mechanical) emitted during interactions involving minors stimulate larger workers to engage in fights, a mechanism that has been demonstrated in other ant species. The integration of size-specific behavioural traits might constitute a very efficient defensive mechanism. • Atta workers show the highest degree of morphological differentiation among ants. • However, the link between body size and aggression is little understood. • We measured aggression towards ant intruders in Atta workers of different size. • We found that the decision to attack an enemy is not related to body size. • Attack persistency, however, is far higher in small than in large workers.