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

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 leafcutting 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.(c) 2022 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Automated summary

The study found that minor leafcutting ants exhibit the most aggressive response against non-nestmate ants. The division of labor in leafcutting ants is based on body size differences, and the nest serves as a strong social contextual stimulus for minors, possibly leading to their increased aggression towards foreign ants.