Specialization and group size: brain and behavioural correlates of colony size in ants lacking morphological castes

Abstract

Group size in both multicellular organisms and animal societies can correlate with the degree of division of labour. For ants, the task specialization hypothesis (TSH) proposes that increased behavioural specialization enabled by larger group size corresponds to anatomical specialization of worker brains. Alternatively, the social brain hypothesis proposes that increased levels of social stimuli in larger colonies lead to enlarged brain regions in all workers, regardless of their task specialization. We tested these hypotheses in acacia ants (Pseudomyrmex spinicola), which exhibit behavioural but not morphological task specialization. In wild colonies, we marked, followed and tested ant workers involved in foraging tasks on the leaves (leaf-ants) and in defensive tasks on the host tree trunk (trunk-ants). Task specialization increased with colony size, especially in defensive tasks. The relationship between colony size and brain region volume was task-dependent, supporting the TSH. Specifically, as colony size increased, the relative size of regions within the mushroom bodies of the brain decreased in trunk-ants but increased in leaf-ants; those regions play important roles in learning and memory. Our findings suggest that workers specialized in defence may have reduced learning abilities relative to leaf-ants; these inferences remain to be tested. In societies with monomorphic workers, brain polymorphism enhanced by group size could be a mechanism by which division of labour is achieved.

Group-wise 3D registration based templates to study the evolution of ant worker neuroanatomy

“…Generating brain templates will accelerate data collection and greatly expand research opportunities in the study of the evolutionary neurobiology of ants and other social insects.Ants (Hymenoptera: Formicidae), renowned for their remarkable diversity and ecological significance [1], typically display extraordinary collective behavior [2]. A key question in evolutionary neurobiology concerns how ant sociality, ecology, and the ability to make accurate group decisions have impacted their brain structure The emergence of eusociality and social complexity are major novelties likely involving rapid behavioral changes that might be reflected in the anatomy of the brain [3,4], although this idea has been controversial [5,6]. The remarkable evolutionary and ecological success of ants is hypothesized to be due to their social organization, which features division of labor, and collective behavior [7].Workers in ant colonies are so intrinsically interdependent that they are considered superorganisms.…”

Social Complexity and Brain Evolution: Comparative Analysis of Modularity and Integration in Ant Brain Organization

“…The varied cognitive challenges associated with group living in insects may lead to complex relationships between social organization and neural adaptation [Lihoreau et al, 2012]. Patterns of brain scaling in eusocial insects therefore may be driven in opposing directions by distinctive aspects of insect social organization [Gronenberg and Riveros, 2009;Riveros et al, 2012;Amador-Vargas et al, 2015;Feinerman and Traniello, 2015;O'Donnell et al, 2015;Kamhi et al, 2016].…”

Cumulative Effects of Foraging Behavior and Social Dominance on Brain Development in a Facultatively Social Bee <b><i>(Ceratina australensis)</i></b>

“…Among social insects, individual differences in brain investment patterns are associated with both complex task performance (e.g. foraging behavior) and social dominance [Farris et al, 2001;O'Donnell, 2007, 2008;Fahrbach and Dobrin, 2009;Riveros and Gronenberg, 2010;Smith et al, 2010;Amador-Vargas et al, 2015]. Whether and how these environmental factors interact in affecting brain development is not known.…”