Form, function, and evolution of worker-worker social carrying in ants

Social insects provide valuable insights into how coordinated group behavior emerges from simple individual rules. In ants, recruitment strategies are diverse, yet some remain poorly understood from a comparative evolutionary perspective. Social carrying, in which one individual physically transports another, is a widespread recruitment strategy associated with several functions, most commonly nest relocation. Despite extensive descriptive work, its evolutionary origins, variation, and ecological correlates remain unclear. We compiled literature data on worker-worker social carrying in ant species from seven subfamilies and integrated these data with phylogenetic analyses to address these questions. Using ancestral state estimation, we reconstructed the evolution of carrying posture and stereotypicality (the rigidity of the behavior). We also tested whether colony structure affects navigation ability after transport. Contrary to previous suggestions, we found that the ancestral form of social carrying was a stereotypical, coordinated interaction that later evolved into less stereotypical and sometimes aggressive forms. Carrying posture was largely subfamily specific. Social carrying occurred in species with colonies ranging from a few dozen to several million individuals, indicating that its use is not constrained by colony size. In addition, some polydomous species were able to navigate after being carried, whereas no monodomous species were. This study provides a broad evolutionary perspective on worker-worker social carrying and highlights new directions for research on recruitment in social insects.