Context-dependent specialization in colony defence in the red wood ant Formica rufa

In many species, specialized defence traits and strategies are crucial for surviving enemy attacks or securing resources. In numerous social insect lineages, a morphologically and behaviourally distinct soldier caste specializes in colony defence, with larger foragers typically engaging most in the aggressive defence of the colony against external threats. We hypothesized, however, that specialization in aggression could show vastly different patterns in the context of the defence against small intranidal parasites that prey on brood. This is because we expected that small, intranidal nurse workers could be better suited to defend against these parasitic myrmecophiles (=ant associates) due to their better matching size, high encounter rate and the high task switching costs that would occur if foragers had to carry out this task. Here, we present data that support this hypothesis from a study on specialization in defence against two parasites in the red wood ant Formica rufa. In particular, we show that small workers displayed the strongest aggressive behaviour towards the parasitic rove beetle Thiasophila angulata and the spider Thyreosthenius biovatus, and present evidence that small workers were better at preventing brood predation than larger workers. In addition, there was worker task specialization in defensive behaviour, with nurses and workers at nest entrances being more aggressive towards T. angulata than extranidal foragers. We argue that this context-dependent specialization in aggression and nest defence was likely to be important in favouring the pronounced worker polymorphism observed in both this and other ant species and discuss our findings in relation to models for the evolution of division of labour and caste polymorphism in insect societies. (C) 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.