Hidden aggression in termite workers: plastic defensive behaviour dependent upon social context

Social insect colonies are self-organized systems that respond to changes in environmental conditions by altering the relative proportions of certain castes or individuals engaged in specific tasks. While termites are known to regulate the ratio of morphologically specialized soldiers in response to the proportion of nestmate castes, the soldier differentiation process requires a relatively long time to be accomplished. Consequently, termites are thought to employ other plastic and flexible mechanisms for colony defence. We quantified the defensive behaviours of soldiers, pseudergates (workers) and reproductives (neotenics) of the damp-wood termite Hodotermopsis sjostedti. When individuals of the caste were exposed to an intruder, soldiers exhibited the most vigorous defensive behaviour, followed by pseudergates and then reproductives. While the aggression levels of soldiers and reproductives were independent of the accompanying castes, pseudergate aggression was more plastic and depended on the other castes present. When paired with reproductives, pseudergates exhibited high levels of aggression towards enemies. However, pseudergate aggression levels remained low when they were paired with soldiers, suggesting that pseudergates moderated their defensive behaviour depending on social context. Plasticity with respect to social behaviours may facilitate rapid and flexible responses required for colony defence.