Observation of plugging behaviour reveals entrance-guarding schedule of morphologically specialized caste in Colobopsis nipponicus

The success of social insects is often attributed to the specialized morphologies and behaviours of workers. One of the most elaborate specializations in ant species is the workers’ plugging behaviour, in which their uniquely shaped heads are used for nest defence. These species generally nest in tree cavities. Without morphologically specialized workers (major workers) plugging the nest entrance, nests are easily attacked and replaced by conspecific and heterospecific cavity-dwelling ants and other predators. Therefore, major workers are expected to defend the nest at all times. However, empirical evidence on the social time investment for nest defence is still lacking. Here, we investigated how major workers of Colobopsis nipponicus achieve nest defence (plugging behaviour) in terms of their work schedule. Our results showed that the nests had more major workers than entrances. The observation of artificial plastic nests over 6 days in the laboratory revealed that the nest entrance was guarded almost continuously by multiple major workers. The entrance-guarding schedule was not equally divided among major workers, and there were no clear shift patterns in the timing of changeovers. We highlight the importance of investigating the time investment of workers to fully understand the defence strategies of this morphologically specialized caste.