Eusocial animals exhibit a sophisticated division of labour. The self-organized colony requires coordination of local interactions at the individual level. Termites are a well-known group exhibiting social organization among castes within a colony, and helper castes have facilitated their ecological and evolutionary success. Termites have soldier castes specialized in defence, but other castes are also known to exhibit defensive behaviour depending on the context. However, it is unclear how the variation in the frequencies of defensive behaviour emerged. Here, we investigate the effects of presence of nestmates on individual defensive behaviour in the dampwood termite Zootermopsis nevadensis. We experimentally prepared three conditions and observed biting behaviour against the ant Camponotus obscuripes. First, the frequency of biting was significantly higher in soldiers than that in pseudergates (i.e., workers) under isolated conditions. Second, the results showed that the frequencies of biting exhibited by soldiers were much higher than those of pseudergates in the pseudergate–soldier pairs. Finally, we investigated the social conditions in the presence of the same castes, that is, pseudergate–pseudergate and soldier–soldier pairs. The frequencies of biting exhibited by first-biting individuals were significantly higher than those exhibited by second-biting individuals in both paired conditions. Our findings demonstrate that defensive behaviour can be frequently observed in first-biting individuals despite the presence of two individuals of the same caste, suggesting that the variation of defensive behaviour may be increased in the presence of nestmates.

Caste polyphenism in social insects is regulated by social interactions among colony members. Trophallaxis is one of the most frequently observed interactions, but no studies have been conducted identifying the intrinsic factors involved in this behaviour and caste differentiation. Dopamine (DA) has multiple roles in the modulation of behaviours and physiology, and it produces species-specific behaviours in animals. Here, to verify the role of DA in termite soldier differentiation, we focused on the first soldier in an incipient colony of Zootermopsis nevadensis, which always differentiates from the oldest 3rd instar (No. 1 larva) via a presoldier. First, brain DA levels of the No. 1 larva at day 3 after its appearance were significantly higher than day 0. Second, DA synthesis gene expression levels were extraordinarily high in the No. 1 larva at day 0–1 after appearance. Finally, injection of a DA receptor antagonist into the No. 1 larva resulted in the inhibition of presoldier differentiation. Behavioural observations of the antagonist or control-injected larvae suggested that brain DA and signalling activity regulate the frequencies of trophallaxis from reproductives and presoldier differentiation. Because trophallaxis is a social behaviour frequently observed in natural conditions, the role of DA should be investigated in other social insects with frequent trophallactic and allogrooming behaviour.