Context-dependent expression of variation in defensive behaviour in the lower termite Zootermopsis nevadensis

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 variation in the frequencies of defensive behaviour is emerged. Here, we investigate the effects of the 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.