Colony fusion in a termite: What makes the society open?

Sociobiological theories assume that insect societies are closed, i.e., members always act aggressively toward alien conspecific individuals. However, introduction of non-nestmate workers into a host colony of a lower termite, Reticulitermes speratus revealed various levels of agonism: from genocide to colony fusion. Members of the host colony attacked and killed the intruders having a higher nymph ratio (the number of nymphs/the number of workers) than the host colony had. In contrast, the intruders were accepted if they had a lower nymph ratio than the host colony. Acceptance of a lower-nymph-ratio colony could be beneficial for the host colony because it would gain additional labor force to feed its own nymphs. However, acceptance of a higher-nymph-ratio colony might be disadvantageous because the workers must rear non-relative nymphs additionally. Here we show that termites facultatively change their agonistic response toward non-nestmates according to the cost and benefit of colony fusion. Contrary to the existent concept, we report for the first time that colony fusion is an adaptive tactic in social insects.