Social structure and nestmate discrimination in two species of Brachyponera ants distributed in Japan

The ponerine ant Brachyponera chinensis was introduced to the USA, where it has become invasive. Although various ecological data have been collected for B. chinensis populations in the USA, most aspects concerning the biology and ecology of native populations in Japan, a presumed origin, remain unknown. Here we investigated the social structure and nestmate discrimination in native populations of B. chinensis and a closely related species, B. nakasujii. Both species showed functional polygyny over seasons. Only in B. nakasujii was there a seasonal change in the numbers of queens and workers per nest. In arena tests, workers of neither species showed aggressive behaviors to conspecific non‐nestmates from the same population, and the mean aggression score did not increase with the distance between nests. However, some differences in non‐aggressive responses were detected between nestmate and non‐nestmate pairs in both species. In an experiment to introduce a single worker into a nest, B. chinensis accepted non‐nestmates with a high probability just like nestmates, whereas in B. nakasujii non‐nestmates were less accepted than nestmates. These findings suggest that native populations of B. chinensis already possess some of the key characteristics shared by many invasive exotic ants in introduced ranges, such as stable polygyny, weak internest aggression and acceptance of non‐nestmates. These tendencies are remarkable in comparison to the closely related B. nakasujii.