A. The two neighbours never fought each other. B. Fights involving three male fiddler crabs have been recorded, but the status of the participants was unknown. C. They pushed or grappled only with the intruder. D. We recorded 17 cases in which a resident that was fighting an intruder was joined by an immediate neighbour, an ally. E. We therefore tracked 268 intruder males until we saw them fighting a resident male.