Mating success in group forming animals largely depend on the ability of same-sex competitors (usually males) to monopolize local resources that are spatially limited and are of importance to their sexual partners. Across taxa, this resource defence behaviour (RDB) is predicted to maximize male reproductive success. Although RDB is widely observed in polygynous societies, its significance as an alternative male mating strategy among randomly mating individuals (i.e., promiscuous) is a less explored topic in ethology, especially within Chiroptera. In this study, we elucidate the relationship between RDB (measured here as male tent defence behaviour) and the harem size associated with adult males of Cynopterus sphinx. From 12 independent resident male removal experiments, we found that inter-male aggressive combats often exclusively occur while defending foliage tents in mating seasons. Further, our harem census data of usurper males (during usurpation) and resident males (post release), suggests a significant bias among females to preferentially roost with the resident males. This preferential association between harem females and the resident males that almost always recovers the foliage tent from the usurpers, suggests that C. sphinx harem males use RDB as an alternative mating strategy to monopolize roosting resources and maintain large harems. However, further research is required to identify the direct effects of RDB on individual male reproductive success.