Social structure of a polygynous tent-making bat, Cynopterus sphinx (Megachiroptera)

The social structure of an Old World tent-making bat Cynopterus sphinx (Meg achiroptera), was investigated in western India. A combination of census an d mark-recapture data over 2 years (1996-98) was used to infer the form of the mating system, compositional stability of social groups and mode of new social group formation. The breeding population of C. sphinx was subdivide d into diurnal roosting colonies, each of which contained one to five discr ete roosting groups and often one or more solitary bats in adjacent roosts. Bats most frequently roosted in stem tents constructed in the flower/fruit clusters of the kitul palm Caryota urens. Temporal variation in social str ucture was assessed using visual census data for a subset of the study popu lation over 3 years (1995-98) spanning six consecutive reproductive periods . The sex and age composition of diurnal roosting groups indicated a polygy nous harem-forming mode of social organization, as groups invariably contai ned a single adult male, 1-37 reproductive females and their dependent youn g (n = 33 harems). Harem size averaged 6.1 adults in the wet season (n = 19 , SD = 3.5) and 13.6 adults in the dry season (n = 14, SD = 8.5). The same harem social configuration was maintained year-round, despite a high degree of synchrony and seasonality in the timing of reproduction. Juveniles of b oth sexes dispersed after weaning and sexually immature bats were never pre sent in harems at the time of parturition. Adult females often remained ass ociated as roostmates from one parturition period to the next, and group co hesion was unaffected by turnover of harem males. Adult females frequently transferred among roosts within the same colony, and harems underwent perio dic fissions and fusions. The founding of new harems most often resulted fr om the fissioning of previously cohesive harems within the same colony. How ever, some harems contained disproportionate numbers of yearling females, i ndicating that new groups are also founded by nulliparous females of the sa me age cohort. A significant degree of heterogeneity in age composition amo ng harems was revealed in the 1998 dry season, but was unrelated to age-str atification of tent roosts. Although formation of new harems may be non-ran dom with respect to age composition of the founders, founding events are no t restricted to newly created tents and often involve recolonization of pre viously occupied roosts.