Conflicts and Strategies in theHarem-Polygynous Mating System of the Sac-Winged Bat, Saccopteryx bilineata

Abstract Daytime roosts of the neotropical sac-winged bat Saccopteryx bilineata (Emballonuridae) include up to 60 individuals and are located in buttress cavities of trees or in well-illuminated sections of caves or abandoned houses. Colonies are structured into harem groups, each of which includes a single male and a varying number of females. In daytime roosts, sac-winged bats do not aggregate into clusters; instead individuals are spaced at least 10 cm apart. Agonistic interactions among colony members are frequent. Males in general almost never win encounters with females, and harem males in particular cannot prevent females from switching between harems. Paternity studies confirmed that harem males are more successful in fathering offspring than nonharem males. Although harem males do not monopolize reproduction within their harem, reproductive success of males increases with harem size. Removal experiments with harem males show that nonharem males are probably queuing for access to harem territories. The importance of joining a queue as soon as possible in a male’s life may have led to male philopatry in S. bilineata. We conclude that the mating pattern of S. bilineata does not resemble a classical harem system because female choice is probably more important than male competition. This has probably led to the evolution of elaborate acoustic and scent courtship displays by male sac-winged bats.