Social Organization and Foraging in Emballonurid Bats: I. Field Studies

1. Five species of emballonurid bats (Rhynchonycteris naso, Saccopteryx leptura, Balantiopteryx plicata, Saccopteryx bilineata, and Peropteryx kappleri), were studied in Costa Rica and Trinidad. Stomach contents suggest that prey size generally increases for bat body size, but within these species there is considerable overlap. R. naso, S. leptura, and P. kappleri each appear to be specialized for foraging in a particular habitat type; B. plicata and S. bilineata are more opportunistic and feed over a variety of habitats during the year. While the other species feed in the proximity of surfaces, B. plicata is further separated from the other species by wing specializations favoring high altitude flight. 2. Foraging dispersion is more closely related to body size than it is to social structure at the roost: small bats group-forage while larger bats feed in solitary beats. In all of the species, food is spatially and temporally variable, and the location of foraging sites changes seasonally in accordance with these locally varying patterns of aerial insect abundance. In the case of S. bilineata, the locations of foraging sites were positively correlated with levels of phenological activity in the underlying plant communities. 3. Colony sizes ranged from small groups of 2-10 bats (S. leptura, P. kappleri), to intermediate colonies of 5-50 bats (R. naso, S. bilineata), to very large colonies with hundreds of bats (B. plicata). 4. R. naso, S. leptura, and S. bilineata colonies have colony-specific annual foraging ranges which are actively defended against conspecifics from other colonies. In most cases, all members of a given colony of one of these species will be found foraging in a common site at any time. In R. naso and S. bilineata, currently used foraging sites are partitioned socially. In the former species, adult breeding females occupy a central area and group-forage while younger non-breeding females and males occupy peripheral foraging areas in the colony territory. In S. bilineata, the colony foraging site is partitioned into individual harem territories defended by harem males and containing the individual beats of all current harem females. For this latter species, details of roost site subdivision are mapped directly onto foraging dispersions. In general, there is a close correlation between day-roost group membership and location of nocturnal foraging sites in all of the study species.