Anim. Behav., 1998, 55, 1717–1732 Greater spear-nosed bats discriminate group mates by vocalizations JANETTE WENRICK BOUGHMAN*² & GERALD S. WILKINSON* *Department of Z oology, University of M aryland, College Park ² Department of Z oological Research, National Z oological Park, Smithsonian Institution ( R eceived 20 M ay 1997; initial acceptance 18 A ugust 1997; final acceptance 23 October 1997; M S. number: 7930) Abstract. Individuals often benefit from identifying their prospective social partners. Some species that live in stable social groups discriminate between their group mates and others, basing this distinction on calls that differ among individuals. Vocalizations that differ between social groups are much less common, and few studies have demonstrated that animals use group-distinctive calls to identify group mates. Female greater spear-nosed bats, Phyllostomus hastatus, live in stable groups of unrelated bats and give audible frequency, broadband calls termed screech calls when departing from the roost and at foraging sites. Previous field observations suggested that bats give screech calls to coordinate movements among group members. Prior acoustic analyses of 12 acoustic variables found group differences but not individual differences. Here, we use the same acoustic variables to compare calls from three cave colonies, and find that calls differ between caves. We also report results from field and laboratory playback experiments designed to test whether bats use acoustic differences to discriminate calls from different colonies, groups or individuals. Results from field playbacks indicate that response depends on the cave of origin, indicating that bats can discriminate among calls from different caves. This discrimination ability may be based, in part, on whether calls are familiar or unfamiliar to the listening bats. Laboratory playbacks demonstrate that bats discriminate calls given by their group mates from calls given by other bats from the same cave irrespective of familiarity. However, these experiments provide no evidence that bats discriminate among individuals. Previous field work indicates that females that forage with social group mates may benefit from shared information about food or mutual defence of feeding sites. Indicating group membership is essential, since these benefits appear to be restricted to group mates.  1998 The Association for the Study of Animal Behaviour Animals that live in stable social groups may benefit by improving their access to food resources, through either group foraging or group defence of resources. We discuss two kinds of groups: long-term social groups that persist over much of an individual’s lifetime, and short-term foraging groups that persist over a single foraging episode. When patchy resources are ephemerally available, food-finding can be enhanced by forag- ing in a group (Krebs et al. 1972; Rabenold 1987; Brown 1988; Wilkinson 1992; Brown & Brown 1996). When food patches persist over time, resource defence becomes advantageous and may be achieved more effectively by a foraging group (Heinrich 1988). If group foraging is beneficial in either case, animals should actively form foraging groups. Foraging groups are likely to be com- posed of social group mates when animals live in stable social groups; therefore, selection may favour a mechanism to ensure association with particular individuals. Vocalizations are a poten- tially effective means of maintaining contact with group mates that travel long distances quickly, especially if they convey social group member- ship. Despite these potential benefits, few studies have demonstrated that animals use vocalizations to identify members of social groups when group mates are not relatives (Cheney & Seyfarth 1982). If calls are the mechanism that promotes for- mation and maintenance of foraging groups among social group mates, then calls must iden- tify social group mates in the foraging context. Both the production of distinctive signatures and their perception are essential components of a Correspondence: J. W. Boughman, Department of Zoology, University of Maryland, College Park, MD 20742, U.S.A. (email: boughman@umail.umd.edu). 0003–3472/98/061717+ 16 $25.00/0/ar970721  1998 The Association for the Study of Animal Behaviour 1717