Function of pipistrelle social calls: field data and a playback experiment

The study aimed to determine whether the so-called social calls that are produced by foraging bats of two phonic types of Pipistrellus pipistrellus serve a social function. First, the relationship between insect availability and the rate of production of social calls was measured at a foraging site of both phonic types. Second, playback experiments of social calls of the two phonic types were conducted in the field to determine the response of foraging bats to these calls, and to determine whether the calls are used in communication within or between phonic types. Two hypotheses are suggested for the function of social calls: that they may be used either to attract other bats to a food patch, or in agonistic interactions between bats in defence of a food patch. At relatively low insect densities, the rate of social call production of both phonic types increased significantly as insect density decreased. When social calls of each phonic type were broadcast, there was a significant reduction in bat activity of the same phonic type. In contrast, playbacks of social calls resulted in no change in activity of the other phonic type. The results supported the food patch defence hypothesis, that social calls are used to warn off other bats of the same phonic type when insects are scarce. The results also supported the hypothesis that the two phonic types are sibling species. Social calls were shown to serve a social function in intraspecific communication, but there was no communication between phonic types. ? 1997 The Association for the Study of Animal Behaviour Feeding bats may communicate by eavesdropping on the echolocation calls of other bats (see review in Fenton 1995) or by means of specific social calls (Fenton 1985). The precise functions of social calls are rarely known. They are often ascribed an agonistic function, implying territoriality, but this is suggested from casual observations and there is little supporting evidence (Fenton 1985). Pipistrellus pipistrellus produces intense so-called social calls during flight near the roost or at foraging sites (Ahlén 1981; Miller & Degn 1981). Social calls are often produced during chases (Miller & Degn 1981; Racey & Swift 1985; Lundberg & Gerell 1986) and most chases are observed at low insect densities (Racey & Swift 1985). It is not known, however, whether the so-called social calls do have a social function. There are two phonic types of P. pipistrellus in Britain, its echolocation calls falling into two distinct frequency bands (Jones & van Parijs 1993). Echolocation calls have a predominant Correspondence: K. E. Barlow, School of Biological Sciences, Woodland Road, Bristol, BS8 1UG, U.K. (email: [email protected]). 0003–3472/97/050991+09 $25.00/0/ar960398 ? 1 99 energy around 55 kHz in one phonic type, and around 45 kHz in the other. The two phonic types occur in sympatry over much of Britain and are probably sibling species (Jones & van Parijs 1993; Barratt et al. 1995). The social calls of the two phonic types of P. pipistrellus differ (Fig. 1); calls of the 45 kHz phonic type consist of more components and are of longer duration and lower frequency than those of the 55 kHz phonic type (Barlow & Jones 1997). We suggest two possible functions of social calls. First, social calls may be produced to advertise food patches to other bats, if there are benefits of foraging in groups. Some bats eavesdrop on the echolocation calls of other individuals to locate feeding areas or potential prey items (Barclay 1982; Balcombe & Fenton 1988; Fenton 1995). Similarly, social calls may be produced specifically to communicate information about food patches to other bats. The costs of advertising a food patch may be small if bats feed on swarms of insects and cannot monopolize the whole swarm (Barclay 1982). The energetic cost of producing social calls, and the cost of attracting 997 The Association for the Study of Animal Behaviour