Information transfer at evening bat colonies

Four lines of evidence indicate that evening bats, Nycticeius humeralis, at nursery colonies in northern Missouri transfer information by following each other to feeding and roosting sites . (1) Daily estimates of insect density from five automated suction traps showed that common prey in evening bat faecal samples, small beetles and flies, occur in rich patches that persist for several days . Bats apparently respond to prey density and variability because these variables independently predict the number of trips and capture success of foraging bats . (2) Videotape records of the time and weight of bats arriving and departing from a colony indicated that adult females leave within 10 s of each other on second and subsequent foraging trips more often than expected within a night . These records also revealed that bats alternate between apparent following and leading over a summer, that unsuccessful foragers follow previously successful foragers within a night, and that the foraging success of putative followers is greater than that of unsuccessful bats which depart solitarily. (3) Radio-tagged bats often returned to foraging sites both within a night and on successive nights . Furthermore, three of 12 radio-tagged bats flew closer to another radio-tagged bat throughout a night than expected if bats foraged independently . (4) Two field experiments in which bats were excluded from their roosts for one night demonstrated that newly volant bats follow adult females to alternate roosts. The possibility that evening bats acquire information passively by monitoring echolocation signals or actively by vocal advertisement is discussed . Animals that live in groups often suffer higher rates of ectoparasite transmission (Hoogland 1979 ; Brown & Brown 1986) and reproductive parasitism (Brown 1984 ; Brown & Brown 1988) as well as more competition for food, mates or other critical resources than solitary individuals . Thus, in the absence of habitat limitation, group living must improve an individual's foraging efficiency, risk of predation, or care of young to offset these costs (Alexander 1974) . Transfer of information about the location or quality of food patches (Ward & Zahavi 1973) is one frequently suggested yet rarely demonstrated (Bayer 1982 ; Weatherhead 1987 ; Mock et al. 1988; Richner & Marclay 1991) advantage to forming a communal roost or colony . Although information transfer has been suggested for communally roosting bats (Fleming 1982), only anecdotal data have been reported (Howell 1979 ; McCracken & Bradbury 1981 ; Wilkinson 1985) . In this study I investigate whether a temperate insectivorous bat, the evening bat Nycticeius humeralis, locates resources, such as feeding and roosting sites, by following individuals from a nursery colony . Most studies purporting to demonstrate information transfer in birds have described coordinated departures from a roost or colony and subsequent arrival at a feeding site, i .e . weaver finches, Quelea quelea (Ward 1965), herons (Krebs 1974 ; Custer & Osborn 1978), terns (Erwin 1978 ; Waltz 1987), hooded crows, Corvus cornix, ravens, C. corax (Loman & Tamm 1980), black vultures, Coragyps atratus (Rabenold 1987), osprey, Pandion haliaetus, (Greene 1987) and cliff swallows, Hirundo pyrrhonota (Brown 1986, 1988a) . However, data which show that leaders alternate roles with followers are provided in only one of these studies (Brown 1986) . Documenting alternation between leading and following is necessary to discriminate between information parasitism and information exchange (Galef 1991) . This distinction is important because whenever more time or energy is required to locate resources independently than that required to wait and follow another forager . following will be more profitable than independent searching. Therefore, when resource patches are difficult to find, following behaviour provides net energetic benefits to participating group members 0003-3472/92/090501 4-18 $08 .00/0 © 1992 The Association for the Study of Animal Behaviour