The use of other individual's echolocation calls by little brown bats, Myotis lucifugus, was tested by observing the response of fi'ee-flying bats to presentations of recorded echolocation calls and artificial sounds. Bats responded by approaching conspecific calls while searching for food, night roosts, nursery colonies and mating/hibernation sites. Response was low or non-existant to other sounds. While searching for prey, M. lucifugus also responded to the echolocation calls of Eptesicus fuscus, a sympatric species with overlapping diet but distinctly different echolocation calls. Subadults were especially responsive to conspecific calls. All four situations in which the bats responded involve patchily distributed resources at which bats accumulate. Concentrations of echolocation calls thus likely serve as cues regarding the location of resources. Individuals approaching feeding groups, for example, could increase prey detection range by up to 50 times over individuals relying solely on their own echolocation. Although the costs associated with eavesdropping may be negligible for M. lueifugus, for other species, particularly territorial ones, being conspicuous may be a disadvantage and the possibility of being overheard by other bats may have been one factor involved in the evolution of echolocation call design.
I tested the interindividual use of echolocation calls using playbacks to free-flying little brown bats, Myotis lueifugus, as they were searching for food, night roosts, nursery colonies and mating/hibernation sites. Observations were made during August and September 1977 and 1979 and from April through October 1980. Three sites near the Queen's University Biological Station, Chaffey's Locks, Ontario, were used : a nursery colony of approximately 1,000 female M. lueifugus and their young located in the attic of an abandoned farmhouse, a nearby woodlot where these bats fed, and a night roost (Anthony et al. 1981) located in a barn. Late summer and fall playbacks were performed at an abandoned mine near Renfrew, Ontario, used by M. lucifugus as a mating and hibernation site.
Echolocation calls were recorded on a Racal (Lockheed) Store 4D tape recorder at 76 cm/s using a broadband microphone (frequency response of the system, 15-150 kHz) (Simmons et al. 1979). Approximately 25 M. lucifugus were recorded while they fed together in a small area (feeding tape); non-feeding M. lueifugus were recorded as they swarmed (Fenton 1969) outside the entrance to the mine (mine tape); and feeding big brown bats, Eptesicus fuscus, were recorded by M.B. Fenton near Milbrook, New York. Other playback sounds were produced from an Exact 126 VCF sweep generator. These included 4 ms signals sweeping from 100 to 40 kHz, designed to imitate the duration and frequency typical of M. lucifugus echolocation calls (Griffin 1958; Fenton and Bell 1979). Playbacks were performed using the prerecorded tapes and the Racal tape recorder or the signal generator. The signal to be presented was amplified through a Dynaco preamplifier and an ultrasonic power amplifier (Simmons et al. 1979). The signal was monitored using a Tektronix 212 oscilloscope and adjusted to 105 dB (SPL re 20 gPa at 10 cm) when played through an 8.5 cm diameter electrostatic speaker.
Playback trials consisted of 2 or 5 min silent controls and test periods. All trials were presented in a double blind manner since the observer scoring the response was not aware whether a stimulus or control was being presented. Stimuli to be compared were presented on the same night to eliminate nightly or seasonal variation in response levels. The response of free-flying bats was observed in most situations in existing light and by backlighting the bats against the sky. At the nursery colony and night roost, bats were also observed using a GBC low light level TV camera with a GE Red Ruby light bulb (wavelength 580-740 nm).
Response criteria were established prior to the playbacks and varied from situation to situation due to differences in visibility. Normally only a small area (diameter about 4 m) around the speaker could be observed and all bats passing through that area were counted and the numbers during controls and tests compared. During feeding area playbacks, however, existing light levels allowed all the bats passing within 10 m of the speaker to be observed. In this situation, I considered responding bats to be those that altered their flight path and approached to within 2 m of the speaker. Thus, the percentage of bats responding at the feeding site could be calculated.