THE SENSITIVITY OF ECHOLOCATION IN BATS

1. The distance at which bats (Myotis lucifugus) react to the presence of a row of small wires has been measured by a photographic determination of the distance at which the pulse repetition rate first increases as the bats fly towards the wires. Distinct changes in this rate were measured in almost every flight towards wires spaced 30 cm. apart and ranging in diameter from 0.18 to 3 mm. 2. The interval between successive pulses averaged 60 to 80 msec as the bats flew along the room towards the row of wires, and dropped to 20-40 msec just before the barrier. The intervals decreased less with the smaller sizes of wire. 3. All but the largest of these wires are well below one wave-length of the emitted sounds of these bats (50-60 kc, or 6-7 mm., at the peak intensity and 120 kc, or about 3 mm., at the very beginning of some pulses). 4. Clear evidence that the wires had been detected was furnished at the point where the interval between pulses first dropped significantly below the level that prevailed before and after the approach to the row of wires. This average distance of first vocal reaction to the row of wires was 215 cm. for 3-mm. wire, 185 cm. for 1.07-mm., 150 cm. for 0.65-mm., 120 for 0.54-mm., 105 cm. for 0.28-mm., and 90 cm. for 0.18-mm. A conservative correction for reaction time and the acoustic delay between the bat and the microphone indicates that the distance of first detection must have been at least 10 cm. greater than these distances of reaction. 5. Since small wires can be detected at distances of as much as 5500 times the wire diameter, and well before the bat gives evidence by its flight pattern that it is aware of them, it appears likely that larger objects are detected at considerably greater distances.