ACOUSTIC ORIENTATION BY FISH-CATCHING BATS.

Captive Noctilio leporinus were unable to detect pieces of fish tissue or small air-filled balloons mounted a few millimeters below the surface of the water. An up-welling of water and small wires projecting vertically above the water surface were readily detected and dipped at, however. Single wires 0.21 mm in diameter extending 5 mm out of the water were regularly detected and 1 mm lengths of this same diameter wire were dipped at well above chance levels. One or 2 cm2 cubes of fish muscle mounted above the water surface were detected at average distances of 148 and 132 cm as judged by the distances at which pulse duration and interval, respectively, first began to decrease. The average distance of detection of 5 mm lengths of 0.21 mm diameter wire was 59 cm as estimated by decreases in pulse interval. Several bats learned to distinguish between a single wire 1.3 mm in diameter extending 5 mm above the surface and a pair of wires 0.9 mm in diameter extending 5 mm above the surface and spaced 2 cm apart. Experienced bats made about 80% of their dips at whichever of these two targets marked the position of the submerged food. A pair of identical wires sloping in opposite directions were not distinguished. Detection and discrimination are independent of vision and olfaction, being accomplished entirely by echolocation. Two well defined pulse types are emitted: an almost constant frequency one at 60 kc/s and one with an initial portion of constant frequency followed by a downward frequency sweep of about one octave. Entirely frequency followed by a downward frequency sweep of about one octave. Entirely frequency modulated pulses are emitted only during difficult maneuvers which require high pulse repetition rates, such as when catching food or landing. When bats flying low over the water approach each other on a collision course, the downward frequency sweep is lengthened to cover about tow octaves. This serves as a warning allowing the bats to swerve off their collision course.

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