Comparative Echolocation by Fishing Bats

The acoustic orientation of two species of fish-catching bats was studied as they negotiated a row of strings or fine wires extending across their flight path. Orientation sounds of Pizonyx vivesi consisted of a steep descending FM sweep lasting about 3 msec. Noctilio leporinus used 8 to 10 msec pulses composed of an initial nearly constant frequency portion followed by a descending frequency modulation. The echolocation of small wires by N. leporimis differed from that of surface fish in that during wire avoidance no nearly constant frequency or entirely FM pulses were emitted, nor was the pulse duration markedly shortened as the barrier was approached. There was extensive temporal overlap at the animal's ear of returning echoes with the emitted cries when the bat was near the barrier—a strong contrast to the apparent careful minimization of such overlap during feeding maneuvers. Noctilio increased its average pulse duration about 2 msec when confronted with a barrier of 0.21 mm, as opposed to 0.51 mm, diameter wires. Pizonyx detected these wires well before pulse-echo overlap began, but at a shorter range than did N. leporinus, suggesting the latter species may have a longer effective range of echolocation.