FISHING AND ECHOLOCATION BEHAVIOR OF THE GREATER BULLDOG BAT, NOCTILIO-LEPORINUS, IN THE FIELD

When hunting for fish Noctilio leporinus uses several strategies. In h igh search flight it flies within 20-50 cm of the water surface and em its groups of two to four echolocation signals, always containing at l east one pure constant frequency (CF) pulse and one mixed CF-FM pulse consisting of a CF component which is followed by a frequency-modulate d (FM) component. The pure CF signals are the longest, with an average duration of 13.3 ms and a maximum of 17 ms. The CF component of the C F-FM signals averages 8.9 ms, the FM sweeps 3.9 ms. The CF components have frequencies of 52.8-56.2 kHz and the FM components have an averag e bandwidth of 25.9 kHz. A bat in high search flight reacts to jumping fish with ''pointed dips'' at the spot where a fish has broken the su rface. As it descends to the water surface the bat shows the typical a pproach pattern of all bats with decreasing pulse duration and pulse i nterval. A jumping fish reveals itself by a typical pattern of tempora ry echo glints, reflected back to the bat from its body and from the w ater disturbance. In low search flight N. leporinus drops to a height of only 4-10 cm, with body parallel to the water, legs extended straig ht back and turned slightly downward, and feet cocked somewhat above t he line of the legs and poised within 2-4 cm of the water surface. In this situation N. leporinus emits long series of short CF-FM pulses wi th an average duration of 5.6 ms (CF 3.1 and FM 2.6) and an average pu lse interval of 20 ms, indicating that it is looking for targets withi n a short range. N. leporinus also makes pointed dips during low searc h flight by rapidly snapping the feet into the water at the spot where it has localized a jumping fish or disturbance. In the random rake mo de, N. leporinus drops to the water surface, lowers its feet and drags its claws through the water in relatively straight lines for up to 10 m. The echolocation behavior is similar to that of high search flight . This indicates that in this hunting mode N. leporinus is not pursuin g specific targets, and that raking is a random or statistical search for surface fishes. When raking, the bat uses two strategies. In direc ted random rake it rakes through patches of water where fish jumping a ctivity is high. Our interpretation is that the bat detects this activ ity by echolocation but prefers not to concentrate on a single jumping fish. In the absence of jumping fish, after flying for several minute s without any dips, N. leporinus starts to make very long rakes in are as where it has hunted successfully before (memory-directed random rak e). Hunting bats caught a fish approximately once in every 50-200 pass es through the hunting area.