Echolocation and foraging behavior of the lesser bulldog bat, Noctilio albiventris: preadaptations for piscivory?

We studied variability in foraging behavior of Noctilio albiventris (Chiroptera: Noctilionidae) in Costa Rica and Panama and related it to properties of its echolocation behavior. N. albiventris searches for prey in high (> 20 cm) or low (< 20 cm) search flight, mostly over water. It captures insects in mid-air (aerial captures) and from the water surface (pointed dip). We once observed an individual dragging its feet through the water (directed random rake). In search flight, N. albiventris emits groups of echolocation signals (duration 10-11 ms) containing mixed signals with constant-frequency (CF) and frequency-modulated (FM) components, or pure CF signals. Sometimes, mostly over land, it produces long FM signals (duration 15-21 ms). When N. albiventris approaches prey in a pointed dip or in aerial captures, pulse duration and pulse interval are reduced, the CF component is eliminated, and a terminal phase with short FM signals (duration 2 ms) at high repetition rates (150-170 Hz) is emitted. Except for the last pulses in the terminal phase N. albiventris avoids overlap between emitted signals and echoes returning from prey. During rakes, echolocation behavior is similar to that in high search flight. We compare N. albiventris with its larger congener, N. leporinus, and discuss behavioral and morphological specializations that can be interpreted as preadaptations favoring the evolution of piscivory as seen in N. leporinus. Prominent among these specializations are the CF components of the echolocation signals which allow detection and evaluation of fluttering prey amidst clutter-echoes, high variability in foraging strategy and the associated echolocation behavior, as well as morphological specializations such as enlarged feet for capturing prey from the water surface.