Vespertilionid bats control the width of their biosonar sound beam dynamically during prey pursuit

Abstract

Animals using sound for communication emit directional signals, focusing most acoustic energy in one direction. Echolocating bats are listening for soft echoes from insects. Therefore, a directional biosonar sound beam greatly increases detection probability in the forward direction and decreases off-axis echoes. However, high directionality has context-specific disadvantages: at close range the detection space will be vastly reduced, making a broad beam favorable. Hence, a flexible system would be very advantageous. We investigated whether bats can dynamically change directionality of their biosonar during aerial pursuit of insects. We trained five Myotis daubentonii and one Eptesicus serotinus to capture tethered mealworms and recorded their echolocation signals with a multimicrophone array. The results show that the bats broaden the echolocation beam drastically in the terminal phase of prey pursuit. M. daubentonii increased the half-amplitude angle from approximately 40°to approximately 90°horizontally and from approximately 45°to more than 90°vertically. The increase in beam width is achieved by lowering the frequency by roughly one octave from approximately 55 kHz to approximately 27.5 kHz. The E. serotinus showed beam broadening remarkably similar to that of M. daubentonii. Our results demonstrate dynamic control of beam width in both species. Hence, we propose directionality as an explanation for the frequency decrease observed in the buzz of aerial hawking vespertilionid bats. We predict that future studies will reveal dynamic control of beam width in a broad range of acoustically communicating animals.A coustic communication plays a major role for conspecific and predator/prey interactions in many animals. Features of emitted sounds, such as time-frequency structure, intensity, and directionality, are central for communication range and direction. Flexibility in acoustic behavior allows for adaptive changes in sound signals to the constraints of a variety of contexts and purposes (1, 2). Directionality defines the angle of attention and is consequently a spatial filter for communication. Thus, directionality is as significant as other acoustic features, but as a result of the methodological challenge in measuring it, directionality has only very rarely been studied in the field (3-6), and almost nothing is known about possible dynamic changes in directionality in response to behavioral tasks. Bats are ideal animals in which to study dynamic changes in directionality, as recording their highintensity echolocation calls allow us to infer, from the bat's adaptive vocal changes to the changing context, which acoustic elements are important for perception through sound.Echolocating bats can hunt and navigate without light, emitting short high-frequency sound pulses to determine the direction, distance, and features of objects in the environment from binaural cues, arrival time, amplitude, and spectrum of sonar reflections (7,8). Bats modify echolocation call parameters such as duration, repetition rate, and...

Echolocating bats use future-target information for optimal foraging

“…We examined which parameter set had a high probability of consecutively capturing two prey items. The sonar beam of the bat was modeled as a circular piston oscillating in an infinite baffle (23,24) (Fig. S1).…”

How the bat got its buzz

“…Beam broadening has been demonstrated in the vespertilionids, Myotis daubentonii and Eptesicus serotinus (figure 2; [5]). In close quarters, even a small positional change on the part of the insect could put it outside the bat's acoustic field of view; hence, sonar beam broadening should improve bats' ability to track evasive prey as target distance decreases [5]. A drop in F 0 and, presumably, a broad acoustic field of view during the buzz describe most species in Vespertilionidae and Molossidae [3,5,8], and the emballonurid, Rynchonycteris naso (figure 1).…”

“…Despite huge variation in echolocation call design (e.g. peak frequency varies from 9 to 212 kHz) between and within families of echolocating bats [1,4], the way call rate increases over the course of the aerial pursuit of prey appears highly conserved [4][5][6]. Laryngeal specializations that allow for production of calls of high fundamental frequencies (F 0 ) and fast, smooth frequency modulation also appear similar across species [6,7].…”

“…Attacks, successful or not, almost always end in a 'terminal buzz', where bats produce calls at rates above 100 calls s 21 and, in some species, up to and beyond 200 calls s 21 (figures 1 and 2) [3,[6][7][8][9]. While the apparent function of the buzz is to quickly update the relative position of moving targets [4][5][6], how bats produce calls at such rates was until recently unknown. We now know that rare superfast muscles power the buzz [6].…”

“…For laryngeal echolocating bats, each call emitted is under active neuromuscular control [6,7]; we therefore assume that all bats that buzz possess superfast laryngeal muscles. During the buzz, some bats (figures 1 and 2) produce a distinct second phase, buzz II, characterized by up to a one-octave drop in the F 0 of their calls [3,5,10]. This drop in frequency has been argued to result from a physiological constraint on producing very short calls quickly (see Schmieder et al [10] for review).…”