NAG/DAGA 2009 - Rotterdam On two possible mechanisms for call directionality steering in the rufous horseshoe bat, Rhinolophus rouxi F. De Mey1;2, D. Vanderelst1, H. Peremans1 1 Departement MTT, Universiteit Antwerpen, Prinsstraat 13, 2000 Antwerp, Belgium. 2 Email: [email protected] a) b) fashion. In this paper, we propose two other possibilities to introduce monaural directionality differences. As the echolocation system's directionality results of a com- kHz bination from the hearing and sending directionality, 77 affecting the call pattern could be as useful to a bat 60 as affecting the hearing directionality. Hence, we put c) 050ms forward two hypotheses explaining possible mechanisms for call directionality steering: a noseleaf movement / Figure 1: a) Snout of the Rhinolophus rouxi (as used in the deformation hypothesis, and a phase-change hypothesis. simulation) containing the noseleaf; b) 3D computer model of the Rhinolophus rouxi's head with an indication of the part The first hypothesis could be stated as follows: used in the simulation; c) Typical call of the Rhinolophus Bats are able to contract and relax the muscles rouxi (Schematic spectrogram). around the noseleaf. These contractions and relaxations result in noseleaf deformations or Introduction movement of the noseleaf, as well as in a nostril The Rhinolophus rouxi, or rufous horseshoe bat, is a shape alteration. Modifying the noseleaf shape member of the family of Rhinolophidae, the Horseshoe would result in changing the call directionality bats. This family of chiroptera occurs throughout the pattern. By applying those modifications simul- temperate and tropical zones of the Old World [1]. The taneous with their CF call an emission system Rhinolophidae usually emit echolocation calls through with a time variant directionality is produced. the nostrils, which are surrounded by a distinct noseleaf Although we have found no evidence of noseleaf move- (see Fig. 1a.). ment in the Rhinolophus rouxi, it has been observed in Being a CF-FM bat, the Rhinolophus rouxi has an other bat species. echolocation call that consists of two components: a The second hypothesis is based on a common principle constant frequency (CF) and frequency modulated (FM) used in antenna design, the dipole antenna. component. In particular, the CF part of the call, situated around 77 khz and about 40 to 50 ms long, is Nose-calling bats have a sound production ap- preceded and terminated by a brief FM component (see paratus with a call site consisting of two emit- Fig. 1c.). Based on observations of various Rhinolophi- ters, i.e., the nostrils. For a CF bat, those dae, it is believed that the CF part of the call serves nostrils are horizontally spaced approximately detection and identification of prey, as well as initial half a wavelength of the CF portion of the call's localization [2, 3]. The relative importance of the CF frequency. This dipole configuration is often and FM portions during flight, in particular during the used in engineering to create an emitter with approach and terminal phase, is still debated. Recent a specific directivity, consisting of a main lobe results [4] indicate that the CF part of the call also plays pointing forward with considerably smaller side a prominent function in the approach phase. lobes. Moreover, one can alter the direction of the main lobe by phase-shifting the emitted FM bats, i.e., bats whose echolocation calls mainly signals relative to each other. This specific contain an FM component, were shown to rely on spectral emitter configuration would enable bats to alter cues for discriminating targets at different elevations (e.g. the azimuthal properties of the call directivity. Eptesicus fuscus [5]). Those spectral cues are a re- sult of the differences between the hearing-directionality In this paper, we test these two hypotheses using a patterns at different frequencies. CF-FM bats cannot Boundary Element Method simulation (BEM) [8]. exploit this effect for the CF portion of their call, as they only have one directionality. However, [6] showed Moving the noseleaf that Rhinolophus ferrumequinum probably introduces directionality differences by means of stereotypical ear To investigate the first hypothesis, in which the bat movements executed while echolocating. As argued in [7] deforms the noseleaf, moves it, or deforms the nostrils to this would allow them to localize targets in an analogous generate a change in the emission pattern, we simulate 625 NAG/DAGA 2009 - Rotterdam a) b) a) b) c) d) 90 0 60 -5 -10 30 -15 0 -20 -30 -25 -30 -60 -35 -90 0 0 0 0 30 30 60 60 90 90 30 60 90 30 60 90 -90 -90 -60 -60 -30 -30 -90 -60 -30 -90 -60 -30 Figure 3: Call patterns at 77 kHz: a) scanned position; b) flexed to the front; c) flexed to the back; d) left nostril Figure 2: a) 3 noseleaf positions in which the call pattern is squeezed. calculated (scanned position in the middle); b) left: scanned 10 noseleaf; right: left nostril squeezed. -8 b) 5 -10 ° °-12 0 -14 a) -16 the various emissions patterns from the Rhinolophus -5 rouxi following the procedure in [9]. 0.9 1 0.8 0.99 0.7 A head of a Rhinolophus rouxi was scanned with a sr 0.98 1 0.6 0.97 Skyscan 1076 microCT scanner. After reconstruction 0.5 c) 0.96 d) 0.4 0.95 of the slices with the reconstruction algorithm accom- 60 62 64 66 68 70 72 74 76 78 80 60 62 64 66 68 70 72 74 76 78 80 panying the scanner, Amira R 2 was used to generate a kHz triangular mesh of the complete head's 3D surface model. Normal Front Back Nostril This model contained over 1,000,000 triangles, but as Figure 4: FM sweep comparisons over frequencies: a) main the BEM simulator can only cope with approx. 35000 lobe azimuth evolution in ◦ ; b) main lobe elevation evolution triangles, extra simplification was needed. Moreover, the in ◦ ; c) main lobe solid angle evolution; d) correlations BEM simulation requires the triangles to edge lengths between the directionalities of the model as scanned and the smaller than 1/6th of the smallest simulated wavelength. deformed models. Thus the ears and the back of the head were removed and the simulation was performed on a nose-only model (see Finally the left nostril is reduced about 50% in width. Figs. 1a. and b.). To obtain the directionality, receivers From Fig. 3d. we conclude that the directionality were placed in the nostrils and sound sources were placed remains roughly the same. A small azimuthal asymmetry on the bat's frontal hemisphere (distance: 1 m). is introduced, which might be useful for localization Four different models of the snout with noseleaf are purposes, if it is sufficiently large for the range of considered: one in which the noseleaf is kept as scanned; biologicaly plausible contractions. More research is a second model, contains a noseleaf flexed to the front; needed on this topic. the third one has the noseleaf flexed to the back (see Summarizing, we conclude that the call directionality at Fig. 2a.); in the last model one of the nostrils is deformed, the frequency of the CF portion of the bat's call is not sig- i.e. the left nostril is squeezed (see Fig. 2b.). nificantly affected by the proposed noseleaf deformations. Figure 3 displays the call directionality of the considered However, since the bat emits short frequency modulated models at the CF-frequency of the Rhinolophus rouxi i.e., sweeps at the beginning and or the end of its call (in the 77 kHz. Figure 3a. displays the directionality for the interval 60 - 77 kHz), we also investigate the influence model in which the noseleaf is not altered with respect of the noseleaf and nostril deformations on the call to the scanned data. The patterns exhibits features that directionality at these frequencies. Figures 4a., b., and are to be expected from a sound source consisting of two c., which display the primary characteristics of the main emitters in the horizontal plane who produce sound in lobe (-3 dB contour, as in [9, 11]), indicate that they are phase with a spacing of about 1/2 of the wavelength: similar for most frequencies. The correlation coefficients a main lobe pointing forward with a notch to the left displayed in Fig. 4d. (≥ 0:955 for all frequencies) further and to the right. The elevation asymmetry results from corroborate our conclusion that the deformations have the vertical asymmetry of the noseleaf. These properties only a small effect on the directionality for all frequencies. are also present in the call directionality of a bat with a As no evidence is available to decide whether the pro- similar noseleaf, notably the Rhinolophus ferrumequinem posed noseleaf deformations occur at all or whether [10]. the effect of the modelled deformations corresponds In Fig. 3b and c. the patterns are shown for the model with reality, this hypothesis is very speculative. More in which the noseleaf is flexed 1 mm towards the mouth detailed measurements of vocalizing CF bats are needed. of the animal and 1 mm backwards respectively. They The present simulation results, however, indicates that are similar to the original one qualitatively as well as the use of noseleaf deformations for call directionality quantitatively. The downward pointing sidelobe (around steering is unlikely. 90◦ elavation) shifts slightly down and up respectively. A dipole emitter 1www.skyscan.be In this section, we propose another mechanism for the 2www.amiravis.com bat to produce enhanced call directionality during the 626 NAG/DAGA 2009 - Rotterdam 10 0 5 90° 180° 135° 0 -135° -5 90° 0° 135° 45° -135° 45° -90° 80 kHz -5 180° -45° Elevation in ° 75 kHz Elevation in ° -45° -90° -10 0° -10 70 kHz 65 kHz -15 60 kHz -20 -15 -40 -20 0 20 40 -30 -20 -10 0 10 Azimuth in ° Azimuth in ° Figure 5: Main lobe azimuth and elevation location of Figure 6: Main lobe azimuth and elevation location of the sending pattern in a horseshoe bat (Rhinolophus rouxi) the echolocation pattern (sending pattern convolved with the performing a phase sweep.