Here we present new results on the SL of type 1 and type 2 signals and on the change of beam direction in B. barbastellus during search and approach flight. In summary, we found that the two types of search signals of B. barbastellus have nearly equivalent SL but are emitted in different directions along the vertical axis. The beams are separated by approximately 70° and the beam directions of type 1 signals are below flight direction whereas those of type 2 signals are above.

The apparent and real sonar beam directions of approach calls resemble those of type 2 calls, and largely point towards the upper area of the array (Figs 2 – 5 ). Even the recordings from a low and a high microphone of the array showed that the high microphone recorded not only type 2 signals but also approach signals with higher amplitude ( Fig 1 ). The similarity in beam direction supports the conclusion that the broadband approach calls are derived from type 2 signals, as evidenced by sonagrams which show how type 2 calls morph into approach calls ( Fig 1 ), [ 15 , 19 ].

Pipistrellus pipistrellus, a species that emits signals only through the mouth, is able to move its head and with it the beam direction [ 20 ]. We therefore assume that B. barbastellus also has the ability to move its head around a roll, pitch and yaw axis and with it the beams of the anatomically fixed nose and mouth emitters. Some of our data indicate that these bats also make head movements. Between succeeding signals beam direction was not only changed in the vertical but also tilted to the side around a roll axis which is indicated by the measured horizontal angles in Fig 4A . The scan path of an example flight in Fig 4B also shows that the bat was capable of turning its head such that the beam moved from right/down to left/up and back. Another hint of head movements around the pitch axis comes from directional changes of the approach signals in Figs 4 and 5 . The direction of these nose signals moves from the upper end of the array more to the center indicating that the bats fixate a target with the nose beam. We predict that such a fixation with the nose beam will also occur in bats approaching an insect.

The switching between mouth and nose emission would explain the reported differences in SPL between call types in single microphone recordings only if the beams of the mouth and of the nose signals point in different directions. When looking at the head anatomy of B. barbastellus we found that the opening direction of the nostrils is nearly perpendicular to the direction of the mouth opening ( Fig 7 ). If we assume that the pointing direction of nose and mouth opening determines the beam directions of the emitted signals we predict that the nose beam should be almost perpendicular to that of the mouth. This prediction corresponds well with our measurements that indicate a separation angle of about 70° in the vertical. We therefore conclude that B. barbastellus vary their emission direction in the vertical of about 70° by most likely emitting type 1 signals through the mouth and type 2 through the nose. The degree of angular separation between nose and mouth beam is determined by the anatomical relation between nostrils and mouth. This anatomical relation is fixed such that the angle between the two emission beams is likely invariable.

From single microphone recordings it was concluded that the amplitude variation between type 1 and type 2 signals indicate changes in beam direction in the vertical, and that they might be caused by up and down head movements [ 15 ]. In 15 high speed videos (by courtesy of Dietmar Nill) and on 39 photos of flying barbastelle bats (by courtesy of Laurent Arthur) we never found obvious changes in head position, which would indicate up down head movements. Another explanation for the observed change in emission direction could be that the upward directed type 2 signals are emitted through the upward pointing nose and downward directed type 1 signals through the mouth. This hypothesis is strongly supported by the observation that the type 1 signals are emitted through the mouth and the type 2 signals through the nose [ 16 ]. The possibility of switching between mouth and nose emission is supported by the finding that even if either the mouth or the nose of bats are plugged they are able to fly, avoid obstacles, and emit echolocation signals [ 17 ]. It was found that B. barbastellus and Plecotus auritus are able to emit signals either through the nose or through the mouth and that the oscillograms of the mouth and nose signals look rather similar [ 17 ]. It was also reported that P. auritus flies with mouth shut and that echolocation calls are emitted through the nostrils, which open upwards [ 18 ].

Adaptive value of emitting two types of search calls with similar SL in different directions

B. barbastellus forages near vegetation during aerial-hawking and preys mainly on tympanate moths [3,4,12]. According to this foraging behavior, barbastelles are attributed to the guild of “edge space aerial-hawking foragers” that search for prey near background targets [6]. Foraging and echolocation behavior of bats have been adapted to the task they have to perform while searching and acquiring food. All edge space foragers share similar adaptations, and their echolocation systems display many similarities. However, this does not exclude species-specific differences that reflect niche partitioning within guilds [6]. If we compare the echolocation behavior of B. barbastellus with that of other edge space foragers we find many similarities, but also some distinct differences which may account for niche differentiation.

The signal pattern of foraging B. barbastellus generated only by type 2 search signals and approach signals is rather similar to that of other edge space aerial-hawking foragers [15,19]. The bats emit shallowly downward modulated search signals of moderate bandwidth (frequency range of 45–32 kHz). The signals are varied in duration (around 3–8 ms), most likely in relation to the distance to the background. The initial more shallow modulated part of the signal improves detection, and the steeper modulated terminal part improves localization of prey [21]. Sometimes the bats skip a sound emission which is indicated by longer intervals. After detection of prey with a long type 2 signal the bats switch to broadband approach signals (frequency range of 52–23 kHz). The first signal of an approach may be even longer than the preceding type 2 signal (transition signal according to [19]) but afterwards signal duration and pulse interval are reduced in the typical way as in other aerial-hawking species approaching prey. The approach ends with a typical buzz which is indicated by pulse intervals below 8 ms. Often the buzz takes quite long and is interrupted by one or several longer pulse intervals (see Fig 3 in [15]). The prolonged buzz with much more signals than in a short buzz most likely indicates that the bats pursue prey which try to escape. In summary, in foraging B. barbastellus, the pattern of only type 2 and approach signals is rather similar to that of other edge space aerial-hawking foragers.

However, we also found distinct differences when comparing the echolocation behavior of B. barbastellus with that of other edge space aerial-hawking foragers. One big difference is that the type 2 and the approach signals are most likely emitted through the nose. The emission of signals through the nose is also found in bats of the genus Plecotus which—together with the barbastelles—belong to the tribe Plecotini. This phylogenetic relationship probably indicates that the two genera had a common ancestor with nose emission [22]. It is striking that the nostril alignment of those two genera differs from the other Vespertilionids in that it contains more cavities with nose openings turned upwards (see also Fig 7) [3,18].

But why are other genera within the family of Vespertilionidae able to forage with mouth-bound signals alone while barbastelle bats have evolved two different emitters and use two different signal types with equal SLs (81–82 dB SPL rms re 1 m) far below those of other aerial-hawking foragers that use SLs of 101–114 dB SPL rms re 1 m [23, 24]? This rather low SL has been interpreted as an adaptation that allows B. barbastellus to hunt successfully for tympanate moths. Many moths can hear and react with escape responses as soon as they detect an approaching bat [2,25,26]. Most edge space aerial-hawking foragers have high SLs which allow for long detection distances. However, these high SLs have the disadvantage that moths can detect the bats early enough to initiate often-successful escape maneuvers. Goerlitz et al. (2010) [4] were the first to propose that B. barbastellus might use some kind of “stealth echolocation” and produces search signals with low SLs that are inconspicuous to eared moths. They determined a SL for type 1 signals of only 94 dB peSPL re 0.1 m which corresponds to 74 dB peSPL re 1 m. We assume that this value is too low as their method probably delivered weaker apparent beam maxima instead of louder real beam maxima. The main peak in the histogram of their SPLs (Fig 3B in [4]) has an upper limit near 104 dB peSPL re 0.1 m which corresponds to 84 dB peSPL re 1 m. It is difficult to compare the peSPL measurements of [4] with our SPL rms measurements. Their peSPL values are related to the rms value of a sine wave with a constant amplitude whereas the SPL rms measurements of our sonarbeam software uses the time-varying call amplitude. The two values would be the same if the bats kept a constant peak amplitude over the entire signal. In barbastelles the call amplitude does not change very much within the signal (see Fig 1 in [15]). According to the envelope of barbastelle signals we estimate that our SPL rms value is between 1–4 dB smaller than the peSPL values of [4]. The subtraction of an estimated 3 dB from the value taken from the histogram in Fig 3B in [4] results in a SL of 81 dB SPL rms re 1 m which corresponds very well to the average SL of the type 1 signals of our measurements.

For B. barbastellus, foraging for large moths with a target strength of -36 dB (re 1 m) the detection distance is about 3 m if we assume a best frequency of 40 kHz, a detection threshold of 20 dB, a temperature of 15°Celsius, and a humidity of 50% (all calculations according to [27]). Under similar conditions, N. leisleri could detect this prey over a distance of 8.5 m if we assume a SL of 107 dB SPL rms re 1 m [27] and a best frequency of 28 kHz. The neural maximum for echolocation detection distances of the moth Noctua pronuba by foraging B. barbastellus and N. leisleri was measured in the field [4]. The more sensitive A1 neuron of the moth always reacted to the search signals of N. leisleri before the bat heard the moth echoes. The moth’s A1 detection distance was with 33 m far beyond the 9 m detection distance of the bat calculated from our data. This early warning gives eared moths the possibility to start with escape movements in time. In foraging B. barbastellus the moth’s A1 neuron detection distance of 4 m was close to the bat’s detection distance of 3 m which we calculated from our data. This suggests that eared moths have far less chance to escape barbastelles by evasive movements. Therefore, our findings lend support to Goerlitz et al. (2010) [4] who stated “B. barbastellus uses a stealth echolocation strategy by emitting low-amplitude calls, a strategy previously suggested by Fenton & Fullard (1979) [28] and by Surlykke (1988) [29] and now supported with field-based measurements”.

In contrast to [4] we do not assume that B. barbastellus use type 1 signals as search signals for prey. We hypothesize that these signals have evolved for a different function. The SLs of the type 1 and type 2 signals of B. barbastellus are 20–25 dB lower than the SLs of other edge space aerial-hawking foragers. This has the advantage of allowing B. barbastellus to approach very close to eared moths without provoking evasive movements, but at the cost of a highly reduced detection distance for prey and background targets. When comparing the calculated maximum detection distances for prey of type 2 signals with those of search signals of N. leisleri [27] we found a lower detection distance by about 1/3rd in type 2 signals (which subsequently reduces the search volume to about 1/27th). Additionally, the detection distance for vegetation in the flight path is strongly reduced from 23 m in N. leisleri to 8 m in type 2 signals of B. Barbastellus (see [27]).

In B. barbastellus foraging above a forest canopy with type 2 signals pointing slightly upward, the detection and the evaluation of canopy echoes are hampered as the directionality of signal emission and also of echo reception substantially reduces the SPL of type 2 echoes from below. If we assume that the perceived SPLs of echoes returning at an angle of 45–60° relative to the beam’s emission direction are at least attenuated by 20–30 dB (estimated according to [30,31]) the detection distance for forest at an downward angle of 45–60° relative to the beam direction would be less than 1–2 m. For echoes returning vertically from below the detection distances would be even smaller. These short detection distances would make it difficult for the bat to forage above vegetation with type 2 signals alone. However, the emission of type 1 signals downward improves the chance to detect and evaluate forest echoes from below. With a type 1 signal that is emitted with a downward beam direction of 45° the detection distance for a forest in beam direction is at about 9 m [27]. This is comparable to a N. leisleri, which can detect forest echoes returning from an angle of 45° below the horizontal emission direction up to a distance of 12 m (assuming a reasonable 20 dB attenuation of the perceived echo SPL). In barbastelles which emit type 1 signals at a downward emission angle of 45° even the echoes returning vertically from below would allow detection distances of up to 2 m for forests, 5 m for meadows, and 10 m for water surfaces (again assuming a reasonable 20 dB attenuation of the perceived echo SPL at an angle of 45°relative to the beam direction). We therefore conclude that the downward directed type 1 signals have evolved to determine the bat’s position in relation to the environment below. Such a strategy has previously been suggested [15]. With downward directed type 1 signals B. barbastellus overcomes the disadvantage of the stealth strategy with the rather short detection distances for the environment below the bat due to low emission SLs of the more upward directed type 2 signals. This strategy does not exclude that in favorable situations echoes of type 1 signals are also able to locate prey such that the overall search volume can be increased. However, the observation that type 1 signals are always rather stereotyped and, in contrast to type 2 signals, do not change according to foraging situation [15] suggests that they are not primarily used as search signals for prey.

A different argument to explain why B. barbastellus evolved two types of echolocation signals was used by Barataud (2004) [19]. He suggested that bats use two signal types which differ in intensity, structure, and frequency to deceive tympanate moths by mimicking the presence of two bats at different distances with sufficiently low repetition rates so as to not provoke the prey’s escape behavior. However, [4] and the present study suggest that B. barbastellus actually use a stealth strategy to improve their hunting success. Given that both signal types have low SLs and cannot be heard by the moths, it is unlikely that the two signal types have evolved to deceive prey.

In conclusion, we found that foraging B. barbastellus emit two signal types of equally low SL in different directions. In relation to flight direction the beams of type 2 signals point upward, and that of type 1 signals downward. The beams are separated by a fixed vertical angle of approximately 70°. Barbastelle bats are able to emit signals through the mouth or the nose and these signals occur at roughly perpendicular angles to one another. This suggests that type 1 signals are most likely emitted through the mouth and type 2 through the nose. In addition, this fixed double emission system can be actively adjusted up or down around the pitch axis, and tilted around the roll axis of the head. We hypothesize that the “stealth” echolocation system of B. barbastellus, with two different signal types of low SL which are emitted in different emission directions, is bifunctional. The upward directed nose signals are used for the search of, and the approach to, prey. Their low SL prevents an early detection of bats by eared moths at the expense of a significantly reduced detection range for the environment below the bat. The more downward directed mouth signals have evolved to compensate for this disadvantage. These second signals are largely used for spatial orientation and biotope recognition.