Bat guilds, a concept to classify the highly diverse foraging and echolocation behaviors of microchiropteran bats

Abstract

Throughout evolution the foraging and echolocation behaviors as well as the motor systems of bats have been adapted to the tasks they have to perform while searching and acquiring food. When bats exploit the same class of environmental resources in a similar way, they perform comparable tasks and thus share similar adaptations independent of their phylogeny. Species with similar adaptations are assigned to guilds or functional groups. Habitat type and foraging mode mainly determine the foraging tasks and thus the adaptations of bats. Therefore, we use habitat type and foraging mode to define seven guilds. The habitat types open, edge and narrow space are defined according to the bats' echolocation behavior in relation to the distance between bat and background or food item and background. Bats foraging in the aerial, trawling, flutter detecting, or active gleaning mode use only echolocation to acquire their food. When foraging in the passive gleaning mode bats do not use echolocation but rely on sensory cues from the food item to find it. Bat communities often comprise large numbers of species with a high diversity in foraging areas, foraging modes, and diets. The assignment of species living under similar constraints into guilds identifies patterns of community structure and helps to understand the factors that underlie the organization of highly diverse bat communities. Bat species from different guilds do not compete for food as they differ in their foraging behavior and in the environmental resources they use. However, sympatric living species belonging to the same guild often exploit the same class of resources. To avoid competition they should differ in their niche dimensions. The fine grain structure of bat communities below the rather coarse classification into guilds is determined by mechanisms that result in niche partitioning.

Keywords: bat; community structure; echolocation; foraging behavior; guild; habitat.

Figure 1

Schematic drawing illustrating the conditions for overlap between emitted signal, prey echo and background echoes a bat encounters when foraging at a distance of 5 m to vegetation. The prey echo overlaps with the emitted signal when an insect flies in the signal overlap zone and with the clutter echoes from the background when it flies in the clutter overlap zone. In the overlap-free window no overlap occurs. The width of the overlap zones depend on signal duration. At durations between 10 and 2 ms, the overlap zones range between 1.70 and 0.34 m, if a sound speed of 340 m/s is assumed. A reduction of signal duration by 1 ms reduces the width of each overlap zone by 0.17 m and thus increases the width of the overlap-free window by 0.34 m.

Figure 2

Echolocation scenes of bats that search for prey in three different foraging habitats with typical foraging modes. The emitted signal (black) and the returning echoes from prey (black) are displayed together with echo trains from background targets (white). In the depicted echolocation scene which covers a time range of 100 ms a bat foraging in open space in the aerial mode perceives a pulse-echo pair consisting only of the emitted signal and the returning echo (both in black) as long as the background is further away than 17 m. Bats foraging in edge space in the aerial mode perceive a pulse-echo pair that is followed by clutter echoes from the background (in white). When foraging in the trawling mode above the water an additional surface echo returns from below immediately after signal emission (in white). In narrow space the target echo is positioned in the clutter overlap zone. Here three different foraging modes are employed. In flutter detecting foragers the echoes of the long CF-FM signals are modulated in the rhythm of the insect's wing beat and can therefore be discriminated from unmodulated background echoes. Passive gleaning foragers use very short signals. They have no chance to find the food echo (black) between the clutter echoes (white) and they rely on other senses for the detection and localization of the food item. Active gleaners exploit favorable short range favorable echolocation situations where the food echo is isolated enough or is so conspicuous that it can be found between clutter echoes.

Figure 3

Border between open and edge space. (A) Repertoire of the echolocation signals of Vespertilio murinus while foraging in open (right to the red line) and in edge space (left to the red line) and (B) isocontour plots of the signal parameters pulse duration and (C) bandwidth as a function of the horizontal and vertical distances to the background. Each dot represents the mean value of a sequence which was emitted at the indicated position. The red line separates open space from edge space according to our definition that bats react to the background in edge space by changing signal structure but not in open space [adapted from Schaub and Schnitzler (2007)].

Figure 4

Foraging habitats of bats. The borders between open and edge space is determined by the echolocation behavior of the bats. Bats react to background targets in edge space but not in open space. The border is species specific. The narrow space begins with the clutter overlap zone which depends on signal duration.

Figure 5

Search and approach signals of a representative species from each guild. The approach sequences of open space and edge space foragers end with a terminal group consisting of buzz I and buzz II. Narrow space flutter detecting foragers maintain the CF-component of the calls even in the shortest signals of the terminal group. The approach sequences of all other narrow space gleaning foragers lack a distinct terminal group. The approach signals of narrow space passive gleaners are often arranged in groups, but grouping is less distinct and pulse intervals are larger than in active and passive/active gleaning foragers. Echolocation is exclusively used for landing control. The approach signals of narrow space active and passive/active gleaning foragers are clearly arranged in groups of two to five. Repetition rate is higher than in passive gleaning foragers. Echolocation is used to approach a stationary identified food item and to evaluate the orientation of the prey in order to grab it.

Figure 6

Search call structure in relation to minimal capture distance (success rate 50%) in 5 sympatric Myotis species. The higher the signal bandwidth of a species the lower is the minimal capture distance for suspended mealworms. The gray block between 24 and 31 cm indicates the range of the outer borders of the clutter overlap zones of the five bats as calculated from the sound durations of the signals. Note that the performance which is an indicator for the masking effect of the clutter echoes strongly depends on signal structure [data from Siemers and Schnitzler (2004)].