Experimental evidence for group hunting via eavesdropping in echolocating bats

Abstract

Group foraging has been suggested as an important factor for the evolution of sociality. However, visual cues are predominantly used to gain information about group members' foraging success in diurnally foraging animals such as birds, where group foraging has been studied most intensively. By contrast, nocturnal animals, such as bats, would have to rely on other cues or signals to coordinate foraging. We investigated the role of echolocation calls as inadvertently produced cues for social foraging in the insectivorous bat Noctilio albiventris. Females of this species live in small groups, forage over water bodies for swarming insects and have an extremely short daily activity period. We predicted and confirmed that (i) free-ranging bats are attracted by playbacks of echolocation calls produced during prey capture, and that (ii) bats of the same social unit forage together to benefit from passive information transfer via the change in group members' echolocation calls upon finding prey. Network analysis of high-resolution automated radio telemetry confirmed that group members flew within the predicted maximum hearing distance 94+/-6 per cent of the time. Thus, echolocation calls also serve as intraspecific communication cues. Sociality appears to allow for more effective group foraging strategies via eavesdropping on acoustical cues of group members in nocturnal mammals.

Figure 1

Mean number of visual observations (±s.d.) of far passes, near passes and approaches contrasting between pre-playback silence (left hand values of each category) and playback of FBs (right hand value). Only the difference between approaches during pre-playback silence and FBs was significant (asterisks, p<0.007; n=14).

Figure 2

Bat foraging movements during one representative evening (15 June 2007); (a) group A (two bats), (b) group B (three bats) and (c) group C (three bats). For explanation of symbols, see (a) (crosses, automated tracking site; circles, hand-tracking site; squares, main bat roost). Points in the figure represent fixes where all the animals' signal was heard from the same direction within a maximum of 30 s.

Figure 3

Bar plot of the fraction of time (no. of times a bat was co-located/total no. of observations) bats from roost 1 (groups A–C, bats 1–8) and roost 2 (group D, bats 9–13) foraged with at least one other bat.

Figure 4

(a) Graphical representation of the structure emerging from the network of co-locations of foraging individual bats. Thick-dashed rectangles cluster animals from the same roost. Thin-dashed rectangles cluster animals from the same group. The shown subdivision is obtained as the maximum value of the network modularity (Q=0.48), and it corresponds to the social structure inferred at capture. The bats are represented by a number in a thick oval shape. Bats that foraged together are connected with a thin line. The number of co-locations between two bats is depicted by the number in a thin ellipse breaking the line connecting them. Co-located bats from the same group are connected by a solid line, bats from different social groups by a dotted line ((i) roost 1 and (ii) roost 2). (b) Two-dimensional representation co-locations of bats during radio tracking, using a non-metric multidimensional scaling analysis. The distance between individual points is proportional to the inverse of the ranked pairwise number of co-locations. Large distances therefore represent a low number of co-locations, while small distances symbolize a large number of co-locations (red circles, group A; blue circles, group B; yellow circles, group C; black circles, group D).