Social-bond strength influences vocally mediated recruitment to mobbing

Abstract

Strong social bonds form between individuals in many group-living species, and these relationships can have important fitness benefits. When responding to vocalizations produced by groupmates, receivers are expected to adjust their behaviour depending on the nature of the bond they share with the signaller. Here we investigate whether the strength of the signaller-receiver social bond affects response to calls that attract others to help mob a predator. Using field-based playback experiments on a habituated population of wild dwarf mongooses (Helogale parvula), we first demonstrate that a particular vocalization given on detecting predatory snakes does act as a recruitment call; receivers were more likely to look, approach and engage in mobbing behaviour than in response to control close calls. We then show that individuals respond more strongly to these recruitment calls if they are from groupmates with whom they are more strongly bonded (those with whom they preferentially groom and forage). Our study, therefore, provides novel evidence about the anti-predator benefits of close bonds within social groups.

A comparison of individual distinctiveness in three vocalizations of the dwarf mongoose (Helogale parvula)

“…Although the level of correct classification for all three calls types was relatively low when compared to that of other studies (Briseño‐Jaramillo, Estrada, & Lemasson, ; Collins et al., ; Gillam & Chaverri, ; Leliveld et al., ), the PIC values were comparatively high (e.g., Bouchet et al., ; Salmi, Hammerschmidt, & Doran‐Sheehy, ). Previous research has shown that dwarf mongooses are capable of discriminating between individuals for each of the three call types (Kern & Radford, ; Rubow et al., ; Sharpe et al., ), suggesting that high levels of call individuality are not a necessity for individual recognition and discrimination. For example, although banded mongoose escort contact calls achieved low correct classification percentages, the acoustic distinction was sufficient to allow mongoose pups to discriminate between individual callers (Müller & Manser, ).…”

“…The dwarf mongoose's snake call also serves a recruitment function, with receivers initially searching for the snake and then banding together to mob the reptile (Rubow et al., ). Individuals are known to respond more strongly to the snake calls of companions (those with whom they most often allo‐groom and forage; Kern & Radford, ), but a signaler is unlikely to benefit directly from receiver response (regardless of its strength) because it is not at risk (i.e., it knows the snake's location, and snakes only catch dwarf mongooses if they can take them unawares; L. L. Sharpe, personal observation).…”

“…Dwarf mongooses prefer to forage in the company of individuals with whom they share strong social ties (Sibun, ). Because these “bonded” individuals are more likely to come to one another's aid (Kern & Radford, ) and dwarf mongooses often intervene when a group member is captured by a predator (successfully rescuing one‐third of all raptor‐victims; Rasa,), signalers will derive a very appreciable benefit from producing identifiable contact calls that allow “bonded” group mates to pinpoint their location and remain close by. Conversely, providing clear identity cues is likely to carry a significant cost in the winter dry season.…”

“…Group members respond by cautiously investigating nearby vegetation and—once the snake is pinpointed—teaming up to mob the reptile (Rubow, Cherry, & Sharpe, ). Dwarf mongooses are able to differentiate between the snake calls of different group members, responding more strongly to callers with whom they share a strong social bond (Kern & Radford, ). The isolation call (Figure c) is a recruitment call emitted when a group member becomes separated from the group.…”

“…We assess one short‐range, intragroup social call (the “contact call”), one predator warning call (the “snake call”, which also has a recruitment function) and one long‐distance call used when a mongoose becomes separated from its group (the “isolation call”). Previous experimental work has demonstrated that dwarf mongooses discern and differentiate between individual callers for all three vocalizations (Kern & Radford, ; Rubow, Cherry, & Sharpe, ; Sharpe, Hill, & Cherry, ).…”