The relative contribution of acoustic signals versus movement cues in group coordination and collective decision-making

Abstract

To benefit from group living, individuals need to maintain cohesion and coordinate their activities. Effective communication thus becomes critical, facilitating rapid coordination of behaviours and reducing consensus costs when group members have differing needs and information. In many bird and mammal species, collective decisions rely on acoustic signals in some contexts but on movement cues in others. Yet, to date, there is no clear conceptual framework that predicts when decisions should evolve to be based on acoustic signals versus movement cues. Here, we first review how acoustic signals and movement cues are used for coordinating activities. We then outline how information masking, discrimination ability (Weber's Law) and encoding limitations, as well as trade-offs between these, can identify which types of collective behaviours likely rely on acoustic signals or movement cues. Specifically, our framework proposes that behaviours involving the timing of events or expression of specific actions should rely more on acoustic signals, whereas decisions involving complex choices with multiple options (e.g. direction and destination) should generally use movement cues because sounds are more vulnerable to information masking and Weber's Law effects. We then discuss potential future avenues of enquiry, including multimodal communication and collective decision-making by mixed-species animal groups. This article is part of the theme issue 'The power of sound: unravelling how acoustic communication shapes group dynamic'.

Keywords: collective behaviour; group cohesion; movement; quorum; vocal communication.

Figure 1.

Schematic diagrams illustrating the role of acoustic signals (red circles) and movement cues (yellow circles) in coordinating activities in group-living animals. Behaviours involving the expression of specific actions or the timing of events tend to rely more on acoustic communication for coordination, whereas behaviours related to directional decisions are typically mediated by movement cues. (a) Contact calls communicate the location and current state of callers, helping to synchronize individuals’ current behaviours, regulate spacing between them and maintain group cohesion. (b) Travel calls communicate the timing of transitions from one group behaviour to another, primarily coordinating departures from a sedentary state. (c) Food-associated calls recruit group members to food sources and initiate foraging behaviours. (d) Mobbing alarm calls rally group members to collectively mob low-urgency predators, prompting a shift from their normal states to anti-predator actions. (e) Flee alarm calls prompt a collective flee response within the group in response to high-urgency predators; once on the move, individuals rely more heavily on movement cues to coordinate their (f) speed, (g) direction, and (h) destination. Black circles indicate individuals, red circles indicate individuals giving acoustic signals and yellow circles indicate movement cues from individuals. Dashed lines indicate movement direction and speed, red lines indicate an individual giving acoustic signals in a specific direction and black thin arrows indicate an individual gaining information from the movement cues of other individuals. Gradient-filled red and green circles indicate different patches.

Figure 2.

Schematic diagrams depicting the discrimination ability for (a) acoustic signals and (b) visual cues, as a function of the difference in the number of individuals communicating each of two preferences (i.e. Weber’s Law). Dark red and dark blue represent high probabilities of selecting either option X or option Y, respectively, corresponding to which option has the highest number of ‘votes’. The yellow area represents where individuals cannot reliably discriminate between the relative number of ‘votes’ and consequently choose at random. In the illustrated example, the greater discrimination ability allows the group to select option X when there are five versus three visual cues for X versus Y, but to choose at random when there are five versus three acoustic signals because they cannot discriminate which option has more votes. The design of this figure is based on Arganda et al. [119].

Figure 3.

Schematic diagrams illustrating the probability thresholds for changing group behaviour based on varying levels of urgency in acoustic signals. The red line represents high-urgency acoustic signals, such as flee alarm calls; the orange line represents mid-urgency signals, such as mobbing calls; and the green line represents low-urgency signals, such as travel calls.