Understanding how animal groups achieve coordinated movement

Abstract

Moving animal groups display remarkable feats of coordination. This coordination is largely achieved when individuals adjust their movement in response to their neighbours' movements and positions. Recent advancements in automated tracking technologies, including computer vision and GPS, now allow researchers to gather large amounts of data on the movements and positions of individuals in groups. Furthermore, analytical techniques from fields such as statistical physics now allow us to identify the precise interaction rules used by animals on the move. These interaction rules differ not only between species, but also between individuals in the same group. These differences have wide-ranging implications, affecting how groups make collective decisions and driving the evolution of collective motion. Here, I describe how trajectory data can be used to infer how animals interact in moving groups. I give examples of the similarities and differences in the spatial and directional organisations of animal groups between species, and discuss the rules that animals use to achieve this organisation. I then explore how groups of the same species can exhibit different structures, and ask whether this results from individuals adapting their interaction rules. I then examine how the interaction rules between individuals in the same groups can also differ, and discuss how this can affect ecological and evolutionary processes. Finally, I suggest areas of future research.

Keywords: Collective behaviour; Collective motion; Interaction rules; Leadership; Social responsiveness.

Fig. 1.

Tracking data can provide information on the relative positions and orientations of individuals in groups. From these data, the distance (d), direction (θ) and differences in heading (φ) between a focal individual and its neighbours (N₁ or N₂) can be quantified. The acceleration, speed or turning angle (α) of the focal individual can then be correlated with these measurements to infer how individuals in groups are responding to each other's movements and positions.

Fig. 2.

The relative density of neighbours around a focal individual for different species. The focal individual is represented by the white silhouette in the centre of each plot (not to scale) and is facing in its direction of motion. (A) Locusts are commonly found 3–10 cm apart and in any direction surrounding a focal individual. Lower densities of neighbours occur within a 1 cm region close to an individual. Adapted from Buhl et al. (2012). (B) Pigeons (Columba livia) are commonly found side by side with respect to their direction of motion. Adapted from Pettit et al. (2013). (C) Female guppies (Poecilia reticulata) in pairs position themselves in front of or behind, and to the left or right of a partner (author's unpublished data).

Fig. 3.

The acceleration and turning responses of mosquitofish (Gambusia holbrooki) and pigeons (Columba livia). (A) Acceleration response of a fish in the centre of the plot and facing right, as a function of distance from its neighbour. There is a region of repulsion close to the focal individual, and attraction occurs at distances further away. Together, these act to maintain distances between individuals within the group. Adapted from Herbert-Read et al. (2011). (B) Pigeons turn towards or away from their neighbour depending on the position and distance of that neighbour. Positive turning angles represent turns to the right, whereas negative turning angles represent turns to the left. Pigeons will turn towards their neighbour when it is >3 m away, and away from their neighbour when it is <3 m away. Adapted from Pettit et al. (2013). The fish and bird silhouettes in the centre of the plots are not to scale.

Fig. 4.

Leader and followership in pairs of ants (Temnothorax alibipennis) and mosquitofish (Gambusia holbrooki). (A,B) Acceleration responses of the leader (A) or follower (B) ant, depending on the distance to their partner. When the pair are <∼1 mm apart (R=1 mm), the leader will accelerate, whilst the follower will decelerate (blue circles left of R). When the pair are between ∼1 and 2 mm apart, the follower will accelerate and the leader will decelerate (blue circles right of R). Both leader and follower show close to zero acceleration when >2 mm apart (red circles). Adapted by permission from Macmillan Publishers Ltd, Nature; Franks and Richardson (2006). (C) Mean change in speed and (D) mean change in angle of mosquitofish that were leaders (solid red curve) or followers (solid blue curve) as a function of their partner's location. In C, negative x regions indicate that the focal individual (either leader or follower) was behind its partner and positive x regions indicate that the focal individual was in front of its partner. In D, negative y regions indicate that the focal individual was to the left of its partner, and positive y regions indicate that it was to the right of its partner. Dashed curves are plotted one standard error above and below all means in each panel. These responses show how individuals differentially adjust their velocity as a function of their neighbours' location. Grey regions represent the regions where the movement responses of leaders and followers are significantly different. Adapted from Schaerf et al. (2016).