Group-living organisms across taxa coordinate their movement to evade threats or predators. However, how information about threats, often available only to a few individuals within the group, efficiently propagates among the group members, and how animals use the information of predator to coordinate their movement, remains less explored. In this thesis, our aim is to study collective escape responses, information propagation and context-dependent hierarchical leadership in collectively escaping groups, using both data and models.

We first investigate the collective responses of a sheep flock (Ovis aries) to a herding dog (border collie). We observed that the sheep flock remained highly cohesive throughout the herding events, consistent with the selfish herd effect, a known mechanism hypothesised to reduce predation risk. Sheep moved faster as the dog increased speed, while being highly polarised but less cohesive. This suggests that cohesion alone may not adequately explain anti-predatory benefits of group-living, especially in groups exhibiting synchronous collective motion as seen in our sheep flock experiments. Using lagged cross-correlation analysis of time series of direction of different individuals, we identified a clear hierarchy among sheep in terms of their directional influence on the flock. We found that the average spatial position of a sheep along the front-back axis of group velocity strongly correlates with its influence on group movement.

To explain these results, we developed a computational model where sheep follow simple interaction rules, namely, repulsion from the dog and a tendency to move towards and align with neighbours. This model can reproduce empirically observed patterns. Consistent with experimental findings, the model predicts that the individuals at the front of the flock had greater directional influence on the group. Furthermore, we developed a null model of herding in which the chasing behaviour of dog is not included. Such a model fails to reproduce the hierarchical information flow, suggesting that the observed empirical patterns are characteristic of collective escape response.

When animals collectively respond to threats, it is difficult to know if the individuals were directly reacting to the threat or to the response of their neighbors. We study high-resolution data from a controlled experimental set up of fish (tiger barbs) where an individual trained to a threat stimulus via aversive conditioning escapes the stimulus, thus precisely controlling the individual reacting to the threat (or thus, having information of the threat). We show that in a group of five fish with only one conditioned fish, the escape behaviour of one conditioned fish could trigger collective escape responses with all the fish. We use lagged cross-correlation analysis of speed of different fish to analyse information propagation and leadership. Under unperturbed conditions, we do not observe any hierarchical leadership. However, when we turn on the green light and the conditioned fish responds to the green light by crossing the barrier, we observe a hierarchical transfer of information from the conditioned fish to the naive ones. Further, by using spatially-explicit agent-based models, we show that the hierarchical transfer of information occurs because, once the green light is turned on, the conditioned fish reduces it’s interaction strength with all the naive fish until it crosses the barrier, while the naive fish respond to the conditioned fish due to its rapid change in speed and direction.

In summary, my thesis reveals that during the initial attack by predators, the information about the threat propagates via sudden changes in the speed of informed individuals. However, when the predator continuously chases the group, information spreads more strongly through changes in the direction of the individuals at the front. Further, we can use computational models to both explain these patterns, as well as make inferences about the broad nature of interactions among group members while they escape threats. Thus, combining results from all these studies, from highly controlled to natural settings, our study revealed some general principles of collective escape dynamics in group-living organisms.