Collective motion of groups of self-propelled particles following interacting leaders

TL;DR

This paper models the collective motion of leader-follower groups in animal herds, reproducing herd features and revealing limitations of simple interaction models in explaining observed group size distributions.

Contribution

It introduces a self-propelled particle model for two-level animal groups and compares simulation results with real herd data, highlighting the need for more complex social interaction models.

Findings

Model reproduces key herd features including harem formation and leader fights.

Emerging group size distribution in the model is normal, unlike observed lognormal distribution.

Highlights the importance of complex social topology in herd formation.

Abstract

In order to keep their cohesiveness during locomotion gregarious animals must make collective decisions. Many species boast complex societies with multiple levels of communities. A common case is when two dominant levels exist, one corresponding to leaders and the other consisting of followers. In this paper we study the collective motion of such two-level assemblies of self-propelled particles. We present a model adapted from one originally proposed to describe the movement of cells resulting in a smoothly varying coherent motion. We shall use the terminology corresponding to large groups of some mammals where leaders and followers form a group called a harem. We study the emergence (self-organization) of sub-groups within a herd during locomotion by computer simulations. The resulting processes are compared with our prior observations of a Przewalski horse herd (Hortob\'agy, Hungary) which we use as results from a published case study. We find that the model reproduces key features of a herd composed of harems moving on open ground, including fights for followers between leaders and bachelor groups (group of leaders without followers). One of our findings, however, does not agree with the observations. While in our model the emerging group size distribution is normal, the group size distribution of the observed herd based on historical data have been found to follow lognormal distribution. We argue that this indicates that the formation (and the size) of the harems must involve a more complex social topology than simple spatial-distance based interactions.