Herding mechanisms to maintain the cohesion of a harem group: two interaction phases during herding

In animal groups, individual interactions achieve coordinated movements to
maintain cohesion. In horse harem groups, herding is a behavior in which
males chase females from behind; it is considered to assist with group
cohesiveness. However, the mechanisms by which the individuals move to
maintain group cohesion are unknown. We applied novel non-invasive methods
of drone filming and video tracking to observe horse movements in the
field with high temporal and spatial resolution. We tracked all group
members and drew trajectories. We analyzed the movements of females and
found two phases of interactions based on their timing of movement
initiation. The females that moved first were those nearest to the herding
male, while the movement initiation of the later females was determined by
the distance from the nearest moving female, not by the distance from the
herding male. These interactions are unique among animal group movements
and might represent a herding mechanism responsible for maintaining group
cohesion. This might be due to long-term stable relationships within a
harem group and strong social bonds between females. This study showed
that the combination of drone filming and video tracking is a useful
method for analyzing the movements of animals simultaneously in high
resolution.