In free-ranging and feral horses, for example, no single individual is consistently the group leader. Instead, an egalitarian social organization appears to be the rule, with any number of horses making decisions and coordinating group movement,1,2 and with relatively little competition or aggression.3
A recent study by Lea Briard, BSci, MSci, PhD, and colleagues,4 “How stallions influence the dynamic of collective movements in two groups of domestic horses, from departure to arrival,” explored what, if any, influence the stallion has on collective movement of the herd. They found that, although the stallion rarely initiates group movement, he does appear to play a unique role in from the rear position by keeping stragglers in line and maintaining vigilance.
Who initiates group movement?
To study collective movement, Briard and her colleagues observed two semi-free-ranging herds of domestic horses in eastern France. The groups were similar in size and composition: one herd included a stallion, nine mares, and eight foals, and the second herd included one stallion, twelve mares, and eight foals.
In this study, leadership was defined as the ability to initiate, recruit, synchronize, and coordinate the movement of others. For more than one hundred group movement events, the researchers recorded the identity of the horse who initiated group movement, and the order, timing, and position of the other horses as they joined as followers. The researchers looked for patterns that supporting either consistent leadership (the same horse initiating group movement and traveling in front) or distributed leadership (several different individuals initiating movement and taking the front position).
The study found distributed leadership in both herds. Nearly all horses initiated group movement and took the front position at least once. Even so, a few horses initiated group movement more often than others; in one herd, a single mare initiated 40% of the group movement events, and in the other herd, two mares initiated a combined 50% of the group movement events.
What is the stallion’s role?
In both herds, the stallion was frequently the last horse to join the group and nearly always ended up in the rear position; stallions rarely initiated collective movement or traveled in front. This pattern has been previously reported in stallions as well as other male ungulates. From the rear position, the stallion has a superior vantage point to scan for threats, monitor the herd, and keep the group together.
By observing the horses’ behavior, the researchers found that both mares and stallions frequently grazed during group movement, but stallions were more likely to remain vigilant, and to stop and look behind them (back-glance). Stallions also urinated and defecated much more often than mares. Researchers have observed these behavioral differences between stallions and mares in other contexts, as well.5
As part of the study, the researchers removed the stallion in one herd with the rationale that if he plays a key role, then his removal should disrupt the dynamics of group movement. Indeed, after the stallion’s removal, following took five times longer and mares were more spatially dispersed. Removing the stallion didn’t alter other herd dynamics during group movement; for example, the same mares were most likely to initiate, and following patterns were not significantly affected.
Social organization and leadership patterns in animal groups can be complex. Horses are polygynous, with one breeding male, several adult females, and their offspring. In polygynous societies, the prevailing belief is that the male holds high status and controls group activities. In horses, this general principle does not hold up. Although the stallion will defend his harem from competitors, a growing body of scientific evidence suggests that, within the herd, the prevailing social structure is egalitarian, in which the stallion isn’t dominant and no mare is boss.
1Krueger, K., Flauger, B., Farmer, K., & Hemelrijk, C. (2014). Movement initiation in groups of feral horses. Behavioural Processes 102, 91–101
2Bourjade, M., Thierry, B., Hausberger, M., & Petit, O. (2015). Is leadership a reliable concept in animals? An empirical study in the horse. PLoS One 10, e0126344. http://dx.doi. org/10.1371/journal.pone.0126344.
3Krüger, K. & Flauger, B. (2008) Social feeding decisions in horses (Equus caballus). Behavioural Processes 78, 76–83
4Briard, L., Deneubourgb, J.-L., & Petita, O. (2017) How stallions influence the dynamic of collective movements in two groups of domestic horses, from departure to arrival. Behavioural Processes 142, 56–63.
5 Sigurjonsdottir, H., Thorhallsdottir, A.G., Hafthorsdottir, H.M., & Granquist, S.M. (2012) The behaviour of stallions in a semiferal herd in Iceland: time budgets, home ranges, and interactions. International Journal of Zoology, 1-7.