The Social Organization of Wild Equine Populations as Seen by Drones

Using drones, researchers from Kyoto University and the University of Strasbourg observed and tracked herds of wild horses in Portugal to study their interactions and the structure of the various social groups.   Human societies are considered complex because they operate on multiple levels. Social organization is structured into several stable groups and subgroups, which can come together to form larger levels of organization. Thus, populations can group together into communities, cities, countries, and so on. Studying and comparing these societies with complex animal societies sheds light on how different groups function, particularly how they maintain themselves within a social system. However, the mechanisms at play remain poorly understood, and quantitative data are lacking. Until now, only numerical methods have been available to evaluate models of social structures, measuring the time groups spend together using non-specific clustering techniques and algorithms designed to reveal different stratifications and associations. However, the distance between individuals should not be the only parameter considered when determining whether they are “together,” as this overlooks the interspecific relationships among animals. This methodology therefore fails to model the full range of social interactions, which can skew meta-analyses. Researchers thus sought to go beyond measuring distances between individuals to develop a more rigorous method for studying the structure of horse groups. To do so, they used spatial data on the composition of equine populations in Portugal—collected using drones—to identify individual social associations within these groups. This individual identification has been carried out in only a few studies and only on small groups. Consequently, most studies that have used drones have focused on temporary social interactions or collective behaviors in behavioral ecology. This time, the researchers sought to add individual information to the positioning data. They identified, among 121 wild horses, 23 units consisting of harems (mixed-sex groups with one or two adult stallions) or bachelor groups (composed entirely of males). However, these units sometimes join together. They then form a higher-level social organization—a herd—likely to provide more effective protection for its members. Aerial observation using drones shows, however, that within herds, the units maintain a certain cohesion. The largest harems tend to occupy a central position, while the smaller ones and the bachelor groups gravitate toward the periphery. This structure is not unusual. It is common to find dominant individuals at the center of social structures. This hierarchical relationship therefore appears to extend to groups of individuals whose dominance correlates with their numbers. The researchers were able to demonstrate the existence of units consisting of several individuals, associations of multiple units forming herds, and stable positions among the units within the herds. These findings have made it possible to identify a complex, multi-level society among wild horses. Scientists hope that their methodology—which combines spatial positioning with long-term tracking of individualized social associations—will be applied to other equines, and even to other animal species, with the aim of drawing comparisons between populations and across species.   https://www.nature.com/articles/s41598-020-79790-1