Multilevel structure of horse society revealed in drone-based study

The image at the left is an example of the results generated from successive aerial images from a drone. Each brown dot on the field is a horse. Circles and arrows represent the units and solitary males partitioned based on the distance distribution. White dots are free-ranging cows. The image at right shows an enlarged view of an orthomosaic (an accurate photo representation of an area, created out of many photos). It captured a harem named Kitakami. All horses on the orthomosaic were identified. The points and labels represent the positions and IDs of each horse.

Evidence from a study using drones strongly indicates a multilevel structure in horse society, researchers report.

The study, described in the journal Scientific Reports, was undertaken in Serra D’Arga, Portugal, where about 200 feral horses live without human care.

Researchers involved in the month-long study took aerial photographs of the horses at 30-minute intervals from 9am to 6pm in two specific zones.

Identifying characteristics of all the horses observed from the ground were used to identify each animal, such as colour, body shape and white markings. These were used to successfully identify more than 100 of the horses in the aerial images.

The researchers, from Kyoto University in Japan and the University of Strasbourg in France, studied the association patterns between the horses in the images to gain an understanding of horse society structure.

The evidence suggested the presence of small social organizations, or “units”, Tamao Maeda and her colleagues reported.

Units contain two types of social organization: a harem composed of one or two adult males and several females and immature individuals; or an all-male unit or bachelor group of adult males who could not attract any females.

A group of feral horses in Serra D'Arga, Portugal.
A group of feral horses in Serra D’Arga, Portugal. Photo by mvcaf on / CC BY-NC-SA

“Inter-unit distances were significantly smaller than those in randomly replaced data, which showed that units associate to form a higher-level social organization, or ‘herd’.

“Moreover, this herd had a structure where large mixed-sex units were more likely to occupy the center than small mixed-sex units and all-male-units, which were instead on the periphery.

“These three pieces of evidence regarding the existence of units, unit association, and stable positioning among units strongly indicated a multilevel structure in horse society,” they said.

Their analysis identified 23 units among the study population — 21 harems and two all-male groups.

They noted that the foraging areas of units largely overlapped; thus, they did not have a territory.

Noting that the larger harems occupied the centre and the all-male units tended to be at the edges of the herd, the authors observed: “In many social animals, dominant individuals often occupy the centre, forcing subordinates to the periphery.

“Applying the dominant-centre rule to group-level social relationships, our data suggest that the hierarchical relationship is between units and correlates with harem size.”

The authors said their study findings contribute to an understanding of the functions and mechanisms of multilevel societies within species.

The study team comprised Maeda, Sakiho Ochi, Monamie Ringhofer, Satoshi Hirata and Shinya Yamamoto, all with Kyoto University; and Sebastian Sosa and Cédric Sueur, with the University of Strasbourg.

Maeda, T., Ochi, S., Ringhofer, M. et al. Aerial drone observations identified a multilevel society in feral horses. Sci Rep 11, 71 (2021).

The study, published under a Creative Commons License, can be read here

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