Drones reveal how feral horse units quietly police their boundaries

Feral horses studied in this research 

 (c) Tamao Maeda

Feral horses are often admired for their apparent freedom, yet life in a wild herd involves constant negotiation. New research using drone technology has now revealed just how precisely horses manage space and social boundaries when living alongside neighbouring groups. It turns out they are far more flexible and strategic than previously understood.

Unlike territorial species that defend fixed areas, feral horses in northern Portugal live in a multilevel society. The basic social unit is a family group, typically led by a stallion with several mares and their offspring. Multiple units aggregate into larger groups, gaining protection from predators and harassment by bachelor males. However, this close proximity also brings rival stallions into frequent contact, creating the risk of costly aggression.

To understand how horses manage this trade-off, a research team led by Kyoto University observed 25 reproductive units in Serra d’Arga, Portugal. Using drones, they conducted 166  detailed aerial observations and applied statistical and spatial analyses to quantify how unit shapes and positions changed as other units approached.

The results show that spacing between units is anything but accidental. When compared with randomised models, real horse units were significantly less likely to be close to neighbouring units than expected by chance. Even when proximity increased, mixing was actively avoided, confirming that horses deliberately manage their spatial relationships to maintain social order.

Crucially, the study found that horses adjust not only where they stand, but how their group is shaped. As neighbouring units came closer, a unit would become more circular and cohesive. This likely reduces the number of individuals exposed along the edge, minimising the risk of accidental boundary crossings or confrontations. When another unit approached extremely closely, the formation changed again: the group elongated, effectively reshaping itself so that unit boundaries did not overlap.

These subtle but consistent adjustments suggest that horses maintain flexible, invisible boundaries rather than rigid territorial lines. By reshaping their group structure in response to their neighbours, they balance the benefits of aggregation with the need to avoid conflict; a sophisticated solution in a fluid social environment.

There was, however, one remarkable exception. Two units, led by stallions named Kobe and Uzumasa, repeatedly broke the rules. Unlike all other pairs, these units frequently crossed boundaries and intermixed, doing so in 21 out of 59 observations. This behaviour was not a one-off anomaly: similar interactions had been recorded as far back as 2016. No other units in this population, or in comparable studies elsewhere, have shown such high levels of mutual tolerance.

This “friendly pair” appears to represent a previously unrecognised social layer within horse multilevel societies, suggesting that long-term, unit-to-unit relationships can exist under certain conditions. Why these two stallions tolerate such close association remains unknown, but it opens new questions about alliance formation, kinship, or individual temperament in horses.

The findings offer a valuable reminder: horses are highly sensitive to space, proximity, and social context. Even in domestic settings, subtle changes in grouping, density, or layout may influence stress and behaviour more than we realise.

Overall, the study highlights that feral horses maintain harmony not through force or fixed borders, but through constant, adaptive spatial negotiation.

 

For more details, see: 

 

Spatial strategies in non-territorial societies: how feral horses maintain boundaries with other groups

Tamao Maeda; Sota Inoue; Monamie Ringhofer; Satoshi Hirata; Shinya Yamamoto  

Proc Biol Sci (2026) 293 (2063): 20252468 .

https://doi.org/10.1098/rspb.2025.2468