Aerial drone observations identified a multilevel society in feral horses

Abstract

The study of non-human multilevel societies can give us insights into how group-level relationships function and are maintained in a social system, but their mechanisms are still poorly understood. The aim of this study was to apply spatial association data obtained from drones to verify the presence of a multilevel structure in a feral horse society. We took aerial photos of individuals that appeared in pre-fixed areas and collected positional data. The threshold distance of the association was defined based on the distribution pattern of the inter-individual distance. The association rates of individuals showed bimodality, suggesting the presence of small social organizations or “units”. 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. The present study contributes to understanding the functions and mechanisms of multilevel societies through comparisons with other social indices and models as well as cross-species comparisons in future studies.

Behavioural synchronization in a multilevel society of feral horses

“…The field site had two large flat areas, Zone 1 and 2, which were visually separated by rocky hills (see Figure 3 of Maeda et al, 2021). We separated these areas because we rarely observed horses moving between them during daytime.…”

“…The minimum frequency, or nadir, between these two peaks was observed at the 12th bin (10.1-11.0m), and we selected this as the threshold distance that divides the intra-and inter-unit association (cf. Maeda et al, 2021).…”

“…We then constructed the social network using inter-individual distance data. The photograph is also used in Maeda et al, (2021) published in Scientific Reports. Table 1.…”

“…Usually, members of a unit stay close together, while the extent of cohesion becomes smaller as the social level increases (Grueter et al, 2012;Maeda et al, 2021;Papageorgiou et al, 2019;Qi et al, 2014;Snyder-Mackler et al, 2012). Some studies have found that different units keep an intermediate distance from each other, staying farther apart than the inter-individual distance within units (Bowler et al, 2012), but closer than random distribution (Maeda et al, 2021). It is argued that this differentiation of social relationships has evolved to balance the advantages of being a large-group and the disadvantages of resource competition with other units (Moscovice et al, 2020;Rubenstein and Hack, 2004;.…”

“…For instance, our use of drones succeed in obtaining positional and behavioural data of a multilevel society composed of more than a hundred of feral horses in Portugal and showed a two layered structure of units (combinations of individuals which stayed closer than 15.5m more than 70% of the time) nested within a herd (i.e. observed inter-unit distance was significantly smaller than that of permuted data sets) (Maeda et al, 2021). In the current study, we further apply this data collection to investigate whether horse multilevel society shows synchrony in resting/moving timing, and if so, whether the extent of synchronization changes within and across units.…”