Is separation anxiety in horses affected by the rank of the departing herd member?

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Researchers in Poland set out to explore what occurs in a herd of horses during a short social separation of individuals differently ranked in the dominance hierarchy.
Researchers in Poland set out to explore what occurs in a herd of horses during a short social separation of individuals differently ranked in the dominance hierarchy. File image by Emma Hodgson

Horses in a herd can experience separation anxiety when one or more are temporarily removed, but does the status of the departing animal have an influence?

Horses in a herd develop and maintain a hierarchy between all individuals, with dominant and submissive members. The herd structure is usually developed and maintained through aggressive interactions.

There are many situations in riding facilities and studs in which horses have to be separated from a group, for the likes of a riding lesson, training or veterinary care.

Anna Stachurska and her fellow researchers at the University of Life Sciences in Lublin, Poland, set out to explore what occurs in a herd of horses during a short social separation of individuals differently ranked in the dominance hierarchy.

The study employed 12 adult Arabian mares kept at a breeding facility. The mares, aged 5 to 10, were kept in individual stalls, but were turned out together for nine hours each day.

The researchers undertook a behavioural test before the main experiment to determine the rank order of the mares in the herd.

The study involved three tests, when a dominant, mixed and submissive three-member group of mares was separated from the herd of 12 for 10 minutes.

The response of the remaining herd was determined by their behaviours, locomotor activity and cardiac parameters.

There were clear changes in emotional arousal within the remaining herd, the researchers found, but the response did not depend strictly on the composition of the separated mares in terms of their dominance in the hierarchy.

“The social herd was not indifferent to the separation of some individuals,” they said. “The remaining horses noticed the incident and expressed their agitation with various behaviours, locomotion and changes in nervous system activity.”

The emotional state of the mares in the remaining herd was evident from a higher rate of many behaviours, including their vocalisations, their high head and tail positions, aggressive postures, play, and ear position, as well as prolonged trotting, cantering, jumping, and an elevated heart rate.

“These behaviours, apart from play, are typical for increased vigilance and agitation.”

The return of the individuals after the short separation elicited a less distinct response in the herd, they said. “It seems that the return of the separated mares began to calm down the herd, which sensed this state as restoring normality.”

They said the differences in the composition of the group of separated mares, which differed in the dominance hierarchy, influenced the response, but inconsistently.

“It may be suggested that the mares in the remaining herd were indifferent to which mares were separated.

“The separation turned out to be stressful for the remaining herd despite the fact that the horses might have been habituated to such a procedure within everyday routine.”

They continued: “Our results indicate horse owners should take into account agitation in the social herd during the separation of some of its members and provide peaceful conditions for such a procedure, whereas the rank of separated individuals does not have to be regarded.”

Despite the fact that separations are constantly repeated in practice, they are stressful for the horses, they concluded. “The inconsistent response to the separation of differently ranked members may mean that both dominant and submissive members of the herd fulfil roles in the hierarchy, and their department deranges the composition of the herd.”

The study team comprised Stachurska, Anna Wiśniewska, Witold Kędzierski, Monika Różańska-Boczula and Iwona Janczarek.

Stachurska, A.; Wiśniewska, A.; Kędzierski, W.; Różańska-Boczula, M.; Janczarek, I. Behavioural and Physiological Changes in a Herd of Arabian Mares after the Separation of Individuals Differently Ranked within the Dominance Hierarchy. Animals 2021, 11, 2694. https://doi.org/10.3390/ani11092694

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

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