Passive riders keep equine stress levels in check, findings indicate

Riders increase the motor laterality of horses but not their sensory laterality, preliminary research shows.
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Passive riders do not appear to affect a horse’s stress levels or mental state, fresh findings suggest.

The tendency toward one-sidedness, or laterality, in animals has been known for almost a century. It has been studied in many species, including horses, and has been linked to factors such as stress and emotionality.

It has been the subject of increasing interest in horses in recent decades. However, the impact of carrying a rider on laterality has yet to be investigated.

Sophie Schwarz and her fellow researchers set out in a preliminary study to determine whether carrying a rider changes the motor and sensory laterality of horses.

Sensory laterality refers to the preferred side of sensory organs, while motor laterality refers to the preferred side for body use.

In their study, reported in the journal Animals, 23 horses and ponies were tested for their lateral preferences in three different experiments.

Motor laterality was assessed by observing forelimb preference when stepping over a pole. Sensory laterality was assessed by observing perceptual side preferences when the horse was confronted with, firstly, an unfamiliar person and, secondly, a novel object.

All three experiments were conducted with and without a rider. During the ridden phases, the rider gave minimal aids and rode on a long rein to allow the horse free choice.

The authors said three key findings emerged from the research.

Firstly, motor laterality significantly increased in strength when the horses were ridden, but it did not change in direction. “This,” they said, “could simply be a function of the person’s weight enhancing motor preferences, but it is possible that other factors that were not in the focus of this preliminary study came into play.”

Secondly, sensory laterality did not change with a rider.

“Other studies … have shown sensory laterality to be connected with stress and emotionality, so the stability of sensory laterality in this study suggests that a largely passive rider does not have a negative impact on a horse’s mental state and stress level.

“This supports the often recommended training practices of taking breaks in training, in which the rider reduces aids, and of using a rider who remains as passive as possible when a horse is ridden for the first time, with a trainer on the ground initiating movement to reduce the chances of a bad reaction from the horse.

“However, it should be noted that all the horses in this experiment were experienced and familiar with their riders, and stress may well be more apparent in young and inexperienced horses or with unfamiliar riders.”

Thirdly, the ponies in the study were less strongly lateralised than the horses in the unfamiliar person test. This finding is consistent with the results of an earlier study, in which ponies also showed the trend of being less strongly lateralised than horses in interactions with others of their kind.

“There are many reasons why this may be the case,” the study team said. “Training has been shown to be a factor in sensory laterality, and while horses are generally handled by adults, and handled predominantly from the left, ponies are more often handled by children who are themselves less strongly lateralised and, therefore, more bilateral in their interactions with ponies.”

Sensory laterality has also been shown to vary between breeds, with less temperamental breeds showing weaker lateral bias.

“As ponies were traditionally used for mining and load carrying, and now for children, their weaker sensory laterality may reflect the calm temperament required for these types of work. However, as there were only five ponies in this study, further research would be needed to investigate this.”

A horses’ sex, age, training frequency and leading side did not influence laterality, the researchers observed.

The authors noted that laterality is found in many animals, with evidence pointing to evolutionary advantages. It can vary in strength, ranging from a weak tendency to an almost universal preference.

“In chicks, strong brain asymmetry has been shown to increase the chances of survival, as the different specialisations of the hemispheres allow them to simultaneously search for food and look out for predators.”

The horse is another example of a lateralised prey species, they said.

“In general, horses are dependent on their ability to be almost permanently alert to possible threats so that they can react quickly and appropriately when they find themselves in danger.”

To this end, horses have special sensory traits, such as laterally placed eyes, which are very sensitive to movement and allow an almost 360-degree visual field.

Laterality also plays a big part in their survival skills. The specialisation of the two hemispheres allows them to spend most of their day seeking and ingesting food while simultaneously moving with their groups and scanning their surroundings for predators.

Today, most horses live as domesticated animals, and their brain asymmetry affects not only equestrian sport but their behaviour with respect to handling and husbandry.

Their laterality has become a focus of scientific research, as it has been proposed that traditional left-sided handling might affect a horse’s lateralisation and, therefore, its training.

Laterality in horses has been linked with different factors. A connection between horses’ laterality and their welfare and stress levels has been made in previous research, with motor and sensory laterality showing a significant left shift under stress, such as a change in housing and initial training.

“This suggests that the right brain hemisphere dominates when horses are stressed or their welfare is compromised.”

The authors said further work is warranted to evaluate whether riding as passively as possible, on a loose rein, reduces the strength of sensory laterality and stress in a training context.

Such research may also seek to distinguish between horses of different sexes, breeds, ages, temperaments and training and may include behavioural and physiological parameters to evaluate stress.”

The study team comprised Schwarz, Isabell Marr and Volker Stefanski, all with the University of Hohenheim in Germany; Kate Farmer, with the University of St Andrews in Scotland; and Katja Graf and Konstanze Krueger, with Nuertingen-Geislingen University in Germany.

Schwarz, S.; Marr, I.; Farmer, K.; Graf, K.; Stefanski, V.; Krueger, K. Does Carrying a Rider Change Motor and Sensory Laterality in Horses? Animals 2022, 12, 992.

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

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