Stress and pressure - links to horse health and wellbeing
I believe my life’s work with horses is about attending to their all-round wellbeing in a way that gives me a deeply mutually beneficial relationship with them. By remaining ever curious about what their bodies and behaviour can teach me, I seem to receive daily life lessons.
My most recent free monthly Q&A call was on the topic of “Stress and pressure - links to horse health and wellbeing”. It was a very interesting discussion on what stress is, what pressure might mean, learning theory in training, and the links to physical, cognitive and emotional wellbeing. We had some excellent questions submitted on the topics of evidence base for lifestyle factors contributing to stress, learning styles and equine welfare, recognising subtleties in behaviours that might signify stress or release of held tension, and the possible contribution of prolonged stress to disease.
Firstly, what is stress? There are a huge number of possible definitions, but in physiological terms, stress can be defined as any condition, whether external or internal, that challenges the homeostasis of an organism (homeostasis is the body’s physiological equilibrium or balance). This therefore could include external factors such as poor nutrition or toxin ingestion, or internal factors such as pain or emotional states. In emotional terms, stress includes feelings of emotional and physical tension and mostly feels unpleasant (but this is complex and both individual- and situation-dependent - see the excellent TED talk by Kelly McGonigal on this, it will come up if you Google it). In short bursts, stress is very useful to an organism in dealing with a threat and surviving, this type of stress can be termed adaptive stress. If the organism is stuck long-term in a state of stress and unable to move out of this, this type of stress can be termed maladaptive, and might have knock-on effects on physiological and cognitive function, posture, and emotional responses.
The first question was on evidence for a link between confinement and stress, and there is indeed a wealth of scientific evidence for confinement increasing stress levels in horses. A good paper to look up is Krueger et al (2021) - “Basic Needs in Horses - A Literature Review”. The overall conclusion of this review is that restricting social contact, free movement and access to roughage (which all happen to varying degrees when horses are confined) increase measures of stress. Many people remark that their horses love coming in to their stable because they are waiting at the field gate when they arrive to bring them in. But we need to be careful about attributing motivations to behaviours. Could it be that actually they don’t have enough access to roughage in their turnout area and that they know they will receive it when they come in? Could it be that their turnout area social group doesn’t make them feel safe and secure? Could it be that, given the choice, they would come into their stable, eat all the forage offered in there, then go outside again? It is important to note that needs and motivations of individuals are complex and fluid.
The second question was on expanding on the use of positive reinforcement for optimizing equine welfare. Positive reinforcement is one of the four quadrants of learning theory, and involves something inherently rewarding or motivating to the horse (an appetitive stimulus) being added when the horse gives the desired behaviour. Negative reinforcement is where something they inherently would move away from (an aversive stimulus) is subtracted when the horse gives the desired behaviour. The most common appetitive stimulus used is food, the most common aversive stimulus used is pressure. There is a lot of nuance to both of these and there lies the crux - the learning quadrant is more of a continuum, and, as mentioned before, motivation is fluid. Is pressure always inherently aversive to horses? If so, is all negative reinforcement training aversive to horses? If an appetitive stimulus is inherently rewarding, does that make all positive reinforcement training also inherently rewarding to horses?
In my opinion, the answer to all of these questions is no, not always. Unhandled horses usually find pressure inherently aversive, even just moving towards them from thirty feet away could constitute an aversive level of pressure to some horses. Other horses that have had agency in their dealings with humans, that is, they have been communicated with in a way where they feel they have a voice and choice, and have been introduced to pressure whilst maintaining this sense, can end up seeing pressure as information and therefore not aversive in the slightest. And there can be everything in between!
In the same vein, using an appetitive stimulus, usually food, to achieve a behaviour can result in feelings of conflict from the horse, especially if the behaviour is something that causes them some degree of physical, cognitive or emotional discomfort, but they know that performing it will bring them the food. Or the process of learning the behaviour can induce a lot of frustration if they don’t understand what it is they need to do and there is withholding of food in an attempt to stimulate them to offer the behaviour. Withholding of food rewards can even tip the scales from the activity constituting positive reinforcement training to it constituting another of the quadrants, negative punishment (the withdrawal of an appetitive stimulus).
Set-up, timing and close observation of the horse’s emotional, postural and cognitive responses are key to the way either a positive or negative reinforcement approach feels to the horse. I am a big fan of using positive reinforcement, partly because it brings out the horse’s curiosity and sense of play more so than negative reinforcement training usually does, especially in the beginning stages. It also, importantly, trains the trainer very effectively - if you don’t use any pressure at all, you can’t rely on hauling the horse along or into a desired posture, you have to shape it gradually, often using your imagination and inventiveness in new ways to set the horse up to be able to offer what you’re looking for. It also gives all sorts of elegant ways of giving horses more methods of two-way communication with us, such as start and end buttons and anchor or “keep going” signals, as long as we listen very consistently to the feedback these allow our horses to give us.
The third question was on whether licking and chewing signifies a release of tension, the presence of human-induced stress, or the horse trying to regulate themselves. There is yet another huge amount of nuance to all sorts of behaviours that can be indicators of either increases or releases of tension. To explain this better I discussed some polyvagal theory (to learn more about this I highly recommend Dr Sarah Schlote’s blog article here: https://equusoma.com/the-polyvagal-theory-and-horses/).
In terms of being able to identify these different states in our horses, I think we need to learn to really slow down and make deeper observations of each individual. Often we don’t give horses enough time to process and give us true feedback, we are living at breakneck speed. Behaviours can have different meanings depending on how they are carried out - for example a dog could be sniffing a trail of scents in a leisurely fashion with a relaxed frame, or dashing about with tail held rigidly out, in a heightened state of arousal, albeit performing exactly the same behaviour. When a horse licks and chews rapidly, perhaps holding tension in their neck and with wrinkled eyes and tight lips, this would likely signify them being closer to a sympathetic state than a full release. When they lick and chew slowly, perhaps bringing their tongue out a long way, breathing deeply and sighing whilst shaking their head and their neck muscles wobbling like jelly, it’s likely to mean a release. We can’t really be absolute about this - they are moving about on a continuum, not stepping into neatly boxed individual states. And we need to keep in mind we can never fully know. But if we repeatedly give them the chance to show us by slowing down and remaining open and ever curious, rather than immediately labelling behaviours as one thing or another, they can start to teach us how to better observe and interpret their behaviours.
Lastly, we had a question about whether equine Cushing’s disease might be associated with prior or prolonged stress. I love this question! I think it’s so important to ask these sorts of questions, and often many fascinating insights can be made when we do. There isn’t necessarily a proven link between prolonged stress and equine Cushing’s disease, also known as PPID or pituitary pars intermedia dysfunction. However there are a few insights from the human field that are interesting threads to think about.
The first is that stress does increase oxygen demand of tissues and free radical production, which is involved in the pathophysiological changes to the pituitary pars intermedia in PPID. This does not explain why this would cause PPID in some horses but not others. The second is that there is an established link between microbiome changes in Parkinson’s disease in people, which shares some pathophysiological features with PPID. Both diseases can be described as neurodegenerative disorders and both are dopamine-associated. The microbiome changes associated with Parkinson’s disease in people might be brought about by chronic stress, and affect the enteric nervous system and ultimately the entire autonomic nervous system and the brain. The equine microbiome also changes in states of chronic stress. The last is the field of psychoneuroendocrinology, explained eloquently by Gabor Maté in his wonderful book “When The Body Says No”. He has found that many chronic health conditions in people have an associated psychological profile. The human psychological profile for Parkinson’s includes some history of not heeding signals from the body to stop unrewarding activities, as there is a role for dopamine interactions not only in reward but in learning when to stop seeking uncertain rewards, in order that the organism does not exhaust themselves or put themselves in danger. I’m wondering whether there would be anything in horses having had a lack of agency contributing here. Perhaps not at all, but worth at least pondering.
It seems that the main take-aways from this discussion were that this subject is indeed complex, that remaining open and curious to our horse’s behaviours and motivations is one of the best forms of learning from them, and that the two-way communication we can set up as a result is just as helpful for us as it is for them.
