How Stress Physiology Drives Reactivity in Dogs

 When a dog "overreacts," we often describe what we see: barking, lunging, freezing, spinning, or shutting down. But those outward behaviors are just the surface. And when you are looking for solutions to reduce reactivity in your dog, you need to understand what’s going on beneath the surface. 

Underneath is a fast-moving surge of hormones, nerves, and brain systems where your dog’s system is deciding whether this moment is survivable.

• Dog reactivity refers to intense, disproportionate responses to specific triggers, such as other dogs, strangers, sounds, or movement, that are driven by stress physiology rather than choice or disobedience. For a broader overview of how reactivity is defined and commonly misunderstood, see What Is Dog Reactivity?

When a dog encounters something that feels threatening, like another dog, a stranger, a sound, or a confined space, their body doesn’t wait for rational thought. The stress response activates first. The sympathetic nervous system fires. Stress hormones surge. Blood pressure rises. The body prepares for action.

Stress responses evolved to keep animals alive, so all of this is completely natural. In short bursts, stress can be useful. It sharpens attention, mobilizes energy, and prepares the body for action. The problem is what happens after the trigger.

When stress becomes intense, prolonged, or poorly resolved, those same systems can undermine learning, regulation, and recovery. The dog’s stress systems are activated in ways that exceed the dog’s capacity to cope (Beerda et al., 1999).

 Physiological stress is not a single switch that flips on or off. It is a cascade involving multiple systems:

• The hypothalamic–pituitary–adrenal (HPA) axis, which releases cortisol
• The sympathetic nervous system, which drives rapid arousal
• The release of catecholamines such as adrenaline and noradrenaline
• Cardiovascular changes, including increased heart rate and blood pressure

These systems interact dynamically. Brief, moderate activation helps dogs adapt to an unpredictable world. But when activation is intense, prolonged, or never fully resolved, it becomes destabilizing.

This is where recent research becomes particularly useful.

A crucial distinction in modern stress research is recovery.

Lensen et al. (2019) examined how dogs’ stress markers changed after exposure to a stress test. They measured both cortisol and chromogranin A, a protein released during sympathetic activation.

What they found was subtle but important:

• Dogs who showed elevated stress markers 10 minutes after the stressor appeared to have an adaptive response.
• Dogs who still showed high levels 40 minutes later were more likely to be experiencing unsuccessful coping.

In other words, stress itself isn't the problem. Lingering stress is. Some dogs startle, recover, and move on. Others escalate and stay "up," even after the trigger is gone. Their nervous systems don’t reset.

And that is where reactivity grows.

One of the strongest predictors of how a dog handles stress is whether their nervous system has learned that support reliably arrives in difficult moments.

Riggio et al. (2022) studied securely and insecurely attached dogs during a test designed to induce stress (called the "Strange Situation" for those who are familiar). Dogs who were insecurely attached with their owner showed significantly higher salivary cortisol after the procedure and higher heart rates than dogs with secure attachments with their owner.

The implication is profound: Dogs who do not experience their human as a reliable regulator carry more physiological load when stressed.

This mirrors human research. In children, insecure attachment is linked to greater cortisol reactivity and sympathetic activation during stress. The body prepares for danger alone.

This also aligns with broader research showing that social context powerfully shapes canine stress physiology. Hennessy et al. (2020) found that the presence, predictability, and behavior of humans can significantly modulate dogs’ stress responses in challenging environments. Dogs are not regulating in isolation; their nervous systems are constantly reading the safety of the social world around them.

Many reactive dogs also show patterns of hypervigilance, difficulty settling, or exaggerated responses to separation, novelty, or uncertainty. Stress physiology and emotional security are intertwined.

Venable et al. (2016) and related work describe the chain of events inside the body when a dog faces a trigger.

• Activation of the sympathetic nervous system
• Release of catecholamines (adrenaline, noradrenaline)
• Elevated heart rate and blood pressure
• Heightened vigilance and motor readiness
• Reduction in fine motor control
• Narrow attentional focus
• Fast, reflexive responses

These changes make sense in a predator-prey world. But in modern environments, like sidewalks, vet clinics, elevators, and parks, they create dogs who are physiologically primed to react.

Once these systems are engaged, access to learned behaviors becomes less reliable. Your dog may technically know a cue, but the physiological conditions that support flexible responding have been compromised. Cortical processing drops and survival circuits dominate. Your dog is not choosing the behavior in any meaningful way. Their body has launched a program that they cannot stop.

This information should reframe reactivity for dog owners. The barking or lunging is not the problem. It is the output of a body that has crossed its coping threshold. Training that ignores stress physiology is working against your dog’s biology.

If reactivity is driven by stress physiology, then effective intervention must do more than suppress behavior.

We have to:

• Reduce baseline stress load
• Improve recovery speed
• Build safety in the presence of triggers
• Strengthen regulation through predictable, supportive relationships

This is why techniques that only aim to "stop" barking or lunging often fail. They don’t change the internal state. They leave the stress system intact.

True change happens when the dog’s body learns that:

• The trigger is not dangerous
• The environment is predictable
• The human is a regulator, not a variable

Counterconditioning and desensitization rely on the dog being able to process new information. That requires a nervous system that can recover. When stress systems are chronically elevated, the window for learning narrows.

Understanding this can help you shift the goal away from suppressing reactions and toward increasing resilience, improving recovery, and reducing the cumulative stress load.

Chronic stress alters how future experiences are processed. Kartashava et al. (2021) describe how repeated or prolonged stress exposure reshapes neuroendocrine functioning and threat evaluation, biasing dogs toward heightened reactivity and reduced flexibility.

In practical terms, this means that reactive dogs are not just responding to the present moment. They are carrying the physiological memory of past stress into every new encounter.

To summarize: Treat your dog’s reactivity as primarily about inner stress-recovery, and less about "bad behavior." You need to be your dog’s rock. Your dog is living under the expectation that the world is unpredictable, so their body stays ready to act. Real change happens when you help their nervous system learn safety again, so your dog can think, recover, and choose, rather than just react. 

Progress is most reliable when behavior work is paired with respect for what’s happening inside their body, instead of just trying to control what your dog does on the outside.  

Beerda, B., Schilder, M.B., Bernadina, W., van Hooff, J.A., de Vries, H.W., & Mol, J.A. (1999). Chronic stress in dogs subjected to social and spatial restriction. II. Hormonal and immunological responses. Physiological Behaviour, 66(2), 243-254. doi:10.1016/s0031-9384(98)00290-x

Hennessy, M.B., Willen, R.M., & Schiml, P.A. (2020). Psychological Stress, Its Reduction, and Long-Term Consequences: What studies with laboratory animals might teach us about life in the dog shelter. Animals, 10, 2061. doi:10.3390/ani10112061

Kartashova, I.A., Ganina, K., Karelina, E., & Tarasov, S. (2021). How to evaluate and manage stress in dogs – A guide for veterinary specialist. Applied Animal Behaviour, doi:10.1016/j.applanim.2021.105458

Lensen, R., Moons, C., & Diederich, C. (2019). Physiological stress reactivity and recovery related to behavioral traits in dogs (Canis familiaris). PLoS ONE 14(9), e0222581. doi:10.1371/journal.pone.0222581

Riggio, G., Borrelli, C., Campera, M., Gazzano, A., & Mariti, C. (2022). Physiological indicators of acute and chronic stress in securely and insecurely attached dogs undergoing a Strange Situation procedure (SSP): Preliminary results. Veterinary Sciences, 9, 519. doi:10.3390/vetsci9100519

Venable, E.B., Bland, S.D., Holscher, H.D., & Swanson, K.S. (2016). Effects of air travel stress on the canine microbiome: A pilot study. International Journal of Veterinary Health Science & Research, 4(6), 132-139. doi:10.19070/2332-2748-1600028

Shawna Baskette is a Canadian-born dog behavior educator with over 20 years of professional research experience and a specialty focus on reactivity. She studies peer-reviewed canine behavior literature, translates evidence into practical steps, and founded The Dog Library and Reactive Dog Reset to help dogs and owners reach calmer lives together.

Do you have a reactive dog and don't know where to start? Then your next read should be What to Do With A Reactive Dog.