Your Dog Isn’t “Just Hormonal”: How Hormones and Brain Chemistry Affect Behaviour at Every Age

When we talk about hormones and dogs, the conversation often begins and ends with adolescence. Your previously attentive puppy reaches puberty and suddenly seems to have forgotten everything you taught them. Recall becomes optional, other dogs become fascinating, confidence can fluctuate and behaviours that appeared to have disappeared months ago suddenly return. We shrug and say, “It’s the hormones.”

There is certainly some truth in that. Puberty involves major hormonal and neurological changes, and adolescence can be a challenging developmental period. But hormones do not suddenly stop influencing a dog once adolescence is over. An adult dog's behaviour is still being affected every day by hormones, neurotransmitters and other chemical messengers. Cortisol changes how the body responds to stress, dopamine influences motivation and reward seeking, oxytocin is involved in social relationships and attachment, and adrenaline and noradrenaline prepare the body for action. Testosterone, oestrogen, progesterone and prolactin can all influence sexual, social and maternal behaviour.

In other words, there is far more going on inside your dog's brain than simply deciding whether or not to do as they have been asked. Understanding this doesn't mean excusing unwanted behaviour. It means understanding the biological animal we are trying to train.

Hormones and brain chemistry are not quite the same thing

Before going further, it helps to separate two terms that are often used interchangeably. Hormones are chemical messengers released into the bloodstream and carried around the body. Cortisol, testosterone, oestrogen and progesterone are examples. Neurotransmitters primarily carry signals between nerve cells, with dopamine and serotonin being two familiar examples. Some substances, including oxytocin and vasopressin, blur the boundaries because they can act within the brain as well as elsewhere in the body.

Together, these chemical systems influence arousal, motivation, fear, attachment, reproduction, appetite, sleep, attention and learning. They don't programme a dog to perform one specific behaviour, but they can affect how strongly that behaviour is expressed. A dog may already have a genetic tendency towards chasing, guarding, investigating movement or being cautious of unfamiliar people, and their current neurochemical and hormonal state can make those tendencies easier or harder to express.

Adolescence: when somebody turns the volume up

Adolescence is particularly noticeable because several things are changing at the same time. Sex hormones increase, the brain continues to mature, social relationships become more important, independence develops and dogs begin experimenting much more with the consequences of their behaviour. This is why a six-month-old puppy who happily followed you everywhere may gradually become a young dog who discovers there is an entire world beyond their owner.

It isn't necessarily that the dog has “forgotten” its recall. The value of competing rewards may have changed. Following a scent, investigating another dog, chasing something moving or exploring independently can suddenly become enormously rewarding, while the dog's ability to regulate excitement and make sensible decisions in highly stimulating environments is still developing.

This is one reason training needs to continue well beyond puppy class. Behaviours learnt during puppyhood need to be practised, generalised and proofed while the dog's brain and body are changing. But adolescence is only the beginning of the hormonal story.

Dopamine: the chemical behind “I NEED to do that again”

Dopamine is sometimes described as the pleasure chemical, but that is an oversimplification. It plays an important role in motivation, reward prediction, attention, learning and goal-directed behaviour. From a training perspective, dopamine helps explain something extremely important: behaviour can become rewarding because of the anticipation of what happens next.

Think about a Border Collie crouching when it sees movement, a spaniel becoming animated as soon as it enters long grass, a Labrador spotting something it could retrieve or a terrier locating the scent of something underground. The activity itself can form part of a powerful reward system, and the anticipation of being allowed to perform that behaviour can be intensely motivating.

This is also why repeatedly rehearsed unwanted behaviours can become so difficult to change. A dog that spends every walk scanning for squirrels, lunging towards dogs or chasing cyclists isn't merely practising the physical behaviour. The whole sequence of scanning, spotting, anticipating and acting can become highly reinforcing. Preventing rehearsal therefore matters just as much as teaching an alternative behaviour.

Breed really can influence brain chemistry

This is where the subject becomes particularly interesting. It would be inaccurate to say something simplistic such as “Border Collies have more dopamine” or “German Shepherds have more cortisol.” We don't have evidence to divide breeds into neat hormonal categories like that. What we do know is that selective breeding has altered the genetic systems involved in how dogs respond to chemical signals.

Research into different breeds has found variations in genes associated with dopamine and oxytocin signalling, and these differences may contribute to variations in attention, sociability, arousal and stress-related behaviour. That does not mean every dog of a particular breed will behave in the same way, but it does show that genetics can influence the biological systems through which brain chemistry affects behaviour.

This is an important point because breed traits are not simply habits that humans have reinforced. They have been deliberately selected over generations. A Border Collie's sensitivity to movement, a spaniel's drive to search and flush, a terrier's persistence and a livestock guardian's independence all have strong genetic components. Hormones and neurotransmitters interact with those inherited tendencies, helping to influence how intensely the dog responds to what is happening around them.

Oxytocin: much more than the “love hormone”

Oxytocin is probably one of the most famous hormones in the dog world. It is often called the bonding or love hormone because it is associated with social relationships, maternal behaviour and attachment. Research has shown that mutual gaze between dogs and their owners can be associated with increases in oxytocin in both species, helping to reinforce the social bond between them.

So when your dog gazes at you, leans against you or seeks physical contact, the relationship isn't purely emotional in the abstract human sense. There are measurable biological systems helping maintain that social connection. However, oxytocin should not simply be translated as “more oxytocin equals a friendlier dog.” Its effects depend on the individual, the social context and the relationship involved.

Different breeds may also respond differently within the oxytocin system. Research into breeds such as German Shepherds and Border Collies has found associations between variations in oxytocin receptor genes and behaviours such as proximity seeking and friendliness towards humans. Interestingly, the same genetic variation does not always appear to produce the same behavioural effect in every breed, which again demonstrates how genetics and brain chemistry interact in complex ways.

Vasopressin: the less famous partner to oxytocin

Vasopressin receives far less attention from dog owners, but it is another important part of the picture. It is involved in numerous physiological processes and also has a role in social behaviour. Studies examining dogs with histories of aggression towards other dogs have found associations between the oxytocin and vasopressin systems and differences in aggressive behaviour.

This does not mean that vasopressin is an “aggression hormone.” Behaviour is rarely controlled by one chemical messenger. Instead, the balance and interaction between different systems may influence how an individual dog responds socially, particularly under stress or conflict.

This is a useful principle to remember whenever we talk about hormones. There is rarely an aggression hormone, a fear hormone or a happiness hormone. Behaviour is produced by many interacting systems, all influenced by genetics, experience and the environment.

Cortisol: not the enemy

Cortisol is another hormone that has developed something of a bad reputation. It is commonly called the stress hormone, which leads people to assume that cortisol itself is harmful. It isn't. Cortisol is an essential part of the body's stress-response system, helping to mobilise energy and prepare the body to deal with challenges.

A short-lived rise in cortisol can therefore be completely normal. The problems arise when a dog experiences repeated or prolonged stress without sufficient opportunity to recover. This is where the concept of trigger stacking becomes particularly relevant.

Imagine a reactive dog that encounters another dog on its morning walk. Later, a delivery driver knocks on the door. Children run past the garden fence, another dog barks during the evening walk and then fireworks begin somewhere nearby. Each event might be manageable individually, but stacked together they can leave the dog's nervous system in a heightened state. The owner may then say, “He reacted to something he normally ignores,” when in reality the dog's physiological state has changed and their threshold has dropped.

This is why sleep, routine, recovery and environment matter so much. Dogs do not experience life in isolation, and their ability to cope with stress is affected by what has happened earlier in the day, how well they have rested and how often they are exposed to situations they find difficult.

Adrenaline and noradrenaline: ready for action

When something important happens, adrenaline and noradrenaline help prepare the body to act. Heart rate increases, attention narrows, muscles receive additional resources and the dog becomes ready to chase, escape, investigate or defend itself. This is incredibly useful from a survival perspective, but less useful when we are asking a highly aroused dog to perform a complicated behaviour.

This is why trying to teach new skills when a dog is already overwhelmed or extremely excited is often ineffective. Their brain is currently prioritising something else. Good training therefore involves gradually teaching dogs to perform familiar behaviours under increasing levels of distraction, rather than expecting them to move instantly from a quiet kitchen to a busy environment and perform perfectly.

Testosterone: it isn't simply the aggression hormone

Testosterone probably attracts more behavioural myths than almost any other hormone. It certainly influences reproductive and sexually motivated behaviour and can affect behaviours associated with sexual interest, competition and roaming. However, it should not be regarded as an on/off switch for aggression, nor should owners assume that castration will automatically cure every behavioural problem displayed by a male dog.

If a male dog is desperately scenting, vocalising, roaming or becoming distracted because a bitch nearby is in season, reproductive hormones may be highly relevant. If that same dog lunges at unfamiliar dogs because he is frightened of them, removing testosterone does not magically teach him that other dogs are safe.

This is why behavioural assessment matters. We need to ask why the dog is behaving as they are rather than treating the behaviour itself as the diagnosis. Hormones may contribute to a behaviour without being its only cause.

Female dogs experience enormous hormonal changes too

Male hormones tend to dominate conversations about behaviour, but intact female dogs undergo dramatic endocrine changes throughout their reproductive cycle. Oestrogen, progesterone and prolactin fluctuate, whether or not the bitch actually becomes pregnant, and these changes can have a significant impact on behaviour.

Unlike humans, non-pregnant bitches naturally experience a prolonged progesterone-dominated phase after ovulation. Later hormonal changes can contribute to pseudopregnancy, sometimes called a false pregnancy. This can produce far more than physical symptoms. Some bitches may begin nesting, collecting toys, mothering objects, becoming unsettled or showing changes in social behaviour. In some individuals, guarding beds, toys or imagined “puppies” can also become an issue.

This is a perfect example of how an adult dog's behaviour can change because their internal chemistry has changed. There is nothing adolescent about it, and it is an important reminder that reproductive hormones can continue to influence behaviour throughout adulthood.

Serotonin and emotional regulation

Serotonin is another neurotransmitter commonly associated with mood and emotional regulation. In dogs, serotonergic function has been investigated in relation to impulsivity, aggression, anxiety and ADHD-like behaviour. Again, there is no simple equation where a certain serotonin level produces a particular behaviour, but it demonstrates that emotional regulation has a biological component.

This becomes particularly relevant when people describe dogs as stubborn, crazy, dominant, naughty or completely unable to listen. Sometimes we place a moral interpretation on behaviour that has very real genetic, neurological and physiological influences.

That still doesn't mean we simply accept the behaviour. It means we train the dog in front of us, taking into account how easily they become aroused, how quickly they recover, what motivates them and which behaviours they have been genetically predisposed to perform.

A Border Collie isn't a Labrador with different fur

Selective breeding matters enormously. Humans have spent generations selecting dogs to notice and respond to very different things. A Border Collie has been selected to detect movement and influence it. A spaniel has been selected to search, flush and investigate scent. A retriever has been selected to locate, pick up and carry objects, while terriers have often been selected for persistence, predatory behaviour and the ability to work independently around prey.

Livestock guardian breeds have been selected for vigilance, independence and the ability to make decisions without constantly looking to a human handler for guidance. Sighthounds have been selected to respond rapidly to visual movement, while many scent hounds have been bred to follow an odour trail with extraordinary persistence.

These behavioural tendencies involve enormous networks of genes, brain structures, sensory processing and neurochemistry. They cannot be reduced to one hormone, but neurochemistry helps power the system. Dopamine contributes to motivation and reinforcement, stress hormones influence how readily a dog responds to perceived threats, and oxytocin pathways contribute to social relationships.

That is why two dogs can stand in exactly the same environment and experience it completely differently. To one dog, the rabbit crossing the field is mildly interesting. To another, it is almost impossible to ignore. To one dog, a stranger approaching the house is irrelevant. To another, it activates generations of selective breeding telling them that unusual activity around the property deserves attention.

Neither response is magic and neither response comes entirely from hormones. Behaviour is the product of genetics, learning, environment, experience and the dog's current physiological state.

Hormones don't remove the need for training

Understanding biology should never mean throwing our hands in the air and saying, “He can't help it, it's his hormones.” Training still matters enormously. In fact, understanding hormones makes good training even more important because it helps us recognise when and where dogs are likely to struggle.

We want well-practised behaviours that remain available to the dog when excitement increases. We want recall to have thousands of successful repetitions behind it before we desperately need it. We want calm behaviours to have a strong reinforcement history, and we want young dogs to learn that seeing something exciting doesn't automatically mean chasing it.

We also need to prevent dogs from endlessly rehearsing the behaviours we are trying to change. You cannot remove a Border Collie's instinct to notice movement, but you can teach them what to do when movement appears. You cannot remove a spaniel's nose, but you can develop engagement and recall strong enough to compete with scent. You cannot prevent adrenaline appearing when something startles your dog, but you can influence what happens next. That is training.

Sometimes a sudden behaviour change needs a vet, not another training session

There is another reason owners and trainers should understand hormones. A dramatic change in an adult dog's behaviour should never automatically be treated as a training problem. The endocrine system is part of the body, just as the nervous, digestive and reproductive systems are, and changes within any of these can affect behaviour.

Pain, neurological disease, reproductive changes and endocrine disorders can all alter the way a dog feels and therefore how they behave. If a previously sociable adult dog suddenly becomes irritable, restless, fearful, unusually tired, aggressive or behaves completely out of character, the first question shouldn't always be, “How do we correct this?” Sometimes it should be, “Why has this changed?”

Behaviour is information.

Your dog's behaviour is the end result, not the starting point

When we watch a dog bark, lunge, chase, jump, freeze, sniff, guard, stare or ignore a recall cue, we are seeing the final visible part of an enormous internal process. Behind that behaviour may be genetics, previous learning, breed tendencies, sleep, stress, pain, experience, reproductive state, environment and the activity of numerous hormones and neurotransmitters.

Dopamine, serotonin, cortisol, oxytocin, adrenaline, testosterone, oestrogen, progesterone and prolactin are all part of the picture, but none of them act alone. This is why dog behaviour is rarely as simple as “reward the good and ignore the bad,” and equally rarely explained by one fashionable label.

Dogs are biological animals. Their brains change with experience, their bodies affect their behaviour, their genetics influence what they find rewarding, threatening or important, and their hormones continue operating long after adolescence has finished.

Perhaps most importantly, different dogs experience the same world differently. Understanding that doesn't make training softer or less important. It makes training smarter, because the better we understand what is driving a behaviour, the better chance we have of teaching the dog what we would like them to do instead.