Testosterone and Aggression in Dogs - Separating Facts from Myths

1. Testosterone - A Brief Neuroendocrine Primer

Testosterone is a steroid hormone produced primarily in the Leydig cells of the testes in males, with smaller amounts produced in the adrenal glands of both sexes. In females, the ovaries also produce testosterone, though at significantly lower levels. Testosterone exerts its effects through two main pathways:

1. Genomic (androgen receptor-mediated) pathway - Testosterone binds to androgen receptors (AR) in target cells, leading to changes in gene expression that alter cellular function over hours to days.

2. Non-genomic pathway - Testosterone acts directly on cell membranes or is converted to estradiol via the enzyme aromatase, exerting rapid effects on neuronal excitability and neurotransmitter release.

Testosterone increases during sexual maturation, but the timing, magnitude, and behavioral significance of this rise vary considerably across individuals, breeds, and developmental trajectories. Rather than following a single fixed timeline, endocrine maturation in dogs appears to show substantial individual variation. A two-stage organizational-activational framework helps explain why gonadectomy (castration) does not exert complete control over sexually dimorphic behaviors: prenatal testosterone exposure has already contributed to the long-term organization of brain structures, and later removal of circulating testosterone cannot reverse those established developmental effects.

2. Testosterone and Aggression - A Modulatory, Not Causal, Role

The relationship between testosterone and aggression is frequently oversimplified in popular discourse. A more accurate understanding requires distinguishing between baseline levels, acute changes, and the concept of behavioral modulation.

2.1 Baseline Testosterone and Aggression - A Weak Association

Studies examining baseline testosterone levels in aggressive versus non-aggressive dogs have yielded mixed results. Dominant and/or aggressive individuals tend to show higher plasma testosterone levels than submissive and/or less aggressive animals, but this association is far from universal. A review by Knol (1989) notes that most literature concerns the effects of hormones on aggression, with little information available on hormonal influences on fear.

Crucially, correlation does not imply causation. Higher testosterone levels in aggressive dogs could be a consequence rather than a cause of aggressive behavior - aggressive encounters themselves can elevate testosterone acutely, creating a feedback loop that is often misinterpreted as a stable trait difference.

A Polish questionnaire study found that castration reduced aggressive behaviors towards other dogs and other animals, but did not change the prevalence of aggressive behaviors towards people. This differential effect suggests that testosterone influences some but not all types of aggression, and that the context and target of aggression matter enormously.

2.2 Testosterone as a Behavioral Modulator

A more sophisticated framework, increasingly supported by veterinary research, is to view testosterone as a behavioral modulator rather than a direct cause of aggression. As O'Heare (2009) notes, it may be appropriate to view testosterone as a behavioral modulator that could help bring about or escalate the aggressive state.

An intact dog may react more easily, escalate any response more quickly, plateau at a higher level of reactivity, return to baseline at a slower rate and possibly alter his baseline to a higher level than would a neutered dog. This framing shifts the emphasis from testosterone as a "cause" to testosterone as an "amplifier" - it lowers the threshold for aggressive responding without being necessary or sufficient for aggression to occur.

2.3 Non-Genomic Mechanisms and the Amygdala

An important study by Jacobs, Van Den Broeck, and Simoens (2006) examined the androgen receptor (AR) in the basolateral amygdala of pathologically aggressive dogs. While aggressive dogs showed a significantly higher number of AR-positive neurons, the researchers concluded that genomic androgen actions mediated through the AR are of minor importance in the testosterone modulation of canine aggression. Instead, the aromatase pathway - in which testosterone is converted to estradiol - is suggested to be a major mechanism through which testosterone exerts its action within the amygdala.

This finding has profound implications. If testosterone's effects on aggression are mediated largely through its conversion to estradiol, then simplistic models focusing solely on androgen receptor blockade or testosterone removal may miss the mark. It also highlights the potential relevance of estrogen in aggressiona topic that has received far less attention.

3. Acute versus Chronic Effects - Timing Matters

Distinguishing between acute and chronic effects of testosterone is essential for understanding its role in aggression.

3.1 Acute Testosterone Fluctuations

Testosterone levels can change rapidly in response to social challenges. A male dog encountering a rival may experience a transient increase in testosterone, which can facilitate escalated responding in that specific context. This acute effect is adaptive in evolutionary terms - it prepares the animal for potential conflict without requiring chronically elevated levels.

Transient flare-up effects after GnRH agonist implantation have been discussed clinically and reported in some contexts. Short-term increases in testosterone levels following deslorelin administration have been associated in some cases with transient exacerbation of intraspecific aggression, particularly in individuals with pre-existing aggressive tendencies.

3.2 Chronic Testosterone Exposure

Chronic exposure to elevated testosterone levels may have different effects, including long-term organizational changes in the brain. However, in dogs, the correlation between baseline testosterone and aggression is relatively weak, suggesting that chronic levels alone are poor predictors of aggressive behavior.

3.3 The Challenge-Persistence Link

One intriguing area of research concerns the relationship between testosterone and persistence in the face of challenges. Lower reward value is associated with increased signs of frustration and lower on-task time in controlled tests. While not directly measuring testosterone, this finding aligns with the concept that hormonal status influences how dogs respond to frustration - a key trigger for many forms of aggression.

For a deeper understanding of how frustration and impulse control interact, see our article on the neurobiology of frustration in dogs.

4. Proactive versus Reactive Aggression - Differential Testosterone Involvement

Not all aggression is the same. A critical distinction in behavioral neuroscience is between proactive (instrumental, goal-directed) aggression and reactive (affective, fear-driven) aggression. These two forms of aggression have different neurobiological substrates and are likely differentially influenced by testosterone.

4.1 Proactive Aggression - Testosterone-Associated

Proactive aggression is planned, goal-oriented, and associated with low physiological arousal. It includes behaviors such as resource guarding, territorial defense, and inter-male conflict over mating opportunities. This form of aggression is more likely to be influenced by testosterone, which is consistent with the observation that castration reduces inter-male aggression in approximately 60-75% of cases.

A seminal survey by Hart and Eckstein (1997) found that at least in intermale aggression, aggression was reduced by neutering in 60% of cases, with rapid reduction in 25% and gradual reduction in 35%. Neilson, Eckstein, and Hart (1997) found that approximately 25% of adult dogs that were aggressive toward humans or other dogs in the household can be expected to have a 50 to 90% level of improvement after gonadectomy.

However, even for proactive aggression, testosterone is not the sole determinant. Marking, mounting and fighting are complex behaviors not wholly controlled by hormones; there is a significant learning component involved in practicing these behaviors that won't be redressed by castration.

4.2 Reactive Aggression - Minimal Testosterone Involvement

Reactive aggression is impulsive, emotional, and triggered by perceived threat or frustration. It is associated with high physiological arousal, fear, and anxiety. Reactive aggression appears to be less influenced by testosterone compared to proactive forms and may even increase following castration in some individuals.

Growing evidence suggests that neutering may increase the risk of fear, anxiety, and panic responses in some individuals and contexts. Studies have reported increases in stress, fear, anxiety, and even certain types of aggression following neutering. The Polish questionnaire study found that castration increased the number of dogs that were fearful of unfamiliar dogs/humans, as well as dogs with sound phobias.

For a detailed examination of reactivity from a neurological perspective, see our article on reactivity in dogs - a neurological perspective.

4.3 Implications for Clinical Practice

The differential involvement of testosterone in proactive versus reactive aggression has direct clinical implications. Castration is more likely to be effective for dogs displaying proactive aggression (inter-male, territorial, sexually motivated) and less likely to be effective - or potentially counterproductive - for dogs displaying reactive, fear-based aggression. Neilson et al. (1997) concluded that with various types of aggressive behavior, including aggression toward human family members, castration may be effective in decreasing aggression in some dogs, but fewer than a third can be expected to have marked improvement.

5. The Behavioral Effects of Castration - A Complex Picture

Despite decades of research, the effects of castration on canine behavior remain incompletely understood and sometimes conflicting. A 2025 comprehensive review by Arroube and Pereira notes that the conclusions drawn from existing research - both in terms of behavior and health aspects - remain inconclusive and sometimes conflicting.

5.1 What Castration Consistently Reduces

Castration consistently reduces behaviors that are directly linked to sexual dimorphism and male reproductive function:

• Roaming - Decreased by approximately 90% in the 1976 Hopkins study

• Male-to-male aggression - Decreased by approximately 75%

• Urine marking - Decreased by approximately 60%

• Mounting - Decreased by approximately 80%

These behaviors are most directly under hormonal control and show the most reliable improvement following castration. A Polish questionnaire study similarly found that castration greatly decreased incidences of roaming, mounting and urine marking as well as the dog's overall activity.

5.2 What Castration Does NOT Consistently Change

Other behaviors show inconsistent or minimal response to castration:

• Aggression toward familiar humans - Studies have found no change or minimal improvement in aggression toward household members

• Fear-based aggression - May increase following castration in some individuals and contexts

• Learned aggressive behaviors - A dog that has practiced aggression or reactivity for months may continue even after hormones drop

• Territorial aggression toward strangers - One study found that dogs neutered between 7 and 12 months were slightly more likely to display moderate to strong aggression toward strangers

A study by Roulaux, van Herwijnen, and Beerda (2020) found that the second most common answer indicated an aggression decrease in dogs castrated to correct unwanted behavior and an increase in dogs castrated for other reasons. This suggests that expectation and context - not just biology - influence perceived outcomes.

5.3 Potential Negative Behavioral Effects

Perhaps most concerning for behaviorally challenged dogs, castration has been associated in some studies with increases in certain problematic behaviors in some individuals and contexts:

• Fear and anxiety - Studies suggest increases in stress, fear, anxiety, and panic responses following neutering

• Sound phobias - Castration increased the number of dogs with sound phobias

• Fear of unfamiliar dogs/humans - Castration increased fearful responses to strangers and unfamiliar dogs

These findings challenge the assumption that castration is universally beneficial for behavior. In some dogs, particularly those with fear-based reactivity, the removal of testosterone may destabilize the nervous system. As O'Heare (2009) notes, hormones also stabilize the nervous system; too little testosterone can cause anxiety, fear, or insecurity in some dogs.

For more on how chronic stress and HPA axis dysregulation affect behavior, see our article on the neurobiology of chronic stress in dogs - cortisol impact.

6. The Influence of Developmental Timing of Castration

The age at which castration is performed has emerged as a critical variable influencing behavioral outcomes. Evidence suggests that early castration - particularly before puberty - may have different effects than castration after sexual maturity.

6.1 Early Castration (Before 6 Months)

Early castration has been associated with increased behavioral risks. A questionnaire study by Zink and colleagues (2014) found that Vizslas neutered at less than six months of age were at higher risk of behavioral anomalies compared to intact dogs.

Another study documented the responses of female dogs to approaches by strangers and other dogs. Female dogs that were neutered at the age of five to ten months responded with significantly greater aggression in these situations five months later than their intact littermates.

6.2 Castration Between 7-12 Months

Dogs neutered between 7 and 12 months show a mixed picture. One study found that dogs neutered at ages between 7 and 12 months were slightly more likely to display moderate to strong aggression toward strangers. However, there does not seem to be any connection between neutering and aggressive behavior toward familiar people and other dogs when different triggering factors are taken into account.

6.3 Castration After 12 Months

Limited evidence suggests that later castration may be associated with fewer negative behavioral outcomes. Dogs neutered at 13-18 months were found to be closest to intact dogs in aggression levels compared to dogs neutered earlier.

However, the evidence base remains limited. A 2023 scoping review on the effect of neutering timing in relation to puberty on female dog behavior concluded that there is a lack of evidence to document the impact of neutering bitches before or after puberty on behavior.

6.4 The Two-Phase Testosterone Surge

Understanding the developmental timing effects requires revisiting the two-phase testosterone surge. The first surge occurs prenatally and organizes male-typical brain structures. The second surge occurs during sexual adolescence and activates these already-established structures.

Castration before the second surge prevents the activational effects of testosterone but cannot reverse the organizational effects that occurred prenatally. Castration after the second surge removes circulating testosterone but cannot erase the behavioral patterns that have already been learned and practiced.

This framework explains why castration is more effective when performed before the full expression of sexually dimorphic behaviors, but also why it cannot completely eliminate those behaviors once they have been established and practiced. There is a significant learning component involved in practicing these behaviors that won't be redressed by castration.

7. Beyond Testosterone - The Multifactorial Nature of Canine Aggression

Perhaps the most important conclusion from the scientific literature is that testosterone alone is a poor predictor of aggressive behavior. Aggression is a complex, multifactorial phenotype influenced by genetics, early experience, learning history, other hormones, neurotransmitters, and environmental context.

7.1 Genetic Contributions

Several genes have been associated with aggression-related traits in dogs, including DRD4 (dopamine receptor), OXTR (oxytocin receptor), and serotonin-related genes such as SLC6A4, HTR1A, HTR1B, and HTR2A. Kim et al. (2024) provide a review of how these genes may influence impulse control, social behavior, and emotional regulation - all of which contribute to aggression risk independently of testosterone.

7.2 Interactions with Other Hormones and Neurotransmitters

Testosterone does not act in isolation. It interacts with cortisol, oxytocin, dopamine, and serotonin to shape behavioral outcomes. Arroube and Pereira (2025) emphasize that the effects of neutering are mediated by the interaction of sexual hormones with other hormones and neurotransmitters, including cortisol, oxytocin, dopamine, and serotonin.

For example, the relationship between testosterone and serotonin is particularly important. Low serotonin levels have been associated with impulsive aggression across species. Testosterone may modulate aggression in part by influencing serotonergic function, rather than through direct effects on aggressive motivation. 

7.3 The Role of Learning and Environment

Perhaps the most significant challenge to hormonal determinism is the role of learning. A dog that has practiced aggression or reactivity for months may continue even after hormones drop. This is because aggressive behavior - like any behavior - is shaped by reinforcement history, not just hormonal status.

If a dog has successfully used aggression to achieve desired outcomes (e.g., making a threat go away, securing a resource), that behavior has been reinforced. Removing testosterone may lower the threshold for aggression, but it does not erase the learned association. Behavioral modification is required regardless of hormonal status.

7.4 The Limitations of Current Research

The scientific literature on testosterone and canine aggression has significant limitations. Most studies rely on owner questionnaires, which are subject to bias and lack experimental control. Many studies lack control groups, are cross-sectional rather than longitudinal, and fail to distinguish between different types of aggression.

As Arroube and Pereira (2025) note, the limitations of current research, the subjectivity of pet owners' evaluations of their dogs' behaviors, the influence of various demographic factors, and the complex relationships between sex hormones and other hormones or neurotransmitters all contribute to the challenges in this field of study.

8. Challenging Oversimplified Hormonal Explanations - A Critical Perspective

Given the complexity of the evidence, oversimplified hormonal explanations for canine aggression should be critically examined and, in many cases, rejected.

8.1 The Myth of Testosterone as the Sole Driver

The popular notion that testosterone is the primary driver of canine aggression is not supported by the evidence. While testosterone modulates aggression in some contexts, particularly proactive, inter-male aggression, it is neither necessary nor sufficient for aggression to occur. Neutered dogs can and do display aggression - sometimes even more than intact dogs, depending on the type of aggression and individual factors.

8.2 The Myth That Castration Is a Universal Solution

The assumption that castration will automatically resolve behavioral problems is not supported by the evidence. As Arroube and Pereira (2025) conclude, neutering is often viewed as a potential solution, but by removing the source of sexual hormones, neutering creates the potential for both beneficial and harmful effects. Castration is not a substitute for behavioral assessment, environmental management, and training.

8.3 The Myth That Aggression Is Purely Hormonal

Aggressive behavior cannot be reduced to a single hormonal cause. Genetics, early experience, learning history, pain, stress, fear, anxiety, and environmental triggers all contribute to aggressive behavior. Addressing aggression requires a comprehensive approach that considers all these factors, not just hormonal status.

For a deeper understanding of how chronic pain can contribute to aggression, see our article on chronic pain and aggression in dogs - osteoarthritis. Pain is a far more common contributor to aggression than testosterone excess, yet it is frequently overlooked in favor of hormonal explanations.

8.4 A Call for Individualized Decision-Making

Given the complexity of the evidence and the variability of individual responses, decisions about castration should be made on a case-by-case basis, considering the individual dog's breed, age, health status, behavioral history, and the specific type of aggression being addressed.

As Arroube and Pereira (2025) emphasize, it is crucial for society to rely on professionals to provide informed advice about whether and when the procedure should be performed on a pet, taking into account the unique risks and benefits of each individual case. Broad generalizations about the behavioral effects of castration are not supported by the evidence and may lead to suboptimal outcomes.

9. Practical Recommendations

Based on the current scientific evidence, the following recommendations can guide clinical practice and owner decision-making:

9.1 For Proactive, Inter-Male Aggression

Castration is more likely to be beneficial for dogs displaying proactive aggression, particularly inter-male aggression and sexually motivated aggression. Approximately 60-75% of dogs show improvement in inter-male aggression following castration.

9.2 For Reactive, Fear-Based Aggression

Castration is less likely to be beneficial and may be detrimental for dogs displaying reactive, fear-based aggression. In such cases, addressing the underlying fear and anxiety through behavioral modification and, when indicated, psychopharmacology should take priority.

9.3 Timing Considerations

If castration is elected, timing matters. Early castration (before 6 months) has been associated with increased behavioral risks in some studies. Later castration (after 12 months) may be associated with fewer negative behavioral outcomes, though the evidence base remains limited.

9.4 Addressing Learned Behavior First

Because learned aggressive behaviors will not be erased by castration alone, behavioral modification should be implemented regardless of castration status. A dog that has practiced aggression for months or years will require systematic desensitization and counter-conditioning, irrespective of hormonal status.

9.5 Comprehensive Behavioral Assessment

Before making decisions about castration, a comprehensive behavioral assessment should rule out other contributors to aggression, including pain, fear, anxiety, frustration, and inadequate socialization. For many dogs, addressing these factors will have a greater impact on aggressive behavior than hormonal manipulation.

9.6 Considering Alternatives to Surgical Castration

For owners who wish to avoid the potential negative behavioral effects of castration while still preventing reproduction, alternatives such as vasectomy or hormone-sparing sterilization may be considered. These procedures prevent reproduction without removing gonadal hormones, preserving the potential behavioral benefits of testosterone while eliminating the risk of unwanted litters.

10. Summary – Testosterone Influence and Castration Effects by Aggression Type

Proactive / Inter-male aggression

• Testosterone influence: Moderate to high (acts as a modulator, not a sole cause)

• Effect of castration: Reduction in approximately 60–75% of cases

Territorial / Resource guarding

• Testosterone influence: Low to moderate

• Effect of castration: Variable; may decrease, but strongly influenced by learning history

Reactive / Fear-based aggression

• Testosterone influence: Low (less involved than in proactive forms)

• Effect of castration: Often unchanged; may increase in some individuals and contexts

Predatory aggression

• Testosterone influence: Very low

• Effect of castration: No consistent effect

Redirected / Frustration-based aggression

• Testosterone influence: Low

• Effect of castration: Unlikely to improve; addressing underlying frustration is essential

Note:
Individual responses vary significantly. Castration is not a substitute for thorough behavioral assessment and targeted behavioral modification.

11. Conclusion

Testosterone plays a role in canine aggression, but that role is far more nuanced than popular discourse suggests. Testosterone is best understood as a behavioral modulator that lowers the threshold for aggressive responding in specific contexts, particularly proactive, inter-male aggression. It is neither the sole cause of aggression nor a universal target for intervention.

The effects of castration on behavior are complex and sometimes conflicting. While castration reliably reduces sexually dimorphic behaviors such as roaming, mounting, and inter-male aggression, it has minimal effect on - or may even increase in some individuals and contexts - fear-based and reactive aggression. The developmental timing of castration influences outcomes, with early castration associated with increased behavioral risks in some studies.

Clinical decisions should never rely on hormonal assumptions alone but require a full behavioral and medical assessment. Aggression cannot be reduced to a single hormonal cause. Genetics, early experience, learning history, pain, stress, fear, and environmental context all contribute to aggressive behavior. Oversimplified hormonal explanations do a disservice to dogs and their owners, leading to ineffective interventions and missed opportunities to address the true underlying causes of behavioral problems.

Current evidence suggests that aggression in dogs reflects the interaction between genetic predisposition, hormonal modulation, learning history, stress physiology, and environmental context, rather than any single hormone or brain region alone. Understanding and managing canine aggression requires embracing this complexity - not reducing it to convenient but inaccurate simplifications.

References

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Roulaux, P. E. M., van Herwijnen, I. R., & Beerda, B. (2020). Self-reports of Dutch dog owners on received professional advice, their opinions on castration and behavioural reasons for castrating male dogs. PLoS ONE, *15*(6), e0234917.

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