Dog neutering (castration for males and spaying for females) is standard practice in many countries, but a growing body of research is challenging the status quo. A 2025 review by Ana Arroube and Alfredo F. Pereira of the Universidade de Évora in Portugal examines the effects of dog neutering on behavior, and shows that it’s not a risk-free intervention. As veterinarians and pet owners become aware of these effects, they’re beginning to reconsider how, when, and even whether to neuter individual dogs.
Why dog neutering became standard practice
According to the World Organisation for Animal Health, the global dog population exceeds 700 million, of which 75% are free-roaming. For decades, dog neutering has been a central strategy for reducing this number and limiting shelter intake.
Dog neutering, or gonadectomy, prevents reproduction by removing the gonads (testes and ovaries). Many shelters and jurisdictions require it, especially for adopted animals, to reduce unwanted litters.
It also gained popularity because of its perceived behavioral benefits. Early studies suggested that neutered male dogs were less likely to roam, mount, urine mark, or fight with other males.
However, much of this early research relied on small samples and owner-reported outcomes. Little research had been conducted on females. As a result, it often overstated the consistency of behavioral improvements.
What new research tells us about behavior after neutering
In the past two decades, research on dog neutering has become more sophisticated, using larger sample sizes, improved behavioral assessments, and better study designs. The 2025 review highlights a key point: dog neutering does not consistently have the positive results that dog owners intend.
Dog neutering, particularly in males, can reduce behaviors linked to reproduction, such as roaming or mounting. In some cases, it may also reduce certain types of aggression, especially when those behaviors are hormonally driven. Because females generally show fewer of those testosterone-driven behaviors to begin with, neutering does not produce the same clear behavioral benefits in them.
At the same time, recent research has identified potential negative behavioral effects. Across multiple studies, neutered dogs were more likely to show:
- Fear and anxiety, including noise phobias
- Stress-related behaviors and increased reactivity
- Certain forms of aggression, often linked to fear or insecurity
The authors highlight that female dogs may be especially sensitive to changes in hormone-regulated systems, which can influence emotional regulation and social behavior. Several studies cited in the review found that neutered females exhibit greater reactivity and vocalization, and higher levels of nervousness and fear-linked aggression.
Hormones and behavior: a complex relationship
One reason for these varied outcomes lies in the complex role that hormones play in regulating behavior. Dog neutering removes the primary sources of testosterone and estrogen, but these hormones influence far more than reproduction.
They interact with key systems in the brain, including those involving serotonin, dopamine, cortisol, and oxytocin. These systems regulate mood, stress responses, motivation, and social behavior.
When these hormonal networks shift after dog neutering, behavioral changes can follow. For example:
- Lower testosterone levels may increase fear and anxiety responses
- Changes in serotonin may affect mood and impulse control
- Reduced estrogen may alter social bonding and stress regulation
- Lower oxytocin levels may increase aggression in females
These biological effects help explain why dog neutering can sometimes increase anxiety or fear-based aggression, particularly in sensitive dogs.
Health effects: benefits and trade-offs
The physical health effects of dog neutering are similarly complex. Neutering prevents unwanted pregnancies and reduces the risk of certain reproductive conditions, such as benign prostatic enlargement in males.
However, the 2025 review also notes associations between dog neutering and increased risks for other conditions, including:
- Musculoskeletal disorders
- Some cancers
- Metabolic changes, such as weight gain
- Urinary and endocrine issues
These risks vary depending on breed, size, sex, and age at neutering. In particular, larger breeds may face higher risks of joint disorders when neutered early.
The evidence for some benefits, such as reduced mammary tumor risk in females, is weaker than previously believed, further underscoring the need for individualized decisions.
Why results vary so much
One reason for conflicting findings is the sheer number of factors that influence dog behavior. These include:
- Traits such as breed, sex, age, and size
- Environmental factors like training, routine, and socialization
- Owner characteristics and lifestyle
Because these variables interact in complex ways, it’s difficult to isolate the effects of dog neutering alone. And the age at which dog neutering occurs appears to influence both behavioral and health outcomes. Some studies suggest that early neutering increases the likelihood of problems.
Alternatives to traditional dog neutering
Given these uncertainties, interest in alternatives has grown. The Parsemus Foundation has been active in raising awareness about options that preserve natural hormones while preventing reproduction.
These include hormone-sparing surgical approaches such as vasectomy and hysterectomy. While these methods don’t reduce hormone-driven behaviors, they may help maintain normal physiological development and stability.
Non-surgical methods like implants can temporarily suppress fertility by turning off the flow of sex hormones. This allows owners to evaluate behavioral effects of the loss of hormones before making permanent decisions.
A more individualized approach
The central takeaway from the 2025 review is that dog neutering should not be treated as a one-size-fits-all decision. The latest research encourages a shift toward individualized decision-making. Factors such as breed, age, lifestyle, and behavioral goals should all play a role in the decision.
In some cases, delaying dog neutering or choosing alternatives may make sense. The best decision is an informed one.
