Behavioral Effects of Spay

HAHD_naughty dog


Many guardians of spayed dogs report significant behavior changes (skittish, aggressive, anxious) after their dog fully recovers from the spay surgery. These are generally guardians who chose to spay their best friend based upon one or more of the following reasons:

  1. the recommendation of their vet that spay would provide the best possible long term psychological and physical health for their dog
  2. the assertion of vets and behaviorists that their dog would behave better and become more manageable once spayed (see section “A New Study Sheds Some Light”  below for further discussion of outcomes)
  3. state law requiring all female dogs adopted from shelters/rescues must be spayed
  4. a contract with a breeder that requires spay at a prescribed time after purchase of the female dog

Alternatively, many people acquire their female dog after she has been spayed and rely on the assertions of shelter staff that once out of the stressful shelter environment and secure in her new home, she will calm down and be better behaved. In either case, guardians may find their dog exhibiting unacceptable behavior, and wonder what it is they have done wrong.

Unfortunately, as observed by Dr. Stanley Coren in Psychology Today, the latest peer-reviewed studies show the behavior changes in female dogs created by spay are not beneficial as generally asserted by vets. In fact, the studies show that spay actually increases aggression, fearfulness and anxiety in the recipient, as well as making them more difficult to train.

In the San Francisco Bay Area, where almost every female dog encountered in public places (e.g., dog parks, neighborhood walks) has been spayed, a relatively new problem is being observed. Specifically, almost every female dog is described by their guardian as “shy” (i.e., fearful, skittish, retreats from new people or situations), or “does not do well with people/dogs she does not know” (i.e., fearful and aggressive towards people or other dogs that scare her). From our perspective, the body language these dogs exhibit is not that of a confident, happy dog. In fact, this type of behavior in dogs seems unnatural and antithetical to the species. For example, who would ever expect to meet a skittish or unfriendly Golden Retriever?

Another common behavior issue guardians of fearful dogs share is an untoward reaction to stressful situations and/or environmental issues (e.g., thunder, fireworks, other loud noises, new places). The behavior their dogs exhibit ranges from mild anxiety to a full-blown “panic attack“. Separation anxiety is another undesirable behavior often exhibited in fearful dogs. Most guardians have no idea why their dog reacts as they do, and most are looking to determine what they can do to help their dogs. As a last resort, some guardians find themselves relinquishing these dogs back to a shelter because the dog’s behavior is not manageable for them. The San Francisco Society for the Prevention of Cruelty to Animals (SF SPCA) is the SF Bay Area’s largest and most progressive adoption agency, and their response to the widespread behavioral problems is to offer specialized “bravery classes for shy dogs“. However, they have one very significant caveat:

These classes are not suitable for dogs who have shown fear-based aggression or reactivity towards people or dogs, including lunging, snapping or having a history or incident of having bitten previously.” 

Can the evidence of significant canine behavior problems here in the SF Bay Area be tied to our high percentage of spayed or neutered dogs? Are the costly SF SPCA bravery classes being filled with behavior impaired students that the SF  SPCA helped to create through their aggressive spay/neuter campaign? Our study of the stress-response system in spayed/neutered dogs leads us to believe spay/neuter adversely affects the dog’s behavior and personality. In fact, the documented changes in the stress-response system would predict an increased level of fear and anxiety, which in some dogs can result in aggression.

When we examined the peer-reviewed studies on the topic of behavior after spay (see below), we find the studies confirm behavior is adversely impacted by spay/neuter and these changes are serious and potentially of epidemic proportions. So let’s see what the experts have to say…


The study “Non-reproductive Effects of Spaying and Neutering on Behavior in Dogs”57 utilizes the “Canine Behavioral Assessment and Research Questionnaire (C-BARQ)”15, the only peer-reviewed, reliable, standardized method for evaluating and screening dogs for the presence and severity of behavioral problems. The overarching conclusion of the study is:

For most behaviors, spaying is associated with worse behavior, contrary to conventional wisdom.

Specifically, the study found:

  • Spayed females are more aggressive towards people
  • Spayed females are more fearful and sensitive to touch/handling
  • Spayed females beg for and steal food more often
  • Spayed females are more aggressive towards other dogs
  • Spayed females are less energetic
  • Spayed females roll in and eat feces more often
  • Spayed females lick people and objects more often
  • Spayed females self-groom and bark excessively

Another study, “Behavioral and Physical Effects of Spaying and Neutering Domestic Dogs” 16, also utilizes C-BARQ. The study concludes:

“The overall trend seen in all these behavioral data was that the earlier the dog was neutered [or spayed], the more  negative the effect on the behavior.”

Specifically, the study found:

  • Spayed females are more aggressive
  • Spayed females are more fearful
  • Spayed females are more anxious
  • Spayed females are more difficult to train
  • Spayed females are less responsive to cues

The study also determined:

“The other three behavioral categories examined (miscellaneous behavior problems, attachment and attention seeking behavior, and separation-related behavior) showed some association with neutering [or spaying], but these differed more substantially depending on the age at which the dog was neutered [or spayed].”

Likewise, a study conducted in 2014, “Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas”,109 concluded the younger the age at gonadectomy (spay or neuter), the earlier the mean age at diagnosis of a behavioral disorder, or fear of storms. It is clear to us that sterilization should not entail removing hormone producing glands, and this study’s conclusions seem to concur:

“Additional studies are needed on the biological effects of removing gonadal hormones and on methods to render dogs infertile that do not involve gonadectomy.”

The study, “Behavioral Assessment of child-directed canine aggression” 56, evaluated dogs who had already bitten a child. The study concludes:

“Historical evidence of fearful or anxious behavior in response to loud noises and thunderstorms or separation from the owner may signal a predisposition to biting in threatening situations related to anxiety or fear…

“…Fear-related aggression was the most common primary behavioral diagnosis in the dogs….Most dogs (93%)…both male and female were neutered [or spayed].  Although our data did not include age at neutering [or spaying] or whether the surgery occurred before or after the appearance of aggressive behavior, it is apparent that neutering [or spaying] does not guarantee a reduction of aggression in dogs.” 

The study, “Behavioural effects of ovario-hysterectomy [spaying] on bitches”58, determined that:

“Spaying is accompanied by the risk of certain behavioural changes. There is a risk of increase in indiscriminate appetite. More importantly, there is a risk of increase in dominance aggression towards family members.”

The study, “Effects of ovariohysterectomy [spaying] on reactivity in German Shepherd Dogs”,  concluded:

“The results revealed that reactivity was increased in the ovariohysterectomy dogs in comparison to the intact group.”59
(Note: in this study the term reactivity refers to barking, growling, snarling, lips lifting or curling, head up, ears forward, staring, widely opened eyes, lunging and/or jumping)

The aforementioned studies document the behavioral changes and diminished cognitive skills observed in spayed female dogs. However, they do not provide a persuasive explanation as to why or how spay creates the changes in behavior. It is our belief that the trauma of the spay surgery itself, as well as the interruption of female sex hormones, primarily but not exclusively estradiol, create structural and functional changes in the brains of spayed dogs. Please see our “Stress Response System” page for a more complete explanation of the structural changes in a spayed dog’s brain.

A 2012 study in Neurodegenerative Diseases explored the association of hysterectomy and oophorectomy [(i.e.,ovary removal)] with the subsequent risk of cognitive impairment or dementia in women. The study concluded:

“Compared with women with no gynecologic surgeries, the risk of cognitive impairment or dementia was increased in women who had hysterectomy alone, further increased in women who had hysterectomy with unilateral oophorectomy [ovary removal], and further increased in women who had hysterectomy with bilateral oophorectomy [ovary removal]. The risk increased with younger age at the time of the surgery…Estrogen deficiency appears to play a key role in these associations, and estrogen therapy may partly offset the negative effects of the surgeries.”191

In 2017, researchers in Italy published the first study to document the adverse effects on cognitive skills when female dogs were subjected to removal of their ovaries. Spatial learning, memory and reversal learning skills were assessed in 64 pet dogs. The findings of the study were:

  • Speed of learning and accuracy was highest in intact females.
  • Success rate in the learning and memory task was highest in intact females.

These results were the first evidence of a non-reproductive effect (i.e. an  effect on brain function) of ovariectomy specifically in dogs.184

There is also a body of research in both men and women with respect to brain chemistry and its effects upon cognitive skills and personality. Consider the findings of this systematic review104 :

“…that responses to estradiol are present in brain regions that are not directly associated with reproductive success but are important for learning, memory, emotional responding, mood, and sensorimotor control…there is substantial evidence for the therapeutic benefits of estrogens in the brain.”

One aspect of this relationship that has been studied extensively is the interaction between estradiol and serotonin. In humans, estradiol supports serotonin production and delays its breakdown in the brain. Functionally adequate levels of serotonin in the brain are important for healthy mood, learning and memory.164 This would explain why mood disorders often accompany the loss of estradiol in menopause.

The loss of female hormones as a result of the spay procedure mimics menopause, however the loss of hormones is much more abrupt and occurs at a time of life that is entirely unnatural. It is not unreasonable to expect the incidence and severity of mood disorders seen in spayed dogs to be unpredictable and most likely much more complicated than that of menopausal women. Consequently, we also find that the traditional medications we use for humans to enhance levels of serotonin (SSRI’s) do not often provide relief and can actually make the situation worse for dogs. Most female dogs are spayed long before they are even fully developed physiologically. Highly potent stressors (i.e., spay surgery) early in life while the dog’s stress response system is still developing, can have detrimental effects on behavior and personality that are permanent.85

We believe the best opportunity to resolve behavior problems and cognitive deficits in spayed dogs is first to restore normal levels of female hormones, i.e. provide hormone replacement therapy.165 On our “Countering the Effects of Spay” page we will explore hormone replacement therapy and additional steps you might take to minimize spay’s effect on your dog’s behavior, cognitive skills, and ability to respond to stress.


Previously, we mentioned that quite often the vet advises the guardian/owner of an intact female dog that spay will improve her behavior. The current peer-reviewed studies explored on this page claim otherwise. However, social media is replete with testimonials where the guardian/owner extols the value of spay surgery for helping their female dog to behave better. We are often left pondering the disconnect between the actual science and some people’s personal experience.

Several of our readers have reported to us that after following our advice to keep their female dog intact, somewhere between the ages of 5-12 months, the dog’s behavior spirals out of control. These formerly well-behaved puppies suddenly seem to have little or no ability to control their impulses. They become blatantly disobedient when their guardian/owner attempts to rein in the errant behavior. Was this even more evidence of  the “Canine-Human Connection? Are female dogs exhibiting bad behavior because they are going through the canine equivalent of human puberty? Human parents are well aware of the difficulty in dealing with young girls at the time they are reaching sexual maturity. However, over time, these young girls become young women and their behavior tends to normalize. Likewise, those guardians/owners who hung tough and resisted the pressure applied by the veterinary industry to spay their puppy, reported that eventually the puppy just seemed to grow out of this bad behavior stage. Perhaps those reporting better behavior after spay had the procedure done at the recommendation of their vet when their female dog was in the throes of the dramatic hormonal changes of canine puberty. The dog’s behavior may seem improved for the immediate circumstance.  However, over time, as hormonal balance was lost, we felt confident their dogs would fit into the study results reported here.

In May of 2020, a new study174 was published in Biology Letters, and discussed by the researchers online at “The Conversation” (a network of not-for-profit media outlets that publish news stories written by academics and researchers):

“…dogs have a passing phase of reduced obedience towards their owners during puberty…Perhaps the most important thing to note for dog owners is that these behaviour changes were a passing phase. By the time dogs were 12 months old, their behaviour had returned to how they were before puberty, or in most cases, had improved…This is crucial for any new dog owner to be aware of, because sadly, adolescence is the peak age when dogs are abandoned and end up in animal shelters. It’s also extremely important that owners don’t punish their dogs for disobedience or start to pull away and disengage from them at this time, as this would be likely to make problem behaviour worse in the long run, as it does in people.”

We believe the results of this new study provide further guidance for dog guardians/owners who decide on hormone-sparing sterilization for their female dog rather than spay.