Should I Neuter or Spay my Dog? The Definitive Guide.

Neutering may be one of the most important ways you can help your dog…or is it?

When you neuter or spay your dog, you’re not just preventing unwanted puppies, you’re also taking steps to help protect their health and happiness. Of course these benefits are not without consequence and there is some debate on the issue.

Like anything argued online, there is a large amount of misinformation floating around on the topic, arguing both for and against the neutering of dogs.

To help clarify the topic, we’ve compiled what we believe is an objective and well researched summary on the issue that should help most dog owners decide whether they should spay or neuter their dog.

Why is a dog photographer writing about this?

As those of you who already follow the Puptrait Studio probably know, we’ve worked with thousands of dogs from all walks of life, handing out belly rubs to street born strays that have never been collared, provided probono image services to foster dogs recently snatched up from death row in hopes to get them adopted quicker, and photographed champion show dogs who are actively being bred on account of how well they represent their breed standard.

The fact is, we’re uniquely incentivized to remain unbiased on the issue and we’re going to do our best to keep this article that way. Meaning, we’re going to treat this as a living document. If you have research or arguments you believe we’ve missed, share it with us in the comments and we’ll update the article if we believe it’s warranted.

That said, we are not veterinarians and the summary included below should not be considered a substitute for medical advice. Only a properly trained and licensed veterinarian is qualified to recommend what is medically best for your pet. All we’re trying to do here is to help fellow dog owners help sort through the noise.

Spaying and Neutering Terminology

For the purposes of our discussion we will be using the term “Neuter” to refer to the surgical sterilization of dogs of both genders using the most common procedures currently performed young healthy dogs in the United States. Specifically for females, this procedure involves removing the ovaries and uterus (hystero-oophorectomy) or only the ovaries (oophorectomy) via an abdominal incision, and not any of the nonsurgical alternatives, tubal ligation or the “Ovary Sparing Spay” method. For males, by neuter we specifically mean castration or the physical removal of the testicles (gonadectomy), and not any of the non surgical alternatives or vasectomy.

Do you intend to breed your dog?

If your answer is “yes, I am definitely breeding my dog” there isn’t going to be a lot for you in this article and you may as well stop reading now. However, if you’re unsure about breeding your pup or even thought, “Yes, I want puppies” this may be of relevance to you. If you don’t understand why “I want puppies” and “I want to breed my dog” are different, definitely keep reading.

Why does anyone care if I neuter my dog?

Pet over population in the United States is an emotional topic for many simply because of how many unwanted dogs are euthanized every year.

Roughly 4 millions dogs enter shelters every year. 26% of these dogs are matched back up with their owners. Another 35% are adopted out to new families. But the remaining 1.24 million dogs are killed simply for lack of resources or being unwanted(1).

And, with only 10% of dogs neutered prior to entry in shelters, there’s an assumption that many of these deaths could be avoided if there were less unplanned dog pregnancies, i.e., the higher the neuter rate the lower the euthanization rate will become.

You might be thinking, “those are stray dogs, what does my dog have to do with any of this?”. The problem of course there being that the vast majority of stray dogs were at one point owned by someone and many of the pets in shelters are lost pets who weren’t properly kept indoors or provided with identification, i.e., pets owned by people who assumed they’d never get out and have a chance to breed.

Besides the public health and over population concerns, are there benefits to neutering a dog?

In fact, there are several. When you neuter your dog, you’re not just preventing unwanted puppies, you’re also taking steps that may help protect their health and happiness.

Sterilized Dogs Live Longer Lives

In a study the included over 40,000 dogs researchers discovered that sterilization was strongly associated with an increase in lifespan (2). That finding in itself wasn’t particularly surprising, as research of other species have demonstrated similar results. What was interesting was how the longevity was achieved. In fact, the researchers recommendation in the abstract was to begin focus on why dogs were dying, rather then when they were dying.

Researchers found sterilization prevented some diseases but encourage others

Not mating has the benefit of decreasing the risk of death from certain obvious cause, such as infectious disease, and has been noted as reducing the risk of certain cancers. But simultaneously also associated with increased risk of other types of cancer, hypothyroidism, and other diseases, such as hormone-responsive dermatoses, a diseased condition of the skin that can lead to hair loss or balding.

But it’s important to remember that despite these trade offs and regardless of any diseases neutering may cause or be associated with, on average neutered dogs objectively live longer.

Neutering Improves Behavior (Sometimes)

When your dog is surgically neutered the testicles or ovaries are physically removed from the body of the dog, eliminating the possibility of your pet producing sperm or eggs. But that’s not all these organs do.

Both testicles and ovaries serve as a secondary roll as glands within your dog’s endocrine system. These organs, along with your dog’s adrenal and pituitary glands, produce and regulate the sex hormones testosterone and estrogen. These sex hormones are identical to the testosterone and estrogen hormones found in humans, in fact, much like human teenagers, dogs also experience puberty, with testosterone levels in male dogs surging at 4-5 months and peaking around 10 months, falling to to adult male levels by 18 months of age(4).

While the pubescent hormonal surge has been shown to have an effect on the anatomy of the brain, increasing the magnitude and frequency of sexually dimorphic behaviors, those behaviors do not originate at that time. Rather, the dog is simply continuing to act on what were already anatomically as the result of an initial prenatal hormonal surge. (4)

Which when you think about it, actually explains what appear to be otherwise two seemingly conflicting arguments on the topic, those of course being:

You should neuter your pet as soon as possible to prevent their brain (and personality) from being shaped by increased levels of hormones. And, it is around the 6 month point that dogs begin to establish what they eventually understand to be socially “normal”, including the concepts of pack hierarchy, dominance and the significance of human commands.
It doesn’t matter when or if you neuter your dog, as either way it is born knowing how to dog. And, the only thing you are stopping a dog from is expressing “roaming” behaviors.

More over, this two stage hormonal surge may also explain why it is not uncommon for prepubescently neutered male dogs to remain reactive to unaltered males. Something I have personally only heard explained with a speculative “maybe they were jealous?” or the fault of failed training. The latter of which may or may not be true, but it should be said that “training” is generally understood as the education of an animal to react in a way differing from its own instincts or reflexes.

In contrast, consideration of a dual stage hormonal process shaping development may explain why most neutered dogs are unreactive and others who may have developed differently prenatally, require more extensive training. With the idea being that behaviors are rarely an on or off pattern, but instead something that falls along a sliding scale or spectrum of reactionary behaviors.

This grey scale versus black and white understanding puts an interesting spin on how we interpret behaviors. As mentioned above, most dog owners and canine experts, despite disagreeing about the benefits of neutering, agree that gonadectomy dramatically reduces or outright eliminates roaming behaviors, such as digging under a fence or hopping a gate to reach a female in heat. Which might suggest that while we may perceive aggressive behavior as more serious, roaming is likely on a more extreme end of the reactionary scale, or alternatively, on an entirely separate scale all together.

Which while not a significant point for this discussion, depending on future research findings, could have interesting, if not outright revolutionary, implications in how we train dogs and understand how to help them overcome their behavioral hurdles.

Neutered Dogs Are Less Likely to Kill

Contrary to that alarmist heading, fatal dog attacks, while unquestionably serious and tragic, are surprisingly rare in the United States.

As we mentioned above, there are roughly 4 million dogs enter shelters every year in our country. But that count only includes surrendered and captured stray dogs. It’s estimated that there are another 70 million dogs homed in the United States (1).

In comparison, there were only 34 fatal dog bite attacks reported in the United States during all 2015 (6). 7 (20%) of those fatal attacks involved breeding on the dog owner’s property, either actively or in the recent past. In comparison, only 3 (9%) of 2015’s fatal attacks involved dogs rehomed by shelters or rescues. 28 (82%) of attacks attributed to “Pit Bulls” (I use quotes here simply because of notoriously inaccurate the “Pit Bull” label tends to be).

It’s important to note, these sample sizes are very small. However, numbers year over year remain fairly consistent in years past.

2014 – 42 total, 13 (31%) involved breeding, “Pit Bulls” contributed to 22 attacks
2013 – 32 total, 5 (16%) involved breeding, “Pit Bulls” contributed to 25 attacks
2012 – 38 total, 5 (13%) involved breeding, “Pit Bulls” contributed to 23 attacks
2011 – 31 total, 9 (29%) involved breeding, “Pit Bulls” contributed to 22 attacks
2010 – 33 total, 11 (33%) involved breeding, “Pit Bulls” contributed to 22 attacks

Data prior to 2015 is scarce for tracking attacks by rehomed or shelter dogs. Which to be fair to our source, may be a new statistic category they are tracking or for all we know it could be a purposeful oversight, as the rescued number doesn’t exactly fit their fairly heavy handed breed specific, “Pit Bulls”, Rottweilers and American Bulldogs are dangerous, narrative. But nonetheless, it’s pretty telling that reported incidents involving breeding, over the course of 5 years, were never lower than the attacks reported involving rehomed shelter dogs – despite 2015 having a higher number number of fatal attacks than 3 of the remaining years.

When you consider it is estimated that anywhere from 30%-40% (~1.5 million dogs) of all adopted shelter dogs are Pit Bulls, a pretty interesting trend emerges from this data.

Shelters vary widely in their procedures – in what kind of animals they’ll accept or adopt out, what types of training or socialization experiences they lend their temporary residents and even when or how often they euthanize animals – but when it comes to neutering policies they are very consistent.

In fact, the vast majority of states, including Alabama, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Iowa, Kansas, Louisiana, Maine, Massachusetts, Michigan, Missouri, Montana, New Jersey, New Mexico, New York, Nevada, North Dakota, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Utah and West Virginia, require sterilization prior to adoption or a promise to sterilize (often with a deposit) to adopt an animal from a pound, animal shelter or pet animal rescue. And those are just state restrictions, that doesn’t include county or municipality guidelines (8).

An adopted dog is a neutered dog, almost without exception.

And, even if you assume the worst about the integrity of our sources data, that it may have been massaged, conveniently recorded or even reported singularly for the purpose of demonizing a breed, neutered dogs still buck the trend and are far less likely to kill a human.

From what we found, dog attack data from the CDC was focused more on breed specific tracking and counting the total number of attacks. However, it should be noted that one of their official recommendations to reduce the risk of attacks is to “Spay/neuter virtually all dogs (this frequently reduces aggressive tendencies).” (9)

The Science Behind These Aggression is Complicated, But Consistent

In male dogs, testosterone is responsible for changes within the amygdala, specifically the basolateral nuclei group of the amygdaloid body. The amygdala is a region of the brain believed to perform a primary role in processing of memory, decision-making and emotional reactions, which is why is commonly colloquially referred to as the “fear center” of the brain. The amygdala determines how we emotionally respond to stimuli and events that we perceive as potentially threatening or dangerous. The testosterone induced changes to this region have been associated by researchers with aggressive behavior in male dogs (5) and promote greater reactivity, meaning they are quicker to respond to averse stimuli and respond more intensely for a longer duration (10).

However, these same studies found that neutering cannot be expected to reduce aggressive behavior in all dogs and could not completely eliminate aggressive behaviors. Which is consistent with the study we mentioned earlier, from Hart, B. L., & Eckstein, R. A., that studied the two part developmental hormonal mechanism that is suspected to begin in dogs prenatally.

Females Are…. Complicated

Female dog behavior follows a similar system to their male counterparts, however the cycling fluctuations in estrogen and progesterone make understanding their hormonal influences slightly more complicated to understand and may have level obvious effects on their dimorphic behaviors. While estrogen increases for a short length of time during each estrous, progesterone remains in the system for much longer, and can have a dramatic effect on the dog. Encouraging behaviors commonly associated with pregnancy, such as nest building, milk production and guarding possessions (11).

In fact, depending on the timing of when a female is neutered, she may actually become more aggressive, encouraging “aggressive dominance” towards guardians and indiscriminate eating. It’s suspected that spaying a female within 2 months of her estrus period (colloquially known and being in “heat”) creates a sudden removal of her source of progestins (which tend to have a calming influence on animals), promoting irritability and at times, aggression (4).

If a female consistently demonstrates dominance related aggression it may actually be a better idea to delay neutering until behavior
modification efforts have achieved more progress, as spaying the dog prior to 12 months, may increase the risk of more aggressive behavior patterns arising in the future.

Sex is Messy

Many first time dog owners fail to realize this, but both male and female dogs can be messy in their uniquely inconvenient and gross ways.

Female Dogs

Female dogs, like human females, follow a fairly regulated reproductive cycle. Roughly every 6 months they enter estrous. Estrous is similar to menstruation with some differences, namely if conception does not occur in a menstruating species, the endometrium (or uterine lining) is expelled from the body, where as an estrous species, such as a dog, will instead reabsorb the endometrium. That isn’t to say that dogs do not present a bloody discharge during their cycles, they simply do so at the beginning of ovulation, rather than afterwards.

Estrous discharge can be messy and it’s not uncommon for unaltered “inside” dogs to wear diapers especially designed for the purpose. Female dogs go into heat typically twice a year, though some breeds may have one or three cycles a year. Bleeding typically lasts 1-2 weeks, depending on the size and maturity of the dog.

Male Dogs

Male dogs don’t ovulate an as such don’t bleed or follow estrus cycles. However, they are constantly producing semen, which can tend to leak even when they are not ejactulating or sexually aroused. Generally not a large amount, at least at any given time. But enough that if your dogs get on your couch or have a favorite pillow, a casual glance will reveal the tell tale signs of an intact male – especially if either is upholstered in a dark fabric.

Pregnancy & Puppies Are Expensive & Stressful

It costs significantly more to responsibly care for a litter of puppies, requiring prenatal care for the mother, a load of shots for the puppies when they arrive and other expenses, than it does to have your dog neutered or spayed. More over it’s important to consider the stress a pregnancy or raising puppies can have on you and your family. You will experience many sleepless nights. Birth is generally a very messy affair that can last for hours, sometimes all day. Your dog could get sick during pregnancy, there are multiple opportunities for complications to arise along the way, and your dog may die in labor.

Even if things go perfectly, it’s important to remember that your puppies will not be fully house trained for upwards of 4 to 6 months. Most people when raising a litter dedicate a walled off room, dedicated for the process than is walled off from the rest of the home. It is recommend that this room  has a nonpermable floor surface, something that can be cleaned (read: mopped and scrubbed) with relative ease several times a day, as you will have anywhere from 4 to 8 animals urinating and defecting throughout the day all over the space.

Worried About the Expense of Neutering?

As we explored above, neutering your pet ales in comparison to the cost of raising puppies. But if you absolutely can’t afford it, there’s hope!

There are a number of resources and programs across the United States that can help dog owners get their dogs spayed or neutered at reduced cost or even at no cost to the owner. In the Baltimore, Maryland area our local animal control center and rescue, BARCS, does free neutering events a few times a year. They’re generally pretty crowded. At the moment, the next appears to be during World Spay Day, which is held on the last Tuesday of every February. This year’s event is Tuesday, February 28th.

For more information about reduced cost or free spay and neutering programs please visit the ASPCA’s website. They have a handy dandy search widget that will allow you to look for programs in your area.

In Summary: Should I get my dog neutered? Here are the big take aways…

  • Millions of dogs are euthanized every year simply because they can’t be homed quickly enough. Consider adoption instead of breeding.
  • Sterilized dogs live longer.
  • Neutering reduces roaming behaviors.
  • Unneutered dogs are not inherently dangerous. Neutering dogs does not completely eliminate aggression.
  • Neutering male dogs help them snap less quickly and be aggressive for shorter times, lessening the severity or chance of an attack.
  • Take caution when spaying female with dominant tendencies. It is generally behaviorally beneficial to delay sterilizing a dominant female until her behavior has been modified.
  • Never spay a dog within 2 months of her estrus period. The progestins produced by the ovaries can have a calming effect and their sudden removal may trigger aggressive behavior.
  • Fatal dog attacks on humans are very rare in the USA. But tend to occur more often where dogs are actively or have recently mated. But are much less likely to involve a neutered or adopted dog.
  • Unaltered males and females can be messy. Birth and puppies are also a mess. Really, if you’re planning on breeding dogs invest in a good mop and upholstery cleaning supplies. You’ll likely need both often.

As promised, we’re going to keep this post up, running and active. If you think we missed something, have research you think we should look at or you operate a nonprofit that helps individuals pay for spay and neutering services or breed responsibly, please tell us about it in the comments. We’d love to hear from you!



  1. http://www.aspca.org/animal-homelessness/shelter-intake-and-surrender/pet-statistics
  2. Jessica M. Hoffman, Kate E. Creevy, Daniel E. L. Promislow. Reproductive Capability Is Associated with Lifespan and Cause of Death in Companion Dogs. PLoS ONE, 2013; 8 (4): e61082 DOI: 10.1371/journal.pone.0061082
  3. Pathirana IN et al (2012) Plasma insulin-like peptide 3 and testosterone concentrations in male dogs: changes with age and effects of cryptorchidism. Theriogenology 77(3):550-557
  4. Hart, B. L., & Eckstein, R. A. (1997). The role of gonadal hormones in the occurrence of objectionable behaviours in dogs and cats. Applied Animal Behaviour Science, 52, 331-344.
  5. Jacobs C et al (2006) Increased number of neurons expressing androgen receptor in the basolateral amygdala of pathologically aggressive dogs. J Vet Med A Physiol Pathol Clin Med 53(7):334-339
  6. http://www.dogsbite.org/dog-bite-statistics-fatality-citations-2015.php
  7. http://btoellner.typepad.com/kcdogblog/2011/05/1-in-600-pit-bulls.html
  8. https://www.avma.org/Advocacy/StateAndLocal/Pages/sr-spay-neuter-laws.aspx
  9. https://wonder.cdc.gov/wonder/PrevGuid/m0047723/m0047723.asp#Figure_1
  10. Heidenberger, E., & Unshelm, J. (1990). Changes of behaviour in dogs after castration. Tierarztliche Praxis, 13(1): 69-75.
  11. Fogle, B. (1990). The Dog’s Mind. New York: Howell Book House.


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