Thanks to strong pressure to reduce the numbers of dogs looking for homes - and often ending up euthanized - in the US, it has become the norm to spay and neuter our pets. These procedures are often done at or before the age of 6 months, so that dogs do not reach reproductive age with gonads intact. But what are the side effects of removing our dogs’ normal reproductive hormones - often before puberty - along with those body parts? During the past 30 years, more and more researchers have been looking at the issue, and showing that it’s time to revise the blanket statement that all “responsible” dog owners get their pets spayed and neutered by 6 months old.
What does the science say?
The article above neatly summarizes the results of nearly all of the relevant research on spaying and neutering as of 2018. It is a must-read for anyone seeking a quick introduction to the topic, with 52 sources for you to then dive into for a comprehensive look at the current science. It includes orthopedic considerations, rates of cancer, behavioral issues, and other health considerations (eg, urinary incontinence, hypothyroidism).
Spay and Neuter Surgery’s Effects on Dog Health, by David Lane, DVM (Nov 2018)
This is another article that summarizes much of the current research as of 2018 - just a different style and different format. It includes an easy-to-read table of pros and cons for spaying and neutering.
“As in previous studies, the new data clearly shows that the positive behavioral effects that were expected from neutering did not occur, and if anything, the behaviors of neutered male dogs tended to be considerably less desirable. Of the 100 behaviors assessed, 40 showed statistically significant differences between the castrated and intact dogs. Only four of these behaviors showed a more positive outcome as a result of neutering...
The pattern of these results is quite clear: Neutering male dogs causes an increase in aggressive behavior, fearful behavior, over-excitability, and a variety of other miscellaneous, undesirable behaviors. What is also clear is that early neutering produces an even greater negative effect on behaviors.”
“The above data is just a small sample of the significant data that were determined in this study. By using large a sample of dogs than any used previously to examine behavior in dogs, we found significant correlations between neutering dogs and increases in aggression, fear and anxiety, and excitability, regardless of the age at which the dog was neutered. There were also significant correlations between neutering and decreases in trainability and responsiveness to cues. The other three behavioral categories examined (miscellaneous behavior problems, attachment and attention-seeking behavior, and separation-related behavior) showed some association with neutering, but these differed more substantially depending on the age at which the dog was neutered. The overall trend seen in all these behavioral data was that the earlier the dog was neutered, the more negative the effect on the behavior. A difference in bone length was found between neutered and intact dogs, suggesting that neutering has an effect on bone growth, which may be related to other orthopedic effects documented in the literature. Examination of changes in bone length of gonadectomized dogs is continuing.”
“Neutered males and spayed females (Figure 2) had the highest prevalence of overweight (32.0% and 32.6%) and obesity (5.5% and 5.6%); intact males had the lowest prevalence of overweight (20.0%) and obesity (3.6%)...
Intact dogs were underrepresented in the overweight and obese categories. The overweight/obesity risk for neutered dogs reported here is also consistent with other study results.2,18 Multiple studies have documented a link between spaying or neutering and decreased energy requirements. 19-23”
Visual impacts of pediatric spay/neuter
There is often a clear visual difference between dogs spayed/neutered before puberty versus well after (or not at all). Dogs neutered early will be taller due to longer leg bones, and with more narrow heads and rib cages. They do not “fill out” the way that a fully mature dog usually does. While aesthetics is a minor consideration compared to the health and behavioral findings above, this is a symptom of the differences in bone growth when sex hormones are removed early in development.
“Pictured are two littermates at 10 months of age: The puppy on the left was spayed at 5 months of age and the puppy on the right is intact. The puppy that was spayed prematurely has longer legs, a leaner build, and less developed skull. The intact puppy has a more proportionate build and his appearance is closer to breed standard.” From Valor K9 Academy.
The same female dog, pictured before and after her first heat cycle. Note the expansion of her rib cage and more developed head/jowls. From Two Gun Kennels.
Changes in coat
In addition, a minority of spayed/neutered dogs get what is colloquially called “spay coat” (this term often gets used for both males and females, but “neuter coat” can be used for males). This is a change in length and texture of their fur, which leads to a fluffy or “woolly” appearance. It may be that the undercoat is growing longer and thicker and/or the guard hairs are more sparse or soft. This change is associated with increased matting and therefore requires more grooming. Again, this is not a serious issue all by itself, but it does reflect the fact that “sex hormones” affect more than just the dog’s ability to reproduce.
So what should you do?
As stated by veterinarian Dr Benjamin Hart, “As we learn the consequences, I think we owe it to our clients to give them that information and the dignity of the decision. Let’s put it on the table and let them decide.”
There are real conveniences to having your dog spayed or neutered. The biggest is peace of mind regarding pregnancy, especially if you have a female dog. No one wants an “oops” litter and the responsibility of raising and rehoming pups they didn’t plan for! Also with females, no dealing with heat cycles, which can be messy and cause mood shifts. Neutered males are less likely to roam, urine mark, and harass sexy female dogs. It is easier to travel with a spayed/neutered dog, and much easier to find a place to board them if needed. City/county licensing fees are often much cheaper for neutered than for intact animals.
So most owners still want to neuter their pets; it’s just a question of when?
UC Davis has published recommendations on the timing of spay/neuter for 35 different breeds, as well as mixed breed dogs divided into 5 weight categories. These recommendations are based on occurrences of joint disorders, cancers, pyometra (in females), and urinary incontinence (in females). However some of the included breeds and the giant mixed breed weight category utilize low numbers (eg, 133 Shetland Sheepdogs, 94 Saint Bernards, 116 Collies, 258 giant mixed breeds) - so in my personal opinion, it’s questionable how much weight should be given to the affected recommendations. It is a good place to start, but it is far from a complete picture.
Note that for the included giant breed dogs - Great Danes, Irish Wolfhounds, Saint Bernard, and mixed breeds weighing 87+ lbs - they state variations of “given the large body size, and physiology of late musculoskeletal development, neutering well-beyond year 1 should be considered.”
Anecdotally, as orthopedic effects are the most visually obvious difference between dogs given pediatric neuters versus left intact, and the related joint concerns were the first ones to be widely raised and spread among the general pet owning populace, many owners are choosing to neuter after their dogs are fully grown and growth plates are closed. X-rays can confirm when an individual dog’s growth plates are closed, but rules of thumb are:
Small dogs (<20 lbs): 6-8 months
Medium dogs (21-50 lbs): 12 months
Large dogs (51-100 lbs): 12-18 months
X-Large dogs (100+ lbs): 24 months
Living with intact dogs
What if you decide that you’d rather keep your dog intact (not spayed/neutered), at least until they’re fully grown? What is it really like to live with an intact dog?
Intact females
Female dogs go into “heat” or “heat cycles” usually twice a year. (Giant breeds and some primitive breeds like Basenjis may cycle less frequently.) This is when their body is preparing to grow puppies and they are fertile. The first cycle can occur as young as 6 months for toy breeds to over 1 year of age for giant breeds. (If you got your pup from a good breeder, they will be able to tell you when females in their lines tend to first come into heat.)
You may notice that her vulva is swollen and that she is occasionally dripping a bloody discharge. (It often shows as little dots where she’s been lying or sitting.) Some females will lick themselves (and even your floor or furniture) clean, while others must wear “panties” or “britches” to keep the mess off your furniture. You may notice that your girl is moody or clingy during her cycle, but others don’t show any behavioral changes.
Your female should not be left outdoors unattended, as there are common stories of males hopping fences or digging under to find the source of the sexy smells. Do not take her to dog parks or other places that males may be off leash. Be careful even with neutered males, as they can still mate and tie, even if no pregnancy would result. (Height differences and inexperience can lead to injury during mating and while tied.)
Once the swelling and discharge have faded away, your female is no longer fertile and can go back to living life as normal. The entire cycle takes 2-4 weeks. If you are taking a training class, ask the instructor what their policy is. In sports classes, it’s common for females in heat to still attend as long as they wear panties.
Separately from dealing with heat cycles, intact girls are at higher risk of mammary cancer and pyometra (a serious infection of the uterus, often requiring an emergency spay).
Intact males
Males don’t cycle; they are continually fertile and ready for action. They are more likely to want to roam and show increased interested in females (sometimes even if the female is spayed). It is important with any dog, of course, that they not be allowed to roam free in the neighborhood.
Intact males are more likely to urine mark (frequently lift legs and pee small amounts), though this is common among neutered males, as well. Sometimes remedial potty training is needed if an adolescent male starts to urine mark in the house.
Anecdotally, many people report that neutered males will be quicker to aggress toward intact males when at dog parks.