Updated: Nov 15, 2021
*This post is still being tweaked in regards to references in grammatical modification*
NOTE: Numbers in brackets denotes the reference which can be found in the reference section at the end of the article.
Neutering is a great topic of debate in our current society. For some, neutering is a no-brainer provided that you are not looking to breed from your dog; for others, it is an unnecessary procedure, which should not be done without reason. By just doing a quick google you will find a wide variety of opinions and reasonings. Traditional thinking that neutering prevents aggression in adult dogs is often at the forefront of many peoples minds, whilst others would argue that there is no true risk to neutering. With a range of conflicting and bias information, it can be difficult to determine what is fact and what is fiction. It can also be difficult to determine who to believe when even advice amongst vets can vary greatly.
To try and help you come to a conclusion of your own, I have looked into a wide range of scientific, peer reviewed research into the area of neutering. What this means is that every piece of research mentioned has been studied from a scientific standpoint, controlling many different factors to ensure that the results found are as accurate as possible. The aim of the article is to remain as objective as possible as I lay out these findings in a concise way. This will hopefully help you with your decision as to whether you want to neuter your dog or not.
The article will only focus on male neutering at this time and will not touch upon the female spay, this is because the findings in this area have significant differences which cannot be applied to both sexes. Due to this, I will be working on an article on female castration after this one.
Disclaimer: I am not a veterinary professional; therefore, I can only comment on the findings of the research to hand. Whilst I have tried to use up-to-date papers, I may also reference some older papers if I feel this is appropriate. I cannot give a conclusive statement as to whether you should neuter or not or at what age to do this but I can present the current available literature on the subject. If you are interested in the research papers, a reference list is provided at the end.
For anybody who has studied academia, you will know the pitfalls of research. Research has flaws due to methodology, samples and other variables. Throughout the article, I will attempt to pinpoint some obvious issues with particular research, but I will discuss this in general at the end.
Scientific research vs Anecdotal evidence
Scientific Research is conducted with a research question in mind. Samples of dogs are collected and controlled for a variety of differences known as variables. This helps to ensure that the results are as accurate as possible and help us to understand any trends and relationships between two things. These findings are then written and checked by a review board. These are a group of experts in the field who may have conducted other research on the same or similar topic. They look for any biases or problems with the way the study was carried out and then approve this for publications. These are then published in scientific journals for other researchers, medical & veterinary professionals and others who are interested in academia to read and understand the latest knowledge in the field. Scientific research is not always conclusive, there are often limitations such as sample size, variables that could not be controlled for or something else may have affected the results which hasn’t been accounted for.
Anecdotal evidence is that has been collected in a more casual nature. For example, this may be someone’s personal experience. This is what is often seen in regards to castration on forums, website articles and Facebook pages. This will often sound like “well my dogs have never been neutered and …..” or “well my dogs have always been neutered and….”. We need to take great care with this kind of information. Because this is more about personal experience, it is not clear whether these dogs would have experienced anything different if their castration status had been different. These samples are usually only a few dogs and it is impossible to assess what other factors were involved in these dogs lives to lead to the expressed outcome. Anecdotal evidence has its place. It can often be the basis that scientific research is based on. Enough people expressing a perceived link can often prompt a researcher to look into this in a more controlled way to see if there really is a link. The problem with anecdotal evidence is there is simply no way to know which anecdotal reports are accurate and which are not. It is not a reliable way to gain information about a medical & behavioural topic because there is no control of other potential factors when the observations were made. This means we cannot be sure what the cause of the observed outcome is.
The most obvious benefit of Neutering
One of the main (and most obvious) benefits to neutering is that by neutering your male dog, you take away the ability for them to reproduce. The risk of your male dog being the cause of an unwanted pregnancy becomes 0 after castration.
It is difficult to find exact figures on the total number of dogs who are euthanised in kennels each year due to lack of space but take a look at rescue pages and websites from around the country and you will see the number of dogs in need of new homes. For many rescues, kennels are full and there are waiting lists for dogs coming in. There are simply more dogs than there are homes for.
Many people worry that they are taking something away from their dog by not allowing them to be a parent. This isn’t how animals think. They have little concept of what could have been and for a dog who has never had a litter or been given an opportunity to tie with a bitch, they do not think that they are missing anything. Considering to breed has a huge financial consideration. Responsible breeding includes ensuring your dog will not pass on genetic conditions onto their offspring. Parents must have sound temperaments and a clean bill of health. For many breeds, this includes expensive testing. I won’t get into the many different cost factors of breeding BUT there is no denying that neutering completely eliminates the risk of any unexpected litters and therefore the financial fallout from this.
For those who do not intend to responsibly breed from their dog, the advice has always been to neuter to prevent the risk of such accidental litters. For multi-dog households which may contain an unspayed bitch, careful management may be something that owners just don’t want to deal with. On top of this, having a bitch in heat in the same house may lead to great stress and frustration for the intact male(37) which can lead to destruction and other unwanted behaviours. The reasons listed above are usually the main consideration when thinking about castration but there are also behaviour and medical effects of neutering. This is what we will be focussing on for the remainder of this article.
A note on alternatives
Something that many owners have not heard about is the canine vasectomy. This is where the testicles are kept intact and instead a small incision is made to cut the tubes. The benefits of this is that the dogs levels of testosterone remain unchanged but the dog is still unable to reproduce. This is no common procedure, however it is gaining more ground. This is something I will be looking into over the coming weeks/months and will hopefully write another article on comparing the two. For now though, I will focus only on the full castration.
Behavioural Benefits of Neutering
As mentioned above, one of the behavioural benefits of neutering can be seen if the dog lives in a multi-dog household with an uncastrated female. For a male dog, this can cause high levels of stress and frustration, leading to undesirable behaviours. Neutering may solve these urges to mate with any household bitches and ensure that unexpected tie does not occur.
Another possible benefit of neutering is to avoid the unwanted attention of neutered males. Whilst the research in this area is scarce, some studies have shown that intact males draw the unwanted attention of neutered males. Neutered males also appear more likley to respond negatively to a neutered male(38). This means that, for those concerned about unwanted attention from other dogs, castration may help with this. In addition, this may prevent your dog experiencing a negative encounter from a neutered male. It must be noted here that this is something that has been highly understudied. It is not yet understood why this would occur between a neutered and intact male. This is one of those topics which has a far greater amount of anecdotal information over any scientific fact. In the training world, this phenomenon is greatly discussed and pondered, yet there are still very little hard facts about this kind of behaviour.
Another behavioural change which is often a factored in to a decision to castrate Is trainability. This is something that has been investigated to assess whether there is any difference between the neutered and unneutered male. Owners were asked to complete a set of questionnaires to assess how “trainable” they found their dogs to be. Those who were neutered scored higher for trainability compared to those who remained intact(1).
This was also found to be the case when researchers studied a group of Shetland sheepdogs and Rottweilers however when this was extrapolated onto other breeds, the same results could not be found(4). Instead, no difference was found between the neutered and intact males. Because of this, it is possible that trainability may be affected through castration, however this may be specific to certain breeds and not others. This is something which would benefit from greater research in the area.
Sexually driven behaviours such as mounting, marking and roaming have all been traditional reasons for neutering. In regard to marking behaviours, older research found that marking, mounting and roaming behaviours could all be reduced through the neutering of the male dog. This is because a reduction in the sex hormone, testosterone, can lead to a desire to continue these sexually driven behaviours(2). When looking at marking behaviours specifically, more recent research has suggested that intact males were more persistent when wanting to mark over a female scent. This suggests that this is a sexually driven behaviour which neutering help reduce. It must be noted, however, that when looking at marking behaviours of intact dogs on the whole, they did not mark any more than those who were neutered. It was simply that they would mark more over a female scent(3). In regards to roaming, it has been suggested that neutering may reduce these behaviours in 90% of cases(3).
For dogs who display aggression prior to neutering, castration may decrease aggression(2). The limitation of this is that this was found in less than 20% of the cases studied by and other research has suggested an opposite effect (more on this later). However, according to questionnaires which were given to owners, marking, mounting and aggressive behaviours were all reduced in around 50-60% of cases when these behaviours were present prior to neutering(6).
We must take care when we are looking at owner questionnaires as there is no standard for these behaviours to be compared to. What I mean by this is that one owners perception of aggressive behaviour may have been very different to another which may have shown a very different effect. This may be the same with marking behaviours with one owner referring to only outdoor marking and another referring to indoor marking. This can lead to results which are difficult to group together and compare.
Castration was found to be effective in decreasing dog aggression that was already present in some dogs, however this was estimated to only result in improvement in less than one third of cases. In addition to this, this had little effect when aggression was due to fear of inanimate objects or unfamiliar people(2). What this means is that whilst it was not found to prevent a dog becoming aggressive in the first place, neutering may help reduce aggression which is already present. The key point to note here is that it did not in 2/3 cases and in some of these cases, the aggression became worse (more on this later).
When looking at 636 dog bites which were reported to animal control, risk factors association with biting were having a purebred status and being an intact male. Other significant factors were having an income lower than in $41,000(26). You may be wondering why income is included here. This is noted as it is a perfect example of how other factors can statistically lead to an increased risk of an aggressive display. A low income does not guarantee a dog bite because they are not directly cause one another but instead, owners who are on a lower income may not pay for formal training or behaviour modification before a bite occurs, may be less likley to seek medical intervention and help or may not be able to afford costly day-care and dog walkers whilst out at work leading to under socialisation. When reading through the research, this is something to keep in mind. Just because there is a link found between neutering and “xyz”, it does not mean that these were directly caused or cured by neutering. This is why its so important to look at all the other factors at play before we come to a final conclusion.
Behavioural Drawbacks of Neutering
A piece of research published last year found that when studying a group of neutered and unneutered dogs, neutered dogs were seen as more “emotionally unstable” in stressful situations. It was suggested that they had a greater inability to cope with stress compared to their intact counterparts. This lead to an increased chance in fear fuelled aggressive behaviours and a reduced display of social behaviours(1). In addition to this, intact males were found to be more sociable and confident compared with their neutered male counterparts.
Whilst we mentioned previously about intact males drawing the attention of neutered males, neutered males may also draw more social attention from intact males(1). The issue that was found with this was that the neutered males were less comfortable with this added attention than their intact counterparts with aggression seen more frequently among the castrated dogs. In addition to this, 15 out of 17 neutered males displayed some level of anxiety whilst out on a walk compared to only 5 Intact males. The traditional view was that intact males were more likley to display aggression but a trend in both research(38) and anecdotal accounts find that this may not be the case. In a large sample study, it was found that aggression & anxiety was seen significantly more in neutered males than intact, irrespective of the age in which neutering occurred(38). This was seen in particular towards those who were still intact.
One possible reason for this is a drop in testosterone after neutering. In humans, links have been made with a drop in testosterone and a rise in depressive episodes(38). In dogs, testosterone has been found to help with confidence which, once taken away may lead to a sudden drop in this confidence. This would therefore make sense why some dogs appear to become more fearful and anxious when they are castrated.(39)
It may be argued that the castration has caused the neutered male to become less tolerant and therefore this is a risk factor to consider for neutering your male dog. We must take care with these statements when drawing our own conclusions however. We do not know these dogs backgrounds in this study. We also do not know why he neutered dogs were castrated in the first place. This may have been in an attempt to reduce such behaviours that they are displaying, and we cannot know whether these behaviours occurred because they were neutered or whether they were present already. This is what we call Causality as we do not know which, if any, of these variables caused the other. Was the dog neutered BECAUSE they were displaying aggressive behaviours or showing anxiety OR was the dog showing aggression and anxiety BECAUSE they were neutered? These are one of the many struggles we face when looking at research into the pros and cons of neutering and something we must be mindful of when we are weighing up the research in front of us.
No difference found:
Often, castration is recommended as a “cure” for many different unwanted behaviours. Whilst we have looked at the research for both for and against, there is also an array of research showing that there is little difference between the neutered and the intact male dogs in regards to many of these behaviours.
Whilst marking and mounting behaviours have been mentioned above, several pieces of research have also highlighted the lack of any difference between a neutered and unneutered dogs behaviour in this regard(1,3). With no significant difference found amongst these pieces of research, it must be noted that neutering cannot be considered a guaranteed fix for these behaviours. This means that whilst castration may stop this behaviour in some dogs, conflicting research has found that this may not be as guaranteed as once thought. From a training standpoint, many dogs do still excessively hump once neutered. This demonstrates that there are other factors other than hormone levels which causes humping – the main examples of this are excitement and stress.
Our researcher above who found a link between neutering and reduced aggression also concluded that this was not found to be significant for dogs who displayed aggressive behaviours towards people(2). Due to this, we must note that again neutering is not a cure-all for unwanted aggressive behaviours and it is more likley dependant on the cause of aggression. Other researchers have found that there was no link at all between neutering and a reduction in aggression(5) so this needs to be a considering factor when looking into neutering for dogs displaying aggressive behaviours.
Benefits of remaining intact – Medical
The age of neutering can seriously affect a growing dog. Research as found that those who were castrated before their growth plates were fully closed led to extended growth plates and a greater risk of joint related problems in the future(7).
When studying a group of Hungarian Vizslas, it was found that those who were castrated at a young age were at a greater risk of cancer compared to those who were neutered later or remained intact. Dogs neutered before the age of 6 months were at the greatest risk of developing mast cell cancer and lymphoma as well as an increased risk of developing behavioural disorders(8).
A study conducted back in 1991 looked at three different groups of dogs.
1. Dogs who were neutered at 7 weeks of age
2. Dogs who were neutered at 7 months of age
3. Dogs who were sexually intact
These three groups were then compared on a range of medical conditions. Group 1 was found to have significantly delayed growth plates leading to an elongation of the plates themselves. Penile development was delayed, and dogs remained more excitable than those in group 3 who were sexually intact(9).
Medical records collected from a teaching hospital in North America found that those who were neutered had a significantly increased risk of prostate adenocarcinoma, urinary bladder transitional cell carcinoma, prostate carcinoma, prostate TCC and prostate tumour(10,11). It was suggested however that there is also likley breed specific differences that made this more significant in some breed more than others.
The table below shows the results of 2,219 dogs who were diagnosed with one of the following conditions. Recorded are their castration status.
From the results, we cannot say that castration has caused any of these problems. The fact that Intact dogs suffer from these also shows there are other factors which have contributed to these problems many of which are likley to be highly unavoidable. However, for each of these conditions, there is a difference between the number of castrated dogs who have experienced these medical conditions and then number of intact dogs. This means we must consider whether castration has lead to an increased risk of these disorders developing. Also take note that these are relatively small samples of each condition with some of the margins between the two figures relatively small. How replicable these findings would be on a greater scale may be questioned in this case. When looking at the total number of dogs who were neutered in this specific study, there was 1387 however there were only 832 intact males used. This is a significantly different sized sample which may lead to these differences in the results.
These results were also taken from vet clinics. There will be many cases who did not visit the vet or died from unrelated causes or undiagnosed prostate cancer. We do not know what these dogs were exposed to in their life, their reasonings for being neutered/left intact and other factors such as veterinary history and diet. Regardless, it is certainly a point to consider before we make the decision to neuter particularly as other research has also supported these findings with some pieces suggesting up to a 4-fold increase amongst neutered males(15).
When 1170 German Shepherds were studied over a 14.5-year period, 21% of dogs neutered before one year old were diagnosed with some form of joint disorder. Cruciate ligament tear was one of the most common problem seen amongst neutered males. German Shepherd can be prone to joint problems which may lead some to wonder whether this was a hereditary issue unrelated to their castration status, however within the same study only 7% of those who were left intact developed any joint disorders(12).
Research has suggested there is a two-fold increase in osteosarcoma compared to intact dogs(13) and a four-fold increase in Rottweilers(14).
On a large study of over 40,000 dogs, neutered males were found more likley to die from cancers compared to intact dogs(16). This study had a very large sample which helps to give a good insight into the effects of neutering. Unfortunately, the study does give little information about the age at which the dogs were neutered.
Neutering has also been found to increase the risk of canine obesity(17). Anecdotally, many owners note that their dogs gained weight after a castration, but this has also been supported by scientific study into this area(17). This may not be an issue for many individuals who are prepared to reduce their dog’s food or increase their exercise but may be a point to note for dogs who are already obese prior to castration.
A study on Golden Retrievers found that 27% of dogs who were neutered before 6 months were diagnosed with joint disorders. 14% of those who were neutered between 6 and 11 months were diagnosed and only 5% of intact males were diagnosed.
In regards to cancer, 15% of dogs who were neutered before 6 months were diagnosed with cancer. 11-15% of those neutered between 6 and 11 months were diagnosed and 11% of dogs who were left intact were diagnosed(17). These numbers do not show a huge difference, but the results found were deemed a significant margin between the three age groups.
These results were then compared with Labrador Retrievers and it was found that the prevalence of cancer was the same for both neutered and intact males regardless of the age at castration. Whilst the risk of joint disorder if neutered before 6 months was twice as high as those who were intact or neutered over the age of 6 months, the difference between those who were castrated at an older age and not castrated at all was little difference.
This may then make us question whether the age at neutering is more significant to the welfare of our dogs (from a medical standpoint) rather than whether to neuter or not at all. Rottweilers neutered before the age of 1 were found to be 4 times more likley to be diagnosed with osteosarcoma. In 2010, it was found that the recommended age to neuter a dog was between 7 – 13 months which, when looking at the evidence presented so far may be a little early(19).
When looking at the research on this topic, a large amount of the research does focus on the risk of cancer in neutered vs intact males. Osteosarcoma was found to be more common in neutered dogs(18, 22,23). In addition to this, neutering was associated with an increased risk in the aggressiveness of prostatic cancer(28, 29, 30), hemangiosarcoma, lymphosarcoma and mast cell tumours(25,33,34,35). When looking at 8 different cancers, neutered dogs were affected more frequently in 7 out of 8 cancers(40).
One piece of research found that there was a link between neutered dogs and cognitive decline(33) and neutered dogs were found to have a greater prevalence of immune disorders(36), epilepsy and cruciate ligament disorder(27).
The findings from this research lead to a real question as to whether, medically, neutering is the most beneficial option for our dogs. Whilst links have been made between these aggressive illnesses, it is also unclear as to exactly why this would be the case.
Benefits of Neutering – Medical
Sexually transmitted infections also exist amongst dogs. Infection of the brucella canis is transmitted through the mating of a male and a female dog. With an increased risk of this occurring in intact males, this may therefore be a medical risk to leaving a dog intact(20). It needs to be noted however that this is not a common bacterium with roughly only 1-18% chance of contracting this. The most common symptom of this is infertility which for many pet owners would not be a priority. However, more serious symptoms such as infection of the eyes, bones and nervous system can occur in some cases.
In intact males, the second most common site of cancer is the testicles. Cryptorchid testicles are more likley to develop tumours in dogs under the age of 10 years(21) and therefore the early removal of this leads to a total prevention of this problem.
Prevention of any testicular problems is often a big factor when considering the medical benefits of neutering. Without the testicles, there is no possibility of these problems developing. Whilst many testicular problems are slow to metastasize leading to only around 15% spreading to other areas, issues such as bone marrow hypoplasia can occur due to tumours increasing the production of oestrogen. In these situations, the cure is often castration(22).
A study looking at a variety of breeds identified several beneficial factors of neutering. These included preventing unwanted litters, preventing testicular neoplasia (most are benign and removed though surgery).
The medical records of over 90,000 dogs were analysed and it was found neutered males were at less of a risk from congenital conditions such as aortic stenosis, early onset cataracts, mitral valve disease, patent ductus arteriosus, portosystemic shunt, ventricular septal defect, dilated cardiomyopathy, gastric dilation volvulus. However, this same study found neutering significantly increased the risk of cancers (hemangiosarcoma, hyperadrenocorticism, lymphoma, mast cell tumour, osteosarcoma), ruptured cruciate ligaments and epilepsy. They found no significant effect on castration status on elbow and hip dysplasia(27).
One important piece of research found that whilst neutered males do have a greater risk of many different cancers, neutering significantly reduced the risk of death from infectious diseases and also significantly increased their overall lifespan by 13.8%(40).
This is an interesting point to note. Whilst many pieces of research, this one included, found a greater risk of various cancers when dogs are neutered, this did not reduce their life expectancy. One point that was raised here was that these cancers may be more prevalent in neutered males because they live longer. In other words, the intact males would equally be affected, but these may occur in old age when the fewer intact males are still alive. This is a very interesting point, especially as most of the research does not factor in the age in which the dogs were diagnosed with cancer. This is an area that requires further research.
Neutered dogs were found less likely to die from trauma, vascular diseases and degenerative diseases but were more likely to die from immune related disorders(40). In regards to infectious diseases, sterilised dogs had significantly lower number in three out of four studied diseases.
No effect – Medical
We have talked a lot about cancer in the medical section of this article. Whilst there is several pieces of research showing the increased risk of cancer in neutered dogs, it is important to mention that some research has found no significant difference between dogs who are neutered and unneutered. This was noted in particular for lymphoma, hemangiosarcoma, osteosarcoma and mast cell tumours(11, 8, 13, 24, 25) . Whilst overall, there is more research suggesting there is a significant difference, the fact that conflicting evidence exists suggests that this is not as cut and dry as it may appear in some of the above research. Differences in methodology and sample can draw up significantly different results.
As we can see, there are many considerations when deciding whether to castrate or not and is not as simple as a right and wrong answer. As with many things, there are pros and cons for either side and it is ultimately down to you to make a decision based on the facts you can find. The questions of “what is best for my dog” is a tricky one and only a decision that you as an owner can make. Whatever decision, remember that none of this research gives a definite yes or no to the effects of castration. This means that just because you leave your dog intact, it doesn’t mean they wont develop some of the cancers more prevalent in neutered dogs. It is important to evaluate whether you are equipped to responsibly control an intact male also. Medical considerations aside, there are times when an intact male needs to be treated differently to a neutered male. Special care needs to be taken around unspayed bitches. If you are a social media user, you will have likley seen this debate countless time with people on both sides reassuring one another they have never seen the negative effects that another has. In many situations, these fallouts on either side are never encountered or experience. Research wholly works on statistics, probabilities and risk factors.
When reading through the research presented above, it is important to remember that in many of the studies, the age of castration has not been controlled for. This means that it doesn’t appear greatly clear as to whether many of these potential negative effects of neutering may be avoided by waiting until full maturity before undergoing the surgery. This is because there is a highly understudied area of canine medicine and whilst there are some interesting findings from the research present, further research in this field would help clarify many of these findings.
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