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Littermate Syndrome in Dogs: Fact or Fiction?

"I recently came across the term “Littermate Syndrome” which seems to have become common parlance and a kind of pseudo-diagnosis in some dog behaviour circles." says Dr Eleanor Parker from Pawly Understood.

This piqued my interest as it was something that I had never come across in my years of study and practice in the field of veterinary behaviour. 
So I started digging deeper to divulge what this curious label was all about.

Broadly, it appears there exists a strong recommendation not to raise two sibling puppies from the same litter together due to the consensus they will inevitably or likely suffer abnormal social development and manifest serious behavioural problems. Behaviour problems that arise in one or more of these siblings are then labelled “Littermate Syndrome”

More specifically, it seems that the behaviour problems reported to surface in siblings homed together fall into four main categories:

1. “Hyper-attachment”

Littermate pups are rumoured to develop an exclusive “super-bond” and become highly co-dependent on each other, making them prone to anxiety and distress when separated.

2. Fearful and anti-social behaviour

Pups reared together are speculated to develop general and social fear because their inward focus on each other hinders their ability to explore, learn about and become socialised to / comfortable with the world around them. This fear may cause anti-social behaviour such as reactivity and aggression.

3. Conflict and aggression towards each other

Litter-mates are thought to be more likely than unrelated pups to show aggression towards each other due to the theoretical concern that they might become anxious and confused over forming a social hierarchy because they are of similar age and size.

4. Poor trainability and compromised obedience

It has been suggested that pups raised together serve as a severe and constant distraction to each other and prioritise interaction with each other over interacting with the humans in their family, rendering them difficult to communicate with, motivate or train.

3-month old siblings Porthos & Aramis
recovering from Parvo at Doggie Rescue
These sentiments have apparently been commonplace and widely accepted in many breeding, rescue and dog training circuits for many years. 

As a result, there are some breeders and shelters who refuse to re-home siblings together into the same family.

However, the existence of a potential “Littermate” phenomenon is actually not substantiated by any science or backed by any research.

It has emerged as a result of anecdote, theory and opinion and as such is contaminated by untruths and biases. Some aspects of the claims have merit, but others are more myth and legend and some parts of the theory are very misguided and anti-scientific. As such, the label “Littermate Syndrome” warrants some discussion, clarification and debunking. 

So what are the facts?

There are explanations as to why behaviour problems may seem more common or obvious among siblings. Behavioural disturbances are contributed to by genetics and are heritable

This means that if one sibling suffers anxiety, the other may be more likely to as well, due to sharing similar genetic predisposition to the disease. It is also possible for pups to exacerbate problems in each other

If one sibling does suffer from anxiety and social impairments, this can be very stressful and detrimental to the development and welfare of the other pup. The normal pup may find it very difficult to navigate interactions with the anxious pup and may be quite disturbed by any abnormal behaviour that is displayed, leading to secondary issues in this pup.

Another factor is simply that getting two puppies at once can generate a lot more chaos, stress, work and frustration for people

This can potentially translate into the pups being met with more animosity at the hands of their human family, which influences their development and sociability. 

Additionally, having two at once almost inevitably means that there will be less time spent with each pup, which may compromise ideal development

In regards to the “hyper-attachment” claim, there is no evidence that there is some kind of abnormal or special bond that develops between siblings. There is also no evidence that siblings are less likely to bond to people in the family due to having each other. 

However, it is possible that if the siblings are never socialised to being alone and are always in each other’s company, that they do not develop the independence, resilience and coping skills to remain calm and confident if they are ever separated. I suspect that observations of this phenomenon are what have given rise to the murmurs of “Littermate Syndrome” but this is not unique to siblings - rather the same risk would apply to any two pups raised together.

An important point and distinction to make is that if true separation anxiety exists (i.e. the two dogs cannot be separated without experiencing acute and severe distress) then this is due to a fundamental underlying disturbance in their physiology and the way their brains process potential threats. This is actually a panic disorder and a serious mental illness. It is not a function of them having “littermate syndrome”.

In regards to the observation that litter-mates in a home are prone to developing global fearfulness and anti-social behaviour, there is no evidence supporting this direct link

It is not true that these pups necessarily have an insular or inward focus on each other which interferes with their willingness or ability to socialise and learn about the world around them. However, I can conceive how this anecdote may have arisen. 
There is a common misconception that having two pups together fulfills their need to socialise and negates the need to actively socialise them to other dogs and the wider world. 

This means that people with two pups may neglect to socialise them well enough and they may have limited social experience during their sensitive developmental period. Then when they reach social maturity these dogs are ill-equipped to face the world and navigate its unfamiliar social complexities. This will translate into experiencing uncertainty, anxiety and overt fear and will precede the development of anti-social coping strategies such as defensive aggression

So it is not the factor of being siblings that pre-determines behaviour disturbances developing but rather a secondary or indirect lack of socialisation

The claim that littermates are more likely to display anti-social behaviour and aggression towards each other is not supported by science. 
There is no such thing as “sibling rivalry” in the human sense among canines

The suggestion that growing up with a sibling generates confusion, competition and conflict over one’s place in a social hierarchy can only have arisen from previous thinking in line with the outdated and thoroughly debunked “dominance theory”. 

This is simply not at all relevant or accurate in regards to explaining any aspects of dog behaviour. Importantly, if inter-dog aggression is a problem between two siblings then invariably this is due to a breakdown in normal social processing and communication, which is caused by underlying anxiety. 

Where two familiar dogs are in chronic conflict, there is always at least one of them (sometimes both) who are abnormal and who suffer an anxiety disorder. The anxiety and associated emotional, cognitive and physiological distortions render them unable to partake in normal perception, processing and reaction to social information. 

To put it another way, it is an anxiety-based disturbance in the brain and body of one (or both) of the individuals which is leading to the aggression – not the fact that the two dogs are related.

In light of the suggestion that siblings are challenging to interact with and train, there is no inherent reason why this would be the case. 

The pups may serve as a distraction to each other but should be able to focus, concentrate, learn and perform when appropriately engaged and motivated by a person

They would certainly need separate time and energy invested in their training by their human family members and would present double the work load, but would respond well when the effort is applied. If training problems are apparent in one or more siblings then the likely reason for this is an issue with the individual such as potential learning impairments due to anxiety.

In Summary

So, when we closely examine what has traditionally been reported as “Littermate Syndrome”, we see that it is actually not an existing syndrome and there is no such entity
It is not a useful, appropriate or accurate label to place on pairs of puppies who are afflicted with behaviour problems. It is not recognised as a diagnosis by experts in the field of dog psychology and behaviour

Being littermates does not automatically or directly predispose puppies to abnormal behavioural development, however there are secondary factors that can contribute to problems. There is a risk involved in getting two puppies due to the facts that:

1) They may both share a genetic predisposition to anxiety and behavioural problems making them both more likely to develop issues individually.

2) If one of the pups has a behaviour problem (especially an anti-social pathology) they may worsen or cause issues in the second pup due to placing social stress on them.

3) They may not have the opportunity to become socialised to being alone (separated from each other) which may render them less resilient and less able to cope when separated.

4) Some people are inclined to socialise them less, incorrectly assuming they can rely on them to socialise each other.

The truth is that raising two puppies of the same litter together will not necessarily lead to behavioural pathologies or disturbances. Neither will obtaining two unrelated puppies or dogs from different litters necessarily be a sure recipe for a harmonious home. Many littermates are able to live long and harmonious lives together if they are mentally normal, healthy and robust

Conversely, many pairs of unrelated companion dogs will manifest issues, due to one or more of them having a mental health problem. It comes down to the individual pair and how mentally normal each dog is (due to the combined influence of their genetics, experience and environment) as to how well they can flourish and navigate the world around them. Ultimately, if littermates are showing any behavioural or social problems then they need to be assessed and treated as individuals for underlying disease.

In light of the above, it is still prudent to emphasize that getting two pups form the same litter is certainly not recommended or encouraged due to the amount of work involved and the challenges it brings

It is a common misconception that getting two pups at the same time will reduce the time and energy needing to be invested in each pup because of the view that they will entertain each other and provide for each other’s social wants and needs – this is not the case. 

Sure, they may entertain each other but they certainly cannot give each other the complete and robust requirements for socialisation. 

What people may not realise is that they still each require the same amount of separate work to be put into them. 

Having two at once makes it more difficult to focus on one at any time, making life harder, not easier. However, there is nothing inherently wrong with it, as long as both individuals are provided with their needs and are appropriately socialised to the world at large (other dogs, people, objects, places, experiences) to foster optimal neuro-behavioural development.

written by Dr. Eleanor Parker, August 2018 for Australian Dog Lover 
(all rights reserved)

For questions, clarifications or further information please contact Dr Eleanor Parker of Pawly Understood.

Dr Eleanor Parker 
BSc BVMS (Hons) MANZCVS (Behaviour)

Elle graduated from Murdoch University in 2010. Starting out in emergency and critical care, she quickly found her passion for behaviour and mental health in animals.

Fascinated by this blossoming field, Elle undertook further study through the University of Sydney in 2015 and sat her membership exams in veterinary behaviour in 2016. 

Elle's behavioural mantra is "compassion, communication, co-operation, cohabitation". She offers private veterinary consulting services to people needing assistance with their pets' behaviour through her practice at Pawly Understood.

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