What is Littermate Syndrome? A Tale of Two Puppies

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What is Littermate Syndrome? 

Littermate syndrome is an anecdotal term that refers to a list of behavioral issues occurring when two puppy siblings grow up in the same household. 

Typically puppy siblings go their separate ways, but in some cases, owners will choose to keep two baby dogs. But is littermate syndrome real, or are you just looking into how to stop sibling puppies from fighting? Keep reading to find out! 

two dogs at home

Is Littermate Syndrome Real?

Ask this question of a group of dog trainers or even veterinarians, and you will get as many answers as there are people. Some will say that littermate syndrome is a myth, and some will say that this dog sibling syndrome is very real indeed. But what does science say? 

Littermate Syndrome Studies

For something to be true in the scientific community, a scientist or researcher needs to obtain consistent results. Still, they also need to get the same results across other studies aiming to answer the same question – each study with its own data. This is known as reproducibility and replicability. 

On top of that, these results will then need to be peer-reviewed to be a valid scientific term. The reality is that to date, no studies have proven that littermates that live together are more prone to behavioral problems compared to dogs raised apart from their siblings. 

Littermate syndrome is, therefore, purely based on anecdotes. Why is it then such a common misconception? Here are some excellent suggestions as to why. 


Quite a few behavioral issues are, at least partly, due to genetics. If you have one dog that suffers from anxiety when left alone, it can be awful, then imagine if you have two at the same time, it can seem a lot worse! 


Leading quite nicely on from the last point, young puppies – and even adult dogs – can exacerbate problems in each other. If one of the dogs is very scared of humans, it can project that fear onto the other dog – making it seem much worse. 


You may have heard the age-old saying that two dogs do not equal two times the work. That saying does not go for puppies, in my experience. A puppy is a lot of hard work – two puppies are often more than twice as hard. Double the mischief, double the training, and double the getting up at night. 

To some owners, even experienced dog owners, it can quickly seem like the issue is that the two puppies come from the same litter. But maybe it is just because it is a lot harder to train two dogs at once? 

No matter what, we must conclude that littermate syndrome is, for the moment at least, purely a theory – albeit a prevalent one, and perhaps one day, it will be proven as a genuine scientific concept. Nonetheless, that will not stop us from looking into this alleged syndrome and, more importantly: How to fix littermate syndrome!

The Effects of Littermate Syndrome in Puppies

We’ve covered that littermate syndrome dogs are anecdotal and a theory, but what is it that might be happening when you have issues adopting littermates? 

The main issue seems to be that many owners report that the littermate syndrome puppies tend to get so attached to each other that they cannot form a proper bond with their human owners. 

When you bring one or several puppies home, you want to bond with them, meaning a lot of owners can feel entirely left out when their dog isn’t interested in them. On top of that, some owners also report that this lack of bonding means it can be exceedingly difficult to train the puppies – as they are more interested in each other. 

The Challenges Relating to Littermate Syndrome 

Although littermate syndrome is not a true scientific term, four issues seem to reappear when discussing littermates and the issues they may have. 

Over-attachment to Their sibling

The most reported issue when bringing home two puppies from the same litter is that the puppies become overly attached, or more like they don’t become particularly attached to their new owners. 

Fearful behavior 

As a response to being overly attached, some puppies can develop fearful and even anti-social behavior, especially when separated. Eventually, this lack of social skills can lead to aggression towards others. Read our article and find out Breed-Specific Canine Aggression.

Conflict Amongst Each other 

Commonly, some dogs show increased aggression towards their siblings when housed in the same home. It is worth noting that many puppies will playfight with any dog of the same age they are close to, but some owners experience that siblings will do this more frequently.  

Compromised Obedience 

Some suggest that dogs raised with their siblings are harder to train, perhaps because they bond more with each other or perhaps because they get distracted by each other. Nonetheless, it results in dogs that are more difficult to train and are, in turn, more disobedient than dogs raised in a single-dog household. 

little girl with two dogs

How to Fix Littermate Syndrome

Any research per se does not back littermate syndrome, but there might indeed be more issues with sibling puppies. However, it is probably more likely to be due to other things than the dogs just being siblings. 

We’ve already talked about how issues can be genetic and inheritable, as well as the increased chaos with two puppies in the house, but let’s dive a little further into this and what you can do to fix the supposed littermate syndrome. 


No evidence suggests that there are some special bonds between siblings or that they are likely to bond less with their family because of each other. However, it is not uncommon for owners with two puppies to let these two play together rather than take them to meet other dogs.  

Two puppies living together cannot socialize with each other properly; they will still need a lot of socialization with other dogs, new environments, people, and experiences to become well-rounded puppies. It may also be beneficial to separate the puppies to ensure that they can handle new experiences on their own. 

It will also help develop a stronger bond between a puppy and its owner. People sometimes forget that two puppies will still individually need the same amount of contact with an owner as a single puppy. Not everything can happen with both puppies present at once; making sure to spend plenty of time alone with the pups will make a better bond.  

Fearful Behavior

Leading on from the last point, it is possible that as siblings spend so much time together that they are likely to seek out each other in new or scary situations rather than their owners. 

But it is also possible when the puppies are on their own, they may start showing more fearful behavior towards new experiences compared to puppies that already spend most of their time alone. 

Some owners with two littermates may also be less prone to socializing their puppies as they tire each other out. To combat any fearful behavior, giving your puppy plenty of positive experiences with lots of treats for good behavior is essential. Remember, socializing is all about quality above quantity!  

Conflict Amongst Each Other

Puppies love to playfight – when they get a little older, their playfight may look like real fighting and conflict. That may be bad enough if your puppy is play-fighting with the neighbor’s dog when they meet in the park, but if it is every day, it may seem like excessive conflict. 

It is expected, as puppies age, that they will test limits – with their owners and dogs in their immediate environment; this can often look like severe conflicts, and, of course, it can sometimes turn into conflicts. 

It is an age-old myth that dogs should “work it out” amongst each other, but it’s our jobs as owners to look out for our dogs – especially if one dog is more “rough” than the other. If you frequently experience your puppies fighting, teach them that when that happens, you will separate them, a.k.a. –the fun ends. 

You can try to put them back together when both puppies are calm. If the too-rough fighting continues, separate them again. Eventually, your pups will learn that fighting, even if it is just rough playing, will lead to the fun ending. 

Compromised Obedience 

Puppies are pretty playful, so they may choose to play with each other rather than stay calm and trainable with their owners. As they mature, they are likely to prioritize training rather than playing. But, as puppies playing is probably on top of their to-do list, which is why it seems like littermates are far more challenging to train.   

To help avoid this, ensure that training sessions sometimes take place away from the other puppy. If you can, split up the dogs between two family members. That is ideal; otherwise, you should just split the training sessions into two, removing the distraction of playtime with their littermate and friend. 

Training puppies separately and keeping one waiting is also a great way to teach the other dog to be alone for a bit and a bit of patience! 

Eventually, you can perhaps do the training sessions with one dog waiting and looking on while you’re training the other. But in the beginning, keep the dogs apart so they can’t see each other, and hopefully, your playful puppy will become a trainable dog in no time. 

Can Dogs Recognize Their Siblings?

It is unclear whether a dog can recognize another dog as their sibling specifically – dogs don’t have the same sense of “family” as we humans do. That said, dogs have an uncanny ability to recognize dogs and humans for years after they last saw them. 

It is, therefore, very possible that although they may not know that another individual dog is their sibling, it is very likely that our dogs realize that they know the dog and recognize them as “friends.”

How to Avoid Littermate Syndrome

Although littermate syndrome is not proven, it is, nonetheless, a widespread idea. The anecdotal evidence suggests that there may be some truth to the idea that littermates who grow up together in the same household have behavioral issues. 

It then goes on to say that the best way to avoid littermate syndrome is not to bring up littermates in the same household. Suppose you’ve already got two littermate puppies at home and feel like you’re experiencing increasing issues with them. In that case, it might be very well worth contacting a professional behavior therapist to help you. 

But, if you’re not quite there yet, keep reading to get some great tips on dealing with the most common behavior issues at home – applicable to littermate syndrome and single puppies. 

Our Top Behavioral Problem Tips

If your puppy has behavioral issues, we recommend you go back and look at our section on ‘how to fix littermate syndrome.’ Many owners report the same troubles in single-puppy households! 

That said, here’s some good advice for any puppy owner on how to curb the worst of behavioral issues. 

  • Quality above quantity: When socializing a puppy, it is more important to have great experiences rather than many experiences.
  • Positive reinforcement: Treats and praise will go a long way with your new family member. 
  • Avoid negativity: Do not shout at your puppy if something goes wrong. If necessary, say a clear “no” and redirect your puppy’s attention to a wanted behavior. 
  • Mental stimulation: Your puppy will, of course, need to be walked and played with, but remember that mental stimulation, like training, searching for treats, and mental games, are what makes for a happy puppy.

The most important advice we can give you: Ask for help! Getting a new puppy can be overwhelming. It is okay to ask for help from friends, family, or professionals if you are struggling and your sweet puppy is more like a furry piranha!

Is Littermate Syndrome Applicable to Cats?

Although described by cat owners, the theory about littermate syndrome is not nearly as common amongst cats as dogs. But perhaps that’s because closely bonded cats are less likely to cause trouble than two closely bonded dogs? 

There have been some reports that adult sibling cats will get along better than adult cats that aren’t siblings, even though they grew up in the same household, so maybe there’s something about littermate syndrome in cats? 

two cats

The Bottom Line

Whether littermate syndrome is an accurate scientific term or merely an anecdotal theory, there’s no doubt that having two puppies at home simultaneously is a struggle for even the most experienced dog owner! 

If you have brought home two puppies, separating them now and then is essential to ensure they receive all the training and love, they need as individuals – and then they can play and enjoy each other’s company all the other hours of the day! 

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With a veterinary master's degree from the University of Copenhagen in 2023, this accomplished writer's academic journey culminated in a thesis focused on the "Feasibility of using ultrasound of the abdomen for early diagnosis of necrotizing enterocolitis in neonatal pigs." Additionally, their dissertation delved into the intriguing topic of "Mercury accumulation in Greenlandic sleddogs." Beyond her academic achievements, her passion for animal health seamlessly merges with her love for writing. She excels in harmonizing clinical precision with literary expression, crafting articles that resonate with the heartbeat of her veterinary profession.