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What is Littermate Syndrome in Dogs? Is It Real?

Littermate Syndrome in dogs is commonly referenced, but is it even a real thing? We explain what it is — and isn’t – and how to deal with it.

Getting a puppy can be a fun, life-changing experience. For some owners, there may be an extra allure to the idea of adopting two puppies: they should be twice the fun, and they will be best friends and keep each other company, right? 

Unfortunately, there are many cases in which adopting a pair of littermates leads to more of a headache than expected. While having two pups can indeed be fun and they can provide some amount of companionship for each other, there is an additional risk that the puppies will not bond as strongly with their new owners as normally, or that other unwanted behaviors may arise.

Whether you’ve just adopted a pair of pooches or you’re planning to, you may be wondering if littermate syndrome in dogs is real, and what potential issues may exist as a result. 

While littermate syndrome is not a scientifically recognized diagnosis, the term is used colloquially to refer to unwanted behaviors that older dog littermates may exhibit as a result of growing up together. Substantial anecdotal evidence suggests that raising littermates together in the same household can create challenges that otherwise wouldn’t exist.

What Is Littermate Syndrome in Dogs?

Littermate syndrome is a term used to describe a number of undesired behaviors that may be exhibited by newly adopted young littermates, including separation anxiety when removed from their sibling, aggressive behaviors toward the sibling, the development of extremely domineering or submissive personalities, and lack of bonding with humans while being unhealthily codependent on each other.

Although “littermate syndrome” is often used to refer to littermates who develop these traits, it may also refer to two unrelated dogs who are adopted together, regardless of age. Older dogs, especially rescues, may have past trauma that exhibits itself in similar ways as the behaviors described as littermate syndrome. 

What Behaviors Cause Dogs to Be Labeled as Having Littermate Syndrome?

Generally, dogs who are said to have littermate syndrome exhibit one or more of the following traits or behaviors:

  • Separation Anxiety when the dogs are away from each other, often evidenced by whining, whimpering, or destructive behaviors. 
  • Dominance or Timidity, with one dog acting more “selfishly” or aggressively and the other acting shy or nervous, especially toward each other.
  • Aggression by one dog toward the other, or by both dogs toward the owner or other humans. This aggression may be in tandem with separation anxiety or a domineering personality. 
  • Aloofness toward the owner or other humans and lack of interest in interacting with (or listening to) humans rather than interacting with the other dog. 
  • Fear of People, which may include the dogs’ human family or strangers. 
  • Housebreaking Issues, which can apply to either one or both of the littermates and is related to their general lack of response to training by the owner.
  • Leash Reactivity, where the dogs overreact to various stimuli.
  • Issues Encountering New Situations, particularly when alone and not with their littermate.

These behaviors and traits become more worrisome when they intensify or cause problems for the owners. Each dog has their own personality and quirks; with puppies, though, these traits are not fully known and develop over time. Because of this, a change in personality or behavior can be unexpected and potentially upsetting. 

dog littermates

For example, a pair of puppies adopted together may be perceived as initially behaving normally. However, over time, one puppy may develop a stronger, more dominant personality while the other becomes shyer and more reserved. If one dog begins to be too aggressive or the other becomes frightened or anxious due to the other dog, these personality traits could become very toxic. 

Some dog breeders or trainers may refer to the preceding situation as a form of littermate syndrome and may suggest separating the dogs. However, this does not always need to be the case, and the labeling of the undesired behavior as a syndrome may make the situation sound hopeless when that is not necessarily so. 

Is Littermate Syndrome Real?

Dogs can definitely experience the behavioral issues referred to as littermate syndrome, but the term itself is used too broadly to provide specific guidance. Additionally, littermate syndrome is not currently recognized in scientific research, and many veterinarians would refer to the particular behavioral problem (e.g., “aggression” or “biting”)  or use a narrower term (“sibling aggression” or “littermate aggression”) rather than labeling it “littermate syndrome.” 

The use of littermate syndrome as a catch-all term makes it easy to blame for behavioral problems that many puppies go through, and in a way, removes agency from the owners. Calling the problem a syndrome (a condition characterized by a set of associated symptoms) may cause an owner to lose hope and believe the issues cannot be fixed, which is often untrue—though making the desired changes happen may in some cases be very difficult. 

Socialization and Training of Littermates

These undesired canine behaviors may be amplified by the lack of proper socialization that sometimes occurs when new dog owners adopt two dogs rather than one. Although dogs can enjoy playing together and may provide a certain amount of companionship and entertainment to each other, it is not correct to assume that the dogs will need less individualized socialization because they were adopted together. 

In fact, adopting two puppies means putting in two times the work (if not more!). Each needs to undergo individual trained as well as training sessions with the other dog, each needs alone time with the owner and other humans in the household in order to foster bonding and discourage codependence between the canine siblings, and when crate training, each should have separate crates. 

Dog trainer Kevin Ryan says he can’t emphasize this point too strongly. “If a client contacts me to train littermates, I will decline the opportunity until they have gotten the puppies to successfully sleep in separate crates. Ideally, they are also often walked separately and introduced to other dogs and people separately as well.”

In essence, dogs adopted together need to be treated as individuals, not as an inseparable unit (similar to how human twins are not the same person but are individuals with different wants and needs). Littermates can still play together and enjoy time together, but if they are never separated or given one-on-one attention, they are more likely to develop unwanted behaviors. 

Ryan believes that even when extra care is taken to raise and train littermates individually, it’s still an uphill battle and should be avoided if at all possible. “There are so many problems that arise in these scenarios that could all be solved by simply not putting yourself in the situation in the first place,” says Ryan. “It’s just so hard to properly socialize littermates when you are raising them together. You’ll always be in competition with their dependency on each other, which goes back to the womb.”

Littermate syndrome does refer to some very real behavioral issues that pups may face; however, referring to these emotional and behavioral problems as a syndrome is a misnomer. It is better to be specific about the behavioral issue(s), because then you will have better luck finding appropriate fixes rather than generic advice.

littermate syndrome in dogs

How Common is Littermate Syndrome?

Because it’s impossible to tie any specific dog behavioral issue directly and exclusively to being raised with a littermate, it’s difficult to cite how “common” what is referred to as littermate syndrom is. But a great many dog trainers see issues with littermates often enough that they strongly recommend against raising two together. 

And as Ryan points out, “any problems that arise directly from raising littermates together only occur when raising littermates together. So it’s only as common as we make it. It’s totally avoidable.”

What Should I Do If My Dog Is Exhibiting Littermate Syndrome Behaviors?

The exact response would depend on a variety of factors, the foremost of which is what type of problematic behavior the dog or dogs express. For instance, problems with aggression between dogs in the same home will need to be dealt with differently than littermates’ separation anxiety.

It may not be a bad idea to consult your vet for advice and an assessment of the behavior in case there is a medical cause behind it; however, especially in the case of young puppies, it is likely that the unwanted behavior has been ingrained as a part of their development and would be better addressed by a dog trainer or behaviorist.

In many cases, such as dealing with jealous dogs or those with seemingly incompatible personalities, additional training will be required. 

Although rehoming one of the dogs is a common suggested solution to littermate syndrome concerns, this does not necessarily have to be the case if you are willing to put in the necessary work to raise each puppy individually. 

That said, because the amount of time, patience, and effort needed for retraining may not be feasible for all dog owners, some may decide to rehome one dog. This may be particularly true in the case of littermates that have been raised and living together for a long period of time.

Final Thoughts

While littermate syndrome is not a scientifically recognized condition, the term refers to a number of unwanted behaviors that may arise in adopted littermates or newly adopted pairs of dogs, including separation anxiety, aggression, aloofness, and personality changes. 

It is a good idea to try to avoid using “littermate syndrome” to refer to these unwanted canine behaviors between littermates and instead be specific about the behavioral problems you are facing when looking for advice from your vet or when perusing helpful information online. 

Adopting two dogs is not an easy task and, in many cases, results in twice as much work, so be aware of the potential pitfalls before committing to two pooches.Many of the concerning behaviors and habits are able to be corrected through additional training, but that will require significant extra time and patience.

Ryan urges owners of littermates to consider the dog’s wellbeing above all else. “People think that separating two littermates raised in the same home will be too traumatic and stressful for the dogs. But it’s actually far more unfair to keep two poorly socialized dogs together, with serious aggression or anxiety issues, and a difficult, challenging process ahead to retrain and rehabilitate them. We need to do what puts the dogs in the best situation to succeed.”

Chelsea Dickan

Chelsea Dickan

Chelsea Dickan is a long-time advocate for animals, especially those that bark or meow. When she isn't writing, she enjoys reading and watching scary movies in which the dog doesn't die.