Thinking of adopting two puppies at the same time? It sounds like the perfect idea-they’ll keep each other company, play together, and grow up as best friends.
Unfortunately, that’s not always what happens.
Many owners are surprised to discover that raising two puppies together can lead to behavioural challenges known as littermate syndrome. While it’s not an officially recognised medical diagnosis, it is a well-documented behavioural pattern seen by trainers, behaviourists, and veterinarians.
If you’re considering bringing home siblings, or two puppies of a similar age, it’s important to understand the risks and how to prevent them.
What Is Littermate Syndrome?
Littermate syndrome refers to behavioural problems that can develop when two puppies of a similar age are raised together, particularly if they spend most of their time with each other.
Despite the name, the puppies do not need to be actual littermates. Unrelated puppies adopted around the same time can develop the same issues.
Instead of forming strong, healthy bonds with their human family, the puppies may become overly dependent on one another, making it difficult for them to develop confidence and independence.
Signs of Littermate Syndrome
Not every pair of puppies will develop littermate syndrome, but common signs include:
- Extreme distress when separated
- Fearfulness or anxiety in unfamiliar situations
- Difficulty bonding with their owners
- Poor responsiveness during training
- Delayed social development
- Competition over food, toys, or attention
- Increasing aggression toward one another as they mature
- Over-reliance on each other for confidence
Many owners notice that their puppies behave well together but become nervous, reactive, or fearful when alone.
Why Does It Happen?
Puppies go through important developmental stages during their first year of life.
During this period, they should learn to:
- Build confidence independently
- Form secure relationships with humans
- Experience new environments
- Learn basic obedience
- Develop emotional resilience
When puppies spend nearly all of their time together, they often rely on each other instead of learning these skills individually.
As a result, their emotional development may be delayed.
Can Littermate Syndrome Lead to Aggression?
Yes.
One of the biggest concerns is that puppies who appeared inseparable as youngsters may begin fighting as they reach adolescence.
This is especially common between dogs of the same sex, although it can occur in any pair.
These conflicts may become increasingly serious because both dogs are maturing and competing for resources within the household.
Are Some Dogs More at Risk?
Certain situations increase the likelihood of littermate syndrome:
- Adopting two puppies from the same litter
- Bringing home two puppies within a few weeks of each other
- Puppies of similar ages raised together
- Limited one-on-one time with each puppy
- Insufficient independent training and socialisation
That said, littermate syndrome is not inevitable. Many pairs of puppies grow into well-adjusted adults with the right management.
How to Prevent Littermate Syndrome
If you already have, or are planning to adopt, two puppies, prevention starts from day one.
Train Separately
Each puppy should receive individual training sessions every day.
This helps them learn to focus on you rather than relying on their sibling.
Walk Them Individually
While walking together is enjoyable, each puppy also needs solo walks to build confidence without the other dog.
Spend One-on-One Time
Play, cuddle, groom, and interact with each puppy separately every day.
This strengthens your bond and helps prevent over-attachment between the dogs.
Socialise Independently
Expose each puppy to:
- New people
- Different environments
- Friendly dogs
- New sounds
- Different surfaces
Importantly, many of these experiences should happen without the other puppy present.
Encourage Independent Rest
Whenever possible, puppies should sleep, nap, and spend quiet time separately.
Many trainers recommend separate sleeping areas/pens to encourage healthy independence.
Attend Training Classes Individually
Taking each puppy to separate puppy classes can dramatically improve their confidence, attention, and ability to cope independently.
Should You Adopt Two Puppies Together?
In most cases, it is recommend to adopt one puppy at a time.
Waiting until your first puppy is mature, usually 12 to 24 months old, before introducing a second dog often results in:
- Easier training
- Better social development
- Stronger human-dog bonds
- Fewer behavioural problems
For first-time dog owners especially, raising two puppies simultaneously can be significantly more challenging and stressfull than expected.
What If You Already Have Two Puppies?
Don’t panic.
Many puppies showing early signs of littermate syndrome improve dramatically with:
- Individual training
- Independent walks
- Separate enrichment activities
- One-on-one attention
- Professional behavioural guidance when needed
The earlier these changes are introduced, the better the outcome tends to be.
Myths About Littermate Syndrome
“They’ll entertain each other.”
While puppies certainly play together, they still need regular interaction, training, and bonding with people. Most puppy classes offer socialisation where puppies can safely play and interact with one another.
“They’re happier together.”
Healthy companionship is wonderful, but excessive dependence can create anxiety when the dogs are apart.
“It’s only a problem for siblings.”
False.
Any two puppies of similar ages raised together can develop littermate syndrome.
Frequently Asked Questions (FAQ) About Littermate Syndrome
Can littermate syndrome happen with unrelated puppies?
Yes. Any two puppies of a similar age raised together can develop the same behavioural issues.
At what age does littermate syndrome appear?
Signs often begin between 8 weeks and 6 months, although aggression may not appear until adolescence, around 6–18 months.
Is littermate syndrome permanent?
No. Many dogs improve significantly with early intervention, consistent individual training, and behaviour modification.
Can adult dogs develop littermate syndrome?
The condition is primarily associated with puppies during development. Adult dogs that are introduced later in life are much less likely to develop it.
Should littermates be separated permanently?
Not necessarily. Most puppies benefit from spending time together and separately. Permanent separation is usually only considered in severe cases where aggression or anxiety cannot be safely managed.
Looking for help from a qualified dog behaviourist?
At Gentled Animals, we’re here to help you and your puppy live harmoniously, with understanding and kindness at the heart of every solution.