Littermate Syndrome in Dogs: Why Raising Two Puppies Together Can Lead to Serious Behaviour Problems🐕‍🦺 🐕‍🦺

Bringing home a puppy is exciting. Bringing home two puppies can feel like double the fun—double the playtime, double the cuddles, and built-in companionship. Many families believe that adopting littermates will make life easier because the puppies will “keep each other company.”

Unfortunately, the reality is often very different.

As a professional dog trainer, I have worked with many families who adopted two puppies from the same litter. Over the years I’ve seen patterns emerge that are commonly referred to as Littermate Syndrome—a collection of behavioural and emotional issues that occur when sibling puppies grow up together and bond more strongly with each other than with the humans raising them.

While not every pair of littermates develops these issues, the risks are significant enough that most experienced trainers and ethical breeders strongly discourage placing siblings in the same home.

Let’s talk about why.

What Is Littermate Syndrome?

Littermate Syndrome refers to behavioural challenges that often develop when two puppies from the same litter are raised together. Because they have grown up side-by-side since birth, they naturally rely on each other for security, confidence, and decision-making.

Instead of forming strong relationships with their human family, they become hyper-bonded to each other.

This can lead to several issues:

• Extreme separation anxiety when apart

• Difficulty bonding with humans

• Delayed or poor training progress

• Fearfulness when separated

• One puppy becoming overly dependent on the other

• Aggression developing between siblings during adolescence

The core problem is simple: the dogs learn to rely on each other instead of their humans.

A Real Example: When the Bond Becomes a Problem

I once worked with a client who had male and female German Shepherd littermates. I also had another client with a brother and sister pair of small mixed-breed dogs.

In both cases, I recommended separating the puppies regularly during development—separate training sessions, separate walks, and dedicated individual time with the owners.

Unfortunately, the advice wasn’t followed. The puppies spent nearly all of their time together.

Over time, something very clear emerged: the males relied heavily on the females for leadership and confidence. The female dogs became the emotional anchors in both pairs.

Then life took an unexpected turn.

The female German Shepherd tragically died at just two years old after being poisoned by lilies that had been planted on the property during landscaping. The mixed-breed female passed away at only three years old due to an aneurysm.

In both households, the remaining male dogs were left completely distraught.

These dogs didn’t just lose a companion—they lost their primary emotional support and leader.

Both dogs went through deep depression. Their anxiety skyrocketed. Their ability to cope with everyday life collapsed because their sense of security had been built around the other dog instead of their human family.

What made the situation so heartbreaking is that much of that stress could have been prevented if the dogs had been raised to build stronger relationships with their people rather than relying almost entirely on each other.

When Littermates Become Rivals

Another common outcome of Littermate Syndrome is the opposite problem: serious aggression between siblings.

I worked with a family who had two female German Shepherd littermates. Everything seemed fine while they were young puppies. They played together, slept together, and appeared inseparable.

Then adolescence arrived.

At around 6–7 months of age, the sisters began fighting. What started as minor disagreements escalated into serious, dangerous aggression.

Once littermates begin fighting at that level, it can be incredibly difficult to restore peace between them. These were not simple squabbles—they were intense conflicts that required immediate management.

Because the family could not bring themselves to rehome one of the dogs, they had to implement a crate-and-rotate system, meaning the dogs could never be out together again.

One dog would be out while the other was crated or behind a barrier. Then they would switch.

This system can work, but it requires constant management for the rest of the dogs’ lives. It’s exhausting for families and limiting for the dogs.

Why Responsible Breeders Avoid This Situation

This is one reason why responsible breeders typically refuse to place two littermates in the same home.

Experienced breeders understand that:

• Puppies need to develop independence

• Each dog needs a strong bond with their human family

• Social development requires individual experiences

Allowing two puppies to go home together increases the risk of behavioural challenges that many families are not prepared to manage.

A good breeder is thinking about the long-term welfare of the dogs, not just placing puppies quickly.

Can Littermates Ever Work?

In rare cases, yes—but it requires a tremendous amount of work and commitment from the owners.

To raise littermates successfully, you must intentionally create independence by:

• Training each puppy separately

• Walking them separately

• Crating them separately

• Taking them to classes individually

• Building strong human-dog relationships with each dog

• Ensuring they spend significant time apart daily

Most families simply underestimate how much effort this requires.

Even Dogs Close in Age Can Struggle

To give another real-life perspective, I actually live with a similar dynamic myself.

I currently have two male dogs that are only a month apart in age. They are not littermates and they do not have littermate syndrome. From the beginning, I was very intentional about preventing that dynamic from forming.

They were:

• Trained separately

• Walked separately

• Fed separately

• Given individual time with me

• Raised to build strong relationships with their humans rather than each other

Because of this, they developed independence and healthy bonds with people.

However, even without littermate syndrome, dogs close in age can still experience social tension.

For a long time my senior dog acted as the clear leader, and both younger males respected that structure. But as the younger dogs matured, the dynamic shifted.

Now, the two males still occasionally struggle with hierarchy between themselves. Neither one naturally wants to defer to the other, and that underlying competition will likely always exist.

Fortunately, I have the advantage of experience, training, and management, so this has never escalated into serious conflict. But the reality is that the tension is always present under the surface. It simply requires awareness and leadership from me.

Interestingly, my female dog—who is three years younger than the males—has absolutely no issue respecting their leadership. She slides into the social structure easily without challenging it.

This really highlights something important about canine social dynamics:

Dogs that grow up close in age, especially same-sex dogs, are far more likely to experience rivalry as they mature.

Why This Matters for Dog Owners

If managing this type of dynamic requires awareness and structure even for a professional trainer, imagine how difficult it can become for the average household.

This is why experienced trainers often recommend:

• Avoid raising same-sex puppies together

• Avoid raising littermates together

• Space dogs out in age when adding a new family member

These strategies dramatically reduce the risk of rivalry, aggression, and instability within the household.

Dogs thrive when the social structure is clear and calm. When two dogs are constantly trying to figure out who should lead, tension can build—even when it isn’t obvious to humans.

My Professional Recommendation

After many years of working with dogs and families, my advice is simple:

If you want two dogs, get them at different times.

Allow the first dog to mature, develop training, and build a strong bond with you before bringing another dog into the home.

This sets everyone up for success and avoids the emotional and behavioural challenges that can come with Littermate Syndrome.

Two dogs can absolutely be wonderful—but timing matters.

Final Thoughts

Watching dogs struggle with anxiety, depression, or aggression because of littermate dynamics is heartbreaking—especially when it could have been avoided with early management or different decisions.

Puppies don’t just need companionship.
They need guidance, leadership, and connection with their humans.

When that relationship becomes the foundation of the dog’s world, they grow into confident, resilient companions.

And that’s the goal we should always strive for.

If you'd like help with puppy training, behaviour concerns, or setting your dog up for success, professional guidance early on can make a huge difference.