Littermate Syndrome: Everything You Need To Know

Littermate syndrome typically describes the situation where two puppies from a litter develop serious behavioral issues, usually aggression between the pair, as a result of being raised together. However, there are other presentations of littermate syndrome and some important details to note.

They Don't Have To Be From The Same Litter

Puppies from different litters but close in age can develop littermate syndrome when raised together. Many dog trainers and behavior specialists advise against raising more than one puppy at a time because of how easy it can be for littermate syndrome to develop between puppies.

The Timeline

The first 3-12 weeks of a puppy's life are incredibly important for developing social connections with other dogs, people, and the world. Many signs of littermate syndrome can appear between 8 and 12 weeks of age, with behavioral problems intensifying between 6 and 18 months of age. Because a dog is most impressionable during their early months and first couple of years, preventing and resolving behavior issues as early as possible is critical for success later in life.

A Range of Behaviors

Here are some of the behaviors that, to varying degrees of severity, can comprise littermate syndrome:

• Intense play that escalates to fighting

• Fights that result in severe injury or even death

• Severe separation anxiety from each other

• Intense resource guarding between the pair that results in fighting

• One sibling becoming significantly less confident and more anxious than the other

• The more anxious/fearful sibling developing reactivity or aggression towards unfamiliar dogs and/or people

Prevention Is the Best Intervention

Littermate syndrome can be so severe and deeply rooted that some dog behavior professionals refuse to work on cases involving littermate syndrome. While not EVERY pair of littermates (or similarly aged puppies) will develop littermate syndrome, the best way to prevent littermate syndrome is to not raise more than one puppy at a time. It's recommended to adopt young dogs with a minimum of a 6-month age gap to give them a chance to develop independently. Larger age gaps (12-18 months) are recommended for the best chances of success for multi-dog households.

If You Must...

If you must raise two puppies at the same time, here are some things to consider to increase your chances for success:

• Male-female pairs tend to do better than same-sex pairs, with female-female pairs having the lowest success rate. A study on inter-household aggression found the following improvement rates following treatment plans: male-female: 75%, male-male: 72%, and female-female: 57%.

  • Note that this study did not focus solely on littermates, thus littermate syndrome may not be accurately represented by this study.

  • Also note that this is just one study. Other studies have found breed to play a role in aggression tendencies. For example, one study found that Cocker Spaniel female pairs had higher rates of aggression while male Terrier pairs had higher rates of aggression.

• Dissimilar pairs tend to do better. Pairing a more outgoing pup with a more laidback pup can result in less competition.

• Raising the puppies with older, behaviorally stable dogs (NOT the dogs' mother) may help with establishing boundaries and developing social skills.

• Giving the puppies as many independent (positive!) experiences as possible until they are out of adolescence (can range from 10-24 months depending on breed) can help. Some specialists even recommend having the dogs live essentially separate lives under the same roof until they reach late adolescence or adulthood; this means they eat, sleep, exercise, and socialize separately.

• Breed can play a role, with low-prey drive breeds (like Golden Retrievers and Pugs) having better success in being raised together.

• Obtaining puppies from highly reputable breeders can also increase chances of success. Reputable breeders will have done genetic and temperament testing on the puppies' parents, will have provided a stable and low-stress environment for the mother during pregnancy (which is crucially important because a mother can pass stress on to her puppies and influence their chemical and hormonal development before birth), and should have started giving the puppies positive experiences before they leave their mother and other littermates which is also very important because early life experiences can influence a dog's adult personality and feelings about the world.

Resolving Littermate Syndrome

If you've found yourself and your dogs in the unfortunate situation of living with littermate syndrome, here are some things you can do to help your situation. Note that complete resolution may not be possible in some cases.

• Establish daily one-on-one time with human guardians

• Introduce more independent activities, like separate training, socialization, eating, enrichment activities, walks, etc.

• Provide separate items for each dog and block access from the other dog, or provide an abundance of items (like toys, dog beds, etc.) to reduce chances of resource guarding

• Actively enforce boundaries - around access to resources, by ending rough play, etc.

• Give each dog separate spaces to rest and relax - essentially give the dogs time and space to "catch a break" from each other

• Gradually increase time apart if they experience separation anxiety from each other

• Introduce stability through routines

• Consult a veterinarian and/or veterinary behaviorist to ensure there are no underlying medical conditions and/or to prescribe anti-anxiety medications

• Keep both dogs completely separate, with at least 2-degrees of separation for safety

• Re-home one of the dogs

• In extreme cases, behavioral euthanasia may be the most humane option

However...

Some experts question the validity and existence of littermate syndrome altogether. There's a lack of scientific, peer-reviewed studies that thoroughly investigate littermate syndrome, with many reports of the phenomenon being personal anecdotes. The experts calling littermate syndrome into question aren't dismissing the phenomenon entirely; they have a different explanation: perceived littermate syndrome isn't caused solely by the puppies being raised together, but is instead caused by a set of conditions that arise when people attempt to raise more than one puppy at the same time.

These conditions include:

• insufficient independent activities

• flawed management systems

• insufficient guardian knowledge of canine behavior and body language

• lack of constructive socialization, especially with other dogs

• failure to meet each dog's needs as individuals

My Take

Personally, my take is more in line with the experts who don't like to call it a "syndrome." Calling it a "syndrome" makes it feel less preventable and less treatable, and can cause some behaviorists and trainers to jump immediately to recommending to re-home a dog or other drastic measures.

In a lot of ways anthropomorphizing dogs can do a disservice to them, but I think drawing a parallel to human nature here can be helpful for dog guardians to better understand their pups. Imagine you're an identical twin and had to share every moment of your formative years with your twin - same bed and/or bedroom, same activities at the same time, same attention from parents, same social interactions, etc. It'd be pretty hard to develop a healthy sense of self and you can probably imagine you'd feel a sense of competition with your twin and/or come to depend heavily on them and have a hard time functioning in their absence. A similar thought exercise can be done regarding dogs of similar ages: a 16-year-old human child is much less likely to throw down with a toddler than they are another teenager.

While breed, sex, age, and other genetic factors certainly play a role, I believe many of the factors in people's control can play an equally important role in the success of two young dogs growing up together. Gathering as much information before bringing home two puppies at the same time and having a thorough understanding of the commitment, risks, preventative measures, and how to recognize and resolve issues early and quickly are crucially important for long-term success in living together.

Need Help?

If you aren't sure how to navigate your potential "littermate syndrome" situation, I'm here to help! Schedule a consultation to take the first step in getting life with your dogs back on track.

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