Health & Environment

Understanding Littermate Syndrome In Puppy Pairs

Two puppies may exhibit certain unfavorable behaviors like anxiety or fear if raised together immediately after weaning from their mother.
By Texas A&M University School of Veterinary Medicine and Biomedical Sciences September 28, 2023

two puppies
A pair of closely bonded puppies tend to be ‘in their own world’ and will likely look to one another for how to respond to new stimuli rather than looking to their owner, says a Texas A&M veterinarian.

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Puppies are so adorable that you may feel tempted to adopt two instead of just one. Even though having two puppies may seem like a good idea, raising them together can cause long-term behavioral problems.

Harmony Diers, a veterinary technician at the Texas A&M School of Veterinary Medicine and Biomedical Sciences, says that while these problems are not a guaranteed outcome of raising two puppies together, nor are they breed specific, littermate syndrome is more common than not when raising two puppies from the same litter, or even two close in age from separate litters.

“Littermate syndrome refers to a specific set of unfavorable behaviors that two puppies might exhibit if raised together immediately after weaning from their mother, like anxiety or fear,” Diers said. “This syndrome occurs when a pair of puppies raised together develop such a close bond with one another during important socialization stages that it hinders their ability to bond with their new owner.”

Recognizing Littermate Syndrome

The socialization stage, or when a puppy is between 3-12 weeks old, is when a puppy’s senses mature, encouraging them to explore their environments and socialize with others. It is also the stage when puppies become aware of their relationship with humans.

“Puppies ultimately learn how to react to things they encounter in their world during the socialization stage,” Diers said. “If bonded closely with their owner, an individual puppy will look to their owner for an answer on how they should react, either to be praised for appropriate reactions or to be redirected away from inappropriate ones.”

For example, if you adopted a single puppy and that puppy chewed on a shoe, their decision to behave that way in the future — or not — would depend on your response to their interesting choice of “snack.” However, if you have two puppies, they might mutually agree that shoe chewing was a great choice without waiting for your response, reinforcing the undesirable behavior.

“A pair of closely bonded puppies tend to be ‘in their own world’ and will likely look to one another for how to respond to new stimuli rather than looking to their owner,” Diers explained. “This gives them the opportunity to feed off of one another’s fear and anxiety, which can develop into unhealthy behaviors.”

Common behaviors that can develop in puppies with littermate syndrome, according to Diers, include:

  • Separation anxiety when away from the other puppy because of a co-dependence on each other
  • Fear of new people, animals, objects, or experiences
  • Fear aggression, which can occur when a puppy encounters a new experience, feels threatened or scared, and reacts by barking, biting or trying to escape
  • Reactivity when on a leash
  • Fighting habits when they reach sexual maturity around 6 months of age
  • Aggression or nervousness in the more dominant puppy; timidity and withdrawal in the less dominant puppy

Intervention As A Prevention Method

 Since littermate syndrome is common and can cause unwanted behaviors, potential dog owners should avoid raising two puppies at the same time. Yet if owners are in a situation where they are raising a pair, owner intervention is preferred to help deter those unwanted behaviors, as long as owners can commit to the process.

“Some experts believe that rehoming one of the puppies is ideal; however, with the high numbers of homeless dogs or dogs that end up in shelters, this is not always the best option,” Diers said. “If a family has two puppies of the same age in their home, owners must take intentional steps to allow their puppies to bond with them and develop as individuals.”

Diers encourages owners to start with crating the puppies separately, either in different rooms or with distance between the crates within the same room.

Another effective way for owners to bond with their pets individually is through walks.

“Puppies should be walked individually and taken on solo outings with their owner within the first few months, gradually moving to each puppy being walked by a separate handler at the same time,” Diers explained. “Once this step is started, owners should practice walking the dogs in opposite directions to monitor for reactivity. Again, the puppy should be most concerned with their owner’s presence and not the lack of the other puppy’s presence.”

Finally, Diers recommends that owners provide separate, one-on-one play times to develop a close bond with each puppy. If a pair of puppies attends obedience training, owners should ensure the puppies are in separate classes, too.

“It also can be beneficial if the pair of puppies join a home with an older dog because in some cases, the older dog can teach boundaries and offer appropriate corrections when necessary,” Diers said. “Nevertheless, you should always seek the advice of your family veterinarian when deciding on bringing a puppy or two home and gather information on behavioral intervention when necessary.”

Having two puppies at the same time can be a challenge. Still, if owners are active in raising the pups, they can prevent behavioral problems resulting from littermate syndrome, ensuring strong human-animal bonds and twice the puppy love.

Bringing home a new puppy? Texas A&M’s Dog Aging Project, the largest observational companion canine health study in the world, allows owners to enroll new puppies and dogs of all ages, breeds, sizes, and health statuses to help discover the factors that lead to longer, healthier lives for dogs. Join the Dog Aging Project Pack at

 Pet Talk is a service of the School of Veterinary Medicine and Biomedical Sciences, Texas A&M University. Stories can be viewed on the web at Suggestions for future topics may be directed to


Media contact: Jennifer Gauntt,

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