The email described a familiar scenario: “We were planning to adopt one puppy, but the breeder said that raising two sisters would be easier. After we brought the girls home at nine weeks, their behavior became increasingly out of control. My husband and I could not get their attention for more than a second or two—it was as if we weren’t even in the same room. And then they started displaying alarming fearfulness of people and other dogs.” I made an appointment for a home visit so I could meet the family and the puppies.

Many dog behaviorists, trainers, breeders and shelters discourage adopting siblings. Anecdotal evidence suggests that behavioral issues may arise during key development periods because the two puppies’ deep bond impedes their individual ability to absorb and grasp the nuances of human and canine communication. Since fear is the canine’s default reaction to odd or unfamiliar stimuli, this muddled understanding of the world around them can lead to impaired coping mechanisms later on.

Of course, many factors influence behavior, and not all siblings raised together will exhibit this problem, which is called “littermate syndrome”; it’s a risk, not a foregone conclusion.

Early Indicators of Littermate Syndrome

Signs of littermate syndrome include fearfulness of unfamiliar people, dogs and other novel stimuli (neophobia); intense anxiety when separated, even briefly; and difficulty learning basic obedience skills. In some cases, the two dogs will fight incessantly.

GET THE BARK IN YOUR INBOX! Sign up for our newsletter and stay in the know. Email Address:

Over lunch, veterinarian and dog behaviorist Dr. Ian Dunbar and I discussed raising sibling dogs. “It’s a disaster waiting to happen for the littermates because they don’t get socialized to other dogs or people, let alone to their owners,” he said. Many owners assume that the dogs’ interactions with one another are adequate, “but when the puppies are five or six months old and meet an unfamiliar dog in a novel setting, they absolutely freak out.”

Dunbar points out that raising littermates necessitates training two puppies, which is particularly challenging when they’re essentially wearing blinders to all but each other. “It’s more than twice the work; it’s exponential. The two combine to produce levels of energy that we can barely measure. Tension develops in training and compliance as they squeeze the owner out of the relationship. They’re always living with an enormous distraction: each other.”

Certified Applied Animal Behaviorist Patricia B. McConnell, PhD shares her idea's against taking in littermates, "They are so busy playing with each other (or squabbling, more on that later), that you become the odd man out... It seems harder to get their attention, harder to teach them emotional control and harder to teach them boundaries" she continues "I have seen some nasty cases of bullying or outright aggression between dogs of the same litter, and it feels as though it is more common than between dogs who come into the family from different litters."

The Tie That Binds

Cohabitating siblings may become so emotionally dependent on each other that even short separations provoke extreme distress. Behavior specialist and author Nicole Wilde recalls a case in which two nine-year-old sibling Huskies attended her group class. “They were so bonded to each other that I literally could not take one and walk a few feet away to practice loose-leash skills because the other would scream.”

Wilde believes the problems are rooted in hyper-attachment, which leads to hindered social development and communication issues. “People assume that having two same-age pups who play together and interact constantly covers their dog-dog socialization needs, but they in fact don’t learn how other [dogs] play and have no idea about social skills with other puppies, adolescents or adult dogs. Perhaps one puppy is a bit of a bully, which his littermate puts up with, but his rude behavior might not be tolerated by a new dog in a new setting.”

During my appointment with the family, we determined that the best course was to rehome one of the 12-week-old siblings. Dunbar agrees that it’s often best to separate littermates, especially if symptoms appear early, so that each has a chance to develop normally as an individual. This is obviously a burdensome decision for the overwhelmed owner to make, a sort of canine Sophie’s Choice, so he recommends that potential new owners meet both puppies and determine which to take home.

Together Forever: What if you already have litter mates?

Those committed to raising a pair of littermates should ensure that the puppies spend significant portions of every day apart so each learns how to be alone—a key lesson in any well-thought-out puppy program. This means feeding, walking and training each puppy separately, with individual crates in different parts of the home. Even trips to puppy socials and the vet should be separate so they learn to incorporate these episodes into their respective psyches without being overly dependent on their littermate.

This separate-but-equal arrangement is timeconsuming, exhausting and seems to defeat the original intent of acquiring siblings. Wilde notes that planned separations must begin immediately. “I’ve been called into homes where four-month-old siblings have been sleeping in the same crate for eight weeks and not purposefully separated by the owners, who had the best intentions but were unaware of littermate issues. Even getting the puppies to sleep in separate crates right next to each other is traumatic for them.”

Dunbar, too, is adamant that one of the key lessons a puppy must master is how to be content with being alone, which is all but impossible with two siblings. “Once we’ve done that, yes, he can live with other dogs and have free run of the house. But if you don’t teach puppies early on how to be alone, and especially with siblings who have always been together, it will be catastrophic when one dies.” Dunbar encourages multiple dog households— “I always like having three dogs”—but the timing, temperament and age that each enters the home is paramount.

Most people have never heard of littermate syndrome, finding out about it while researching their dogs’ problematic behaviors. Increasingly, however, trainers and behavior professionals recognize that the cons of adopting siblings far outweigh the pros. “The only advantage I can think of is a short-term gain of the puppies being less lonely in the first month of life,” says Dunbar. “Everything else is a loss.”

Exceptions and Hope

While there are indeed struggles in raising siblings—including ongoing dog aggression and fighting often seen between same-gendered littermates— there are also well-adjusted cohabitating pairs. A common thread seems to be that littermates are more likely to thrive when introduced into a household with an older dog, who perhaps acts as an arbiter and stabilizing influence.

Myriad factors affect dog behavior, including genetics, early life experiences and owner engagement. As University of California, Davis, veterinary behaviorist Dr. Melissa Bain points out, “Two fearful littermates very well may be genetically predisposed to fear.” Bain is less inclined to apply the term syndrome to the set of symptoms. “It makes you think all littermates have problems, which is not the case.” She also emphasizes that the level of owner involvement is key, saying, “The symptoms escalate when the owners treat them as one dog with eight legs.” When conflict ensues between the pair, Bain believes it’s due to the dogs being similar in size, age and gender. “This uniformity makes it difficult for the siblings to delineate a hierarchy,” she said.

After one of the siblings had been rehomed, I received an email from the owner describing how the remaining puppy began to thrive under a remedial socialization program. “Dora has blossomed in the last three months into a delightful household companion, and she continues to improve. She now approaches people out of curiosity. We know she would still be fearful had we not separated the two before it got any worse. Dora has become more confident with all kinds of dogs, and successfully completed a group obedience class.”

Increased Awareness

Recognition of the risks of dual adoption appears to be spreading, with many breeders and shelters declining to place siblings together. Shelley Smith, adoption center manager at Pets Unlimited in San Francisco, said her shelter stopped placing siblings together after a particularly disturbing case. “A Dachshund mix named Thelma was returned to the shelter because her sibling repeatedly attacked her; she had multiple injuries by the time the heartbroken family returned her to us. Thankfully, we were able to rehome Thelma, but it’s almost certain the fighting and anxiety could have been avoided had the two littermates not been placed together. We now separate siblings and inform adopters about the rationale for our policy.”

While siblings blessed with extraordinary genes and socialization-forward owners may avoid littermate syndrome, the consensus among canine professionals is that it’s not worth the risk. Most would encourage new owners to adopt a single puppy who suits their lifestyle and to focus on the training and socialization that strengthens the interspecies bond unique to humans and dogs. Once your puppy is a dog, by all means, get a second, since the two will be at completely different stages, and the older one may very well emerge as a great life teacher to the younger.