There’s No Scientific Reason to Believe Littermate Syndrome Exists

Written by Kayla Fratt, CDBC

Searching Google Scholar for “littermate syndrome” + “canines” yields no relevant results. If the scientific community is silent on the issue of “littermate syndrome,” why do behavior consultants keep preaching about it?

I started to think more critically about “littermate syndrome” this January because I received a submission to my “Ask a Behavior Consultant” page on my website that detailed escalating aggression between 9-month-old Husky siblings. As I worked through my answer to the email, I posted on Facebook asking for people’s opinions.

What followed was a fascinating discussion of so-called littermate syndrome in dogs. Several people offered up the fact that they’d owned or raised several pairs of littermates, pointing out that most breeders do this all the time without issue.

So, why does it feel like behavior consultants see so many “sibling rivalry” aggression cases? What about the shelter dogs that panic, screeching and thrashing at the end of a leash if they’re separated from a sibling? Is there something to “littermate syndrome” or not?

When behavior consultants use the shorthand term “littermate syndrome,” they generally mean one of two behavioral patterns:

  • The two dogs appear panicked if separated. They may also be extremely neophobic, especially if separated.
  • The siblings start to develop intense aggression toward one another, especially beginning around social maturity.

The problem is that while these two behavior patterns are not mutually exclusive (I’ve heard reports from clients of siblings who fight viciously but also panic if separated), they are not one and the same. As usual, when using behavioral shorthand (labels), we need to define the behavior before going forward.

One of my favorite sayings is that the absence of evidence is not the same as the evidence of absence. Does the lack of scholarly research on littermate syndrome mean it doesn’t exist, or just that scientists haven’t gotten around to studying it? Or is this just another example of the file drawer problem?

Since the actual term “littermate syndrome” (and any science-y synonyms that I could come up with) seems to be one that exists solely outside of the scientific literature, our only options are to:

1) Rely on self-selected anecdotes of littermates being raised together.

2) Give up and stop attempting to understand littermate syndrome (until better research comes out).

3) Attempt to look at the problem indirectly through research.

I have chosen to rely most heavily on the third approach. Arguably, there’s a fourth option: get out there and study it yourself (if you’ve got funding, please go for it!). Without the resources required to do that, though, the third option is probably our best shot.

Littermate syndrome in pet dogs: An indirect view through research

First, let’s investigate the aggression that is often reported as a risk of raising sibling dogs together. We’ll talk about the fear-based issues next.

Does breed play a role in sibling aggression?

Dominance relationships (as determined by which dog from a pair maintained access to a bone for at least 8 minutes out of a 10-minute trial period) varied widely between breeds in a study by Scott and Fuller (1974) on littermates. In this study, puppies were paired up and a bone placed between them for 10 minutes regularly, starting at 5 weeks of age and continuing through1 year of age. Puppies varied in their responses, which included sharing the bone, taking turns with it, fighting for it, or one puppy primarily possessing the bone while the other puppy was sidelined. The researchers also tested to see what happened if the bone was given to the puppy who had not possessed the bone in earlier trials rather than placed in a neutral place between them. If the subordinate puppy kept the bone when it was given directly to them, but could not gain or keep it when it was placed in neutral space, the relationship was classified as incomplete dominance. Some breeds showed a tendency toward consistent hierarchies, others showed mostly incomplete dominance, and others had no discernable hierarchies.

If a litter displayed a more complete dominance hierarchy, Scott and Fuller expected to see less fighting. However some breeds, such as basenjis, continued to fight each other at increasing levels from 5 to 11 weeks old, then showing another spike in fighting at a year of age. Other breeds, such as Shetland sheepdogs, fought the most at the age of 5 weeks, with fights being reduced to zero by the age of 11 weeks. Before we label Shelties as peaceable, though, the authors point out that the Shelties actually showed intense barking and chasing of littermates in order to gain access to open space and human attention. This behavior was generally led by just one or two littermates that controlled access to outdoor space. Often, one or two Sheltie puppies couldn’t leave the indoor enclosure without being relentlessly chased and barked at by siblings. The authors state that there was no clear dominance order in relation to food in Shelties, but rather one based on space.

Fox terriers fought so infrequently that Scott and Fuller couldn’t determine any dominance relationships within the litter—but they showed the most complete dominance of all breeds over the bone. Beagles and cocker spaniels also almost never fought, but they also were unlikely to have a clear dominance hierarchy (Scott and Fuller, 1974).

Finally, this same study found more examples of complete dominance (which also came with reduced fighting) in male-female pairs. In male-male pairs, the heavier puppy was likely to be dominant. In female-female pairs, there was no clear weight correlation.

This study shows us that it’s possible that the risk of aggression in siblings raised together may be have the dogs’ breed as a contributing factor. One might expect the basenjis from this experiment to show up at a behavior consultant’s office for aggression, while the beagles may cohabitate peacefully. Of course, this study is limited to a specific dominance test for the first year of the puppy’s life—hardly an adequate comparison for a lifetime of variety in a person’s home.

Does sex play a role in social intraspecific aggression?

Since no studies that I found directly compare sibling pairs to non-sibling pairs, I also chose to investigate intrahousehold aggression through the lens of sex. If a given sex pairing is likely to produce aggression in non-siblings, that trend likely holds true in siblings as well. In other words, perhaps my clients’ 9-month-old huskies were having aggression issues due to sex rather than family relations.

Animal behavior databases show that male dogs consistently are presented for aggression more often than females (Beaver, 1999; Association of Pet Behavior Counsellors, 2005; Bamberger and Houpt, 2006). Males are also significantly more likely to display intrasexual aggression toward unfamiliar dogs—79.2% of 180 cases of intrasexual aggression were male-male in a study conducted by Fatjo et al. in 2007. In that same study of 1,040 cases of aggressive dogs in Spain, social intraspecific aggression (aggression between two familiar dogs) was observed at comparable rates in both males and females (Fatjo et al., 2007), but other studies (Beaver, 1999 and Bamberger, 2006) have found higher rates of social intraspecific aggression between females. Again, the study in Spain found differences between breed. Cocker Spaniel female pairs were more likely to have aggression issues, while Terriers were most likely to be aggressive if the pair were both male (Fatjo et al., 2007)

It’s possible that same-sex sibling pairs, therefore, are more likely to have aggression issues—especially if it’s two sisters. Female-female aggression cases are also likely to be harder to resolve than male-female pairs or male-male pairs (Wrubel et al., 2011).

What about siblings that are especially neophobic?

Another common subset of the problems called “littermate syndrome” is the apparent lack of social skills or confidence in one or both of the siblings. This aspect of so-called littermate syndrome is even less well-researched.

It’s possible that the perceived increase in neophobia in siblings raised together is due to the owners doing less thorough socialization with a pair than with a single puppy.

Siblings that panic when separated are probably not much different from any other pair of dogs who have never experienced the world apart from one another. When I worked at Denver Dumb Friends League, we’d see a case or two of these so-called “bonded pairs” per month. These dogs would scream, flail, and alligator-roll on their leashes if separated, even if just by a few meters.

There’s not much in the scientific literature to explain these hyperbonded dogs. In fact, there’s little research on dog social bonding at all that isn’t human-centric or based on free-roaming dogs. Despite what I’ve seen in a shelter setting, intense dyadic pair bonds are unrepresented in scientific literature for dogs.

It’s possible—probable, even—that these intense interdependent relationships can only arise if the dogs are raised together for long periods of time, rarely separated, and rarely introduced to other dogs (or even people). My perception of most of the hyperbonded dogs I met at the shelter was that they seemed relatively socially deprived except for each other. If true, this explanation offers some context for why hyperbonded dogs (siblings or unrelated) are unstudied. There just aren’t very many of them, and it would be hard to recreate the relationship in a lab setting.

What does this mean for our clients?

Clearly it’s not true that all littermates will inevitably experience aggression or neophobia. Many breeders routinely keep siblings or littermates as show or sport prospects. These dogs generally grow up to fit nicely into the show or sport world, free of serious aggression or neophobia. Anecdotally, it seems that sibling aggression or neophobia is mitigated somewhat by having an older third dog in the home—which would be the case in any breeder’s home. But is that the only key? Unlikely.

My personal hypothesis is that perceived littermate syndrome is actually generally a result of several specific conditions that often arise when people attempt to raise siblings together. Of course, all of these observations are, by necessity, anecdotal and based on personal experience.

  • Inadequate socialization, especially with other dogs. Many unknowing owners assume that letting their two puppies play together is an adequate replacement for dog-dog socialization. This misunderstanding is particularly understandable when the two puppies are the same age and breed. In other words, it’s particularly easy to fall into this trap when raising siblings. The owners I personally know who have successfully raised sibling pairs took pains to introduce the puppies and teenage dogs to other dogs, both together and separately.
  • Inadequate environmental management. It also seems that some owners are more likely to slip up on environmental management (removing food bowls, managing access to resting places) when the dogs are perceived as “best friends who have never been apart”—as is the case with siblings.
  • Insufficient “alone time” training. Many of the hyperbonded dogs I met at the shelter were crated together, walked together, taken to the vet together, and so on. The owners sometimes reported that they had “never been apart.” And therein may lie the problem. Just like we’d expect to see separation anxiety if a dog had never been more than three feet from their owner, it’s not surprising to see extreme distress in these adult siblings that have never been taught how to be apart. I’ve found that most of the owners that successfully raise and keep siblings do things with those dogs. They go to training class, shows, trials, and more with just one dog at a time. At the very least, the dogs are used to being trained and crated separately.
  • Failure to meet the dogs’ needs. Many of the cases of sibling aggression that I’ve seen are also paired with a clear lack of mental and physical enrichment for the dogs. In conversations with the owners, I often realized that they assumed that the two siblings could keep each other company. The owners didn’t see a need for puzzle toys, training games, long walks, and so on because the dogs “have each other.”

As always, we need to start with the bottom of the Humane Hierarchy. If the dogs are not having their basic needs met, we can’t start to blame their problems on an unsupported diagnosis of littermate syndrome. The dogs’ needs for socialization, mental enrichment, training, environmental management, and physical exercise all must be addressed before we can start grasping for other explanations.

What’s the harm in calling it littermate syndrome?

Again, it’s possible that siblings raised together are more likely to show aggressive behavior toward each other or display neophobia—there haven’t been any studies published to show us either way. Tendencies toward aggressive behavior in particular may be further explained by breed or sex.

So, what’s the issue with a shorthand label of “littermate syndrome”? When we call something a “syndrome,” it automatically sounds very real and quite serious. Owners may perceive this label as an abdication of responsibility. Oh, it’s a syndrome, they may think. So there’s nothing I could have done. This perception stymies our attempts to make real improvements in the lives of our clients and their dogs.

Potentially worse, calling it a syndrome also makes it easy for us, the behavior consultants, to make another dangerous mental leap. I’ve seen dozens of conversations about littermates on social media where the prevailing response was, “It’s littermate syndrome. You have to rehome one of the dogs.” While it’s certainly unsurprising to see blunt and simplistic behavior suggestions being doled out on social media, the jump to rehoming seems, in my experience,  to come especially quickly when an owner admits their dogs are siblings.

As behavior consultants who value the Humane Hierarchy, we can do better than crying “littermate syndrome!” and jumping straight to rehoming.  Rather than immediately suggesting that struggling owners give up one of their puppies, perhaps we should focus on:

  1. Teaching our clients about basic canine body language.
  2. Helping our clients understand the importance of training, walking, and socializing the dogs separately at times.
  3. Implementing management strategies for our clients, such as crate-and-rotate and muzzle training, when necessary.
  4. Educating our clients on the necessary steps of desensitization, counterconditioning, and teaching alternative behaviors.

Behavior consultants should at least attempt to address the dogs’ social, physical, and mental needs. When necessary, it is our responsibility to implement environmental management, conduct counterconditioning, and teach the dogs obedience skills before suggesting rehoming.


Bamberger, M., Houpt, K.A., (2006). Signalment factors, comorbidity, and trends in behavior diagnoses in dogs: 1.644 cases (1991-2001). Journal of the American Veterinary Medical Association 229, pp. 1591-1601.

Beaver, B., (1999). Canine Behavior: A Guide for Veterinarians. Saunders, St. Louis, pp. 137-199

Fatjo, J., Amat, M., Mariotti, V. M., Jose Luis Ruiz De La Torre, & Manteca, X. (2007). Analysis of 1040 cases of canine aggression in a referral practice in Spain. Journal of Veterinary Behavior, 2:5, pp. 158-165.

Scott, J. P., & Fuller, J. L. (1974). Genetics and the social behaviour of the dog. Chicago: University of Chicago Press.

Wrubel, K. M., Moon-Fanelli A. A., Maranda L. S., and Dodman N. H. (2011). Interdog household aggression: 38 cases. Journal of the American  Veterinary Medical Association 238, pp. 731-50.


Kayla Fratt owns Journey Dog Training, where she provides remote behavioral support to dog and cat owners around the world via email, text, video chat, online courses, and e-books. She also works as a freelance writer. She holds a degree in Organismal Biology and Ecology from Colorado College and is a Certified Dog Behavior Consultant with the IAABC. She is currently driving from Denver to South America with her Border collie and boyfriend. She’s obsessed with conservation detection dogs and loves trick training.