The Truth About Littermate Syndrome

When you are preparing to adopt a puppy, it can be tempting to want to take home two at once. Who could blame you? Those clumsy, floppy little angels just tug on our hearts with their big, sweet eyes, cuddly little bodies, and endless puppy kisses. If you are adopting from a shelter or breeder who knows their stuff, however, you might find that they advise you against adopting two puppies from the same litter, which could leave you wondering why. Wouldn't it be better for siblings who are already bonded to have each other to grow up with? Not necessarily.

This is because of a phenomenon known as littermate syndrome. In short, littermate syndrome is what experts call the extreme aggression and competition that can sometimes occur between puppies of the same litter when they remain together in one household beyond the usual puppy stage and into adolescence and maturity (via Better Pet). This issue can sometimes become so serious that the dogs are at risk of injuring each other and being truly unable to coexist peacefully, which can break the hearts of owners who adore both of them. So why does this happen?   

While scientific studies and dog experts haven't ever found a simple reason for littermate syndrome, they have noted that it is a common phenomenon and feel it has to do with the natural instincts of dogs as pack animals, dating back to their pre-domestication time as wolves (via Better Pet). 

Interestingly, littermate syndrome can play out in the opposite way as well, but create just as many, if different, behavioral issues (via Great Pet Care). Rather than becoming competitive and aggressive toward each other, dogs who display littermate syndrome can instead become so intensely bonded to one another that it prevents them from interacting normally with other animals or even their own human owners. 

This can look like extreme anxiety when the two are separated, leading to excessive whining and barking to anxiety-triggered destruction of property. It can also cause the dogs never to properly bond with their owners the way they usually would, making training difficult. This is why many breeders and shelters recommend against adopting two puppies from the same litter. If, however, you have done so and are seeing signs of these issues, speaking to your vet and to a professional trainer could help you help your dogs coexist more successfully (via Canine Behavioral Services).