Littermate syndrome (also known as sibling aggression or littermate aggression) is an anecdotal, unscientific term that refers to a whole range of behavioral problems that tend to occur when canine siblings (littermates) are raised in the same household beyond 8 to 10 weeks of normal age, when puppies are usually. As we mentioned in our “Choosing a Puppy” article, littermate syndrome is a serious behavioral condition. The condition develops when two young dogs end up getting too attached to each other. It may seem ideal when dogs are such close friends, but major problems can arise in the future.
The problem is that their relationship with each other can prevent them from creating adequate links with humans and hinder their social development. As the name suggests, littermate syndrome usually exists in two puppies from the same litter, however, it can also occur when two puppies of a similar age are raised together. Although littermate syndrome doesn't develop every time two puppies are raised together, it's common enough to warn against having two puppies at the same time. My brother and I (we live in the same house) adopted Rottweiler puppies named Bach and Beethoven (both males) and for the past few days, it seemed that they were having fun playing together until recently they started fighting hard, where both my brother and I were scared that one of them might get hurt.
As mentioned earlier, littermate syndrome sometimes causes puppies to focus on the other puppy rather than the humans in the household. Littermate syndrome can affect dogs of any breed, and it can also affect unrelated puppies that are adopted at the same time and raised together. Shelters also have stories of couples (or one of a couple) who were returned because the adoptive owner feared for the welfare of the sibling who was being bullied. If you have misbehaved sibling puppies (or are thinking of training two puppies from the same litter together), keep reading to learn everything you need to know about littermate syndrome in dogs.
Another sad result related to this is that when one of the sister dogs dies, the other can't cope and lives the rest of his days in torment. Countless stories can be found of puppy siblings apparently suffering from the common symptoms of littermate syndrome. Puppies with littermate syndrome only interact with each other and become highly dependent on each other for a sense of security and normalcy. Many experts in canine behavior believe that it is best to rehouse one sibling when a partner shows early signs of littermate syndrome, so that both puppies have the opportunity to grow separately and become stable, balanced adults.
Speaking of rescues, many shelters don't even allow two puppies to be adopted at the same time, even if they're from different litters. Training two puppies from the same litter may take longer than expected because puppies are very distracted from each other.