The only thing more exciting than bringing home a new puppy? Bringing home two new puppies. But as fun as the overload of cuteness may seem at the start, there can be drawbacks to adopting puppy siblings at the same time, including the risk of littermate syndrome, or the development of a sibling bond that is so strong, it can cause anxiety and aggression in dogs. 

Here, experts explain the signs of littermate syndrome in dogs, how canine temperaments can create behavioral issues when adopting two puppies from the same litter and what dog owners can do to prevent anxiety and aggression in siblings.

What is littermate syndrome in dogs?

Littermate syndrome, also commonly referred to as littermate dependency, is a concept that describes a range of dependency behaviors when puppies from the same litter are raised in the same household. “Littermate syndrome is the idea that puppies can develop behavioral issues related to their extreme attachment to their littermate,” says Stacy Choczynski Johnson, a doctor of veterinary medicine with over two decades of experience. 

However, Johnson notes that in her 12 years of clinical practice, she most often attributes behavioral issues like separation anxiety and aggression in dogs to other factors, such as socialization and training, and many dog behaviorists agree.

“Most owners do not take the time to train them separately or take them out for socialization separately which then causes the breakdown of the dog’s bonding to the humans in the home and bonding more to their sibling,” explains professional dog trainer and behaviorist Sally Grottini with JustAnswer who has over 30 years of experience working with canines.

“You will likely find that without the alpha sibling around, a fearful sibling will suffer from separation anxiety and destroy things in the home.”

—Sally Grottini, professional dog trainer and behaviorist with JustAnswer

Littermate syndrome in dogs vs. cats

Unlike littermate syndrome in cats, littermate syndrome in dogs is more recognized by animal behaviorists and is also known as littermate dependency, Grottini explains. This is due to the research that exists around canine temperaments, or the biological differences in a dog’s behavior that exist even between siblings.

“[Dogs] are born with an instinct and temperament that develops as they age,” Grottini says. These temperaments vary from shy to socially dominant, or alpha, and can cause unhealthy littermate dynamics, she explains. 

“You will likely find that without the alpha sibling around, a fearful sibling will suffer from separation anxiety and destroy things in the home,” Grottini says, noting that these behavioral issues can then be exacerbated later in life. Have two alpha dogs? That is when fighting can start as they get older, she says. 

“Owners are usually shocked to see their loving siblings who got along so well for the first year suddenly go at each other,” Grottini says. “But this is [a common] outcome when you adopt two siblings or even two same-age puppies, that are not actual littermates.”

Is it good to adopt two puppies from the same litter?

“There are really no pros to adopting two [dogs] from the same litter,” Grottini says. “Some people get lucky where the dogs do get along for a while, but there are more times than not that as they grow, their dog instincts and their temperaments kick in.”

Another downside of adopting two active puppies at the same time is that working one-on-one with each dog is simply more challenging, Johnson notes. Since training two dogs at the same time can be burdensome, she recommends spacing out adoption by at least six months or until the first puppy is fully socialized and house-trained.

What are the signs of littermate syndrome in dogs? 

The first sign of littermate syndrome dog owners will notice is that the siblings are very attached to each other, Grottini explains. This can happen as early as 8 to 10 weeks. “They may not want to leave the other dog and you may see signs of anxiety and stress when they are separated,” she says. These early signs of littermate syndrome can then develop into behaviors such as aggression, separation anxiety and issues with training independently, explains Grottini. 

How to prevent littermate syndrome when adopting two dogs 

Preventing littermate syndrome is challenging, and much depends on dog owners, Grottini says. From separate training classes to walks, here are the best ways to avoid littermate syndrome and behavioral issues when adopting two dogs from the same litter:

Wait to adopt puppies until at least 8 weeks of age

“Puppies should stay with the litter until they are 8 to 10 weeks old as this is the best time frame for them to learn some impulse control and bite inhibition,” Grottini advises. 

According to the American Kennel Club, bite inhibition is the awareness a dog develops to know they shouldn’t bite too hard when interacting with others. This is best learned when a dog’s mother or littermates reacts instinctively with a yelp if a puppy’s bite inflicts pain. This teaches them that biting too aggressively and impulsively will have a negative effect.

“Taking [a dog from a litter] too soon can make it harder to teach them these things for the owner,” Grottini says. At 8 to 10 weeks old, puppies are also independent of the mother and can eat on their own as well as take in new environments, she adds.

Socialize puppies early and individually 

“Early in life, a puppy should be exposed to other dogs — when fully vaccinated — and a variety of environmental stimuli and people of all ages,” Johnson says. “Growing up with a littermate can help with socialization without exposure to outside pathogens.” 

But only relying on a littermate to socialize your pup may prevent them from bonding with other humans and getting comfortable and confident with their surroundings, adds Johnson. 

To set up dogs for success, the American Kennel Association recommends these tips when it come to puppy socialization which should be done separately for each dog, according to the experts we spoke with:

• Introduce your puppy to as many different types of people, places, noises and even textures like different floors and landscapes as as possible. 
• Make new experiences positive by giving treats and praise when they are exposed to the feeling of seeing or interacting with something that is unfamiliar. 
• Remain calm when introducing your puppy to new people and dogs — if you are nervous, your pet will be nervous, too, and may become fearful of strangers in the future.
• Start slow with new introductions — taking your puppy to a very busy public place can be overwhelming and result in a fearful response to groups of strangers in the future.

Separate littermates as much as possible at the start

To prevent behavioral concerns, Johnson recommends doing even basic activities around the house separately. “For example, you may feed the puppies in separate rooms to allow them the time and space to eat — and to prevent food-related aggression.” 

Training should also be done separately no matter whether it’s inside or outside your home, according to our experts. Johnson recommends that leash training be done one-on-one initially. “You may even choose to do obedience classes separately,” she says. “At a minimum, two handlers should attend obedience class if both puppies will attend.”

“Early in life, a puppy should be exposed to other dogs — when fully vaccinated — and a variety of environmental stimuli and people of all ages. Growing up with a littermate can help with socialization without exposure to outside pathogens.” 

—Stacy Choczynski Johnson, a doctor of veterinary medicine

Can littermate syndrome be cured?

By the time an owner realizes their dogs have suffered from littermate syndrome as a puppy, the dogs are usually in graphic fight episodes, Grottini warns. “Even doing strict routine training won’t guarantee a good outcome, and training will likely need to be for the rest of the dog’s lives,” Grottini says. 

If you suspect your dogs are showing signs of littermate syndrome, speak with your veterinarian who can recommend the best course of action, which often will include a new training regime and routines at home to mitigate fighting, our experts advise.

The bottom line on littermate syndrome in dogs

Littermate syndrome is when two puppies are adopted together, and they bond too strongly with their littermate rather than the humans and other pets in the home. This can cause behavioral issues like dependency on one another, aggression toward others and separation anxiety. 

To prevent littermate syndrome, dog owners should start separate training classes, socialization and play and walk outings as soon as they get the puppies home, advises Grottini, and these should continue to the maturity of the dog. She concludes, “You must treat them as individual dogs to develop them to their full individual potential.”