top of page

Seeing Double! The dangers of littermate syndrome

By Hannah Burton

Believe me, I am familiar with the temptation to take home every puppy I encounter! While it can be so tempting to bring home every bundle of joy you encounter, if you bring home littermates it can be potentially very damaging for those puppies’ development.

Trust me, I know it sounds backwards, given that dogs are social animals and having two puppies means they can play together and give each other company. While a good idea in theory, it can often lead to heartbreak and in some cases, severe aggression.

This is a concept known to trainers and behaviourists as littermate syndrome. It is sometimes referred to as sibling syndrome, though this is less common.

It is important to note that littermate syndrome doesn’t just occur with puppies from the same litter! Any two dogs introduced to live with each other during young puppy-hood, a crucial developmental stage, could start to show symptoms of littermate syndrome.

What is littermate syndrome?

Littermate syndrome is a descriptive term that refers to one of two behavioural patterns:

1. The puppies struggle to be separated from each other, appearing visibly panicked when apart. There is also anecdotal evidence that these dogs appear to be particularly nervous of new items, people, and dogs, especially when separated from their littermate.

2. Siblings become aggressive towards each other, with this aggression beginning around social maturity.

Though there are many trainers and behaviourists who swear that they have encountered littermate syndrome in the field, it has never been studied and verified scientifically. Empirical evidence still has great value but it is important to consider the scientific literature around this topic.

What are some of the common behaviours associated with littermate syndrome?

Within the umbrella term of “littermate syndrome”, there are some common behavioural patterns you may expect to see. These include, but are not limited to:

1. Hyper-attachment: This term refers to the puppies’ relationship with each other. Left unchecked, puppies from the same litter than remain together past 10 weeks become completely dependent on each other. It is not uncommon for puppies who have grown up like this to be unable to be more than a few feet apart. This also leads to an “us against the world” mentality, leading to the pups more likely to get together to cause mischief!

2. Poor trainability: The previously mentioned “us against the world” mentality often results in puppies with littermate syndrome having a very short attention span. Playing and interacting with their littermate is often considered to be much more important than any form of training!

3. Personality stunting: This hyper-attachment often results in the puppies fitting into one of two personality types. The first will become overconfident and is likely to be the instigator of trouble. The second is likely to be slightly more reserved and less outgoing, completely dependent on the first puppy. Unfortunately, this second puppy is unlikely to ever reach their full potential.

4. Neophobia: Siblings raised together can often be terrified of new items or novelty. This is especially true if they are ever separated for any reason.

5. Aggression: Despite their inability to be separated and love of interaction with each other, interactions can quickly grow to be nasty. Wrestling and fighting can often lead to incidents that draw blood and/or result in serious injury.

Guide Dogs Experiment

In order to improve the efficacy of their puppy raiser programme, Guide Dogs began an experiment whereby they sent each raiser two puppies instead of one.

However, soon after its inception, the organisation found an interesting trend. In every household, one of the puppies became shy and timid, and the other became incredibly outgoing. The littermates would become very dependent on each other and struggled to be separated. In some cases, they showed extreme aggression towards each other, despite the fact they couldn’t be separated.

None of the puppies involved were suitable for work and were all retired at just six months old. As a result, Guide Dogs abandoned the experiment after just a few months, and it has not been attempted again.

Does a puppy’s gender make a difference?

Interestingly, littermate aggression has never been compared between sexes, although anecdotal evidence suggests it is worse between two single sex pairs, especially between two female puppies!

There have been many studies looking at aggression in dogs that have found that males are on average, more aggressive that females (Fatjo et al., 2007). Male dogs are much more likely to show aggression towards other male dogs, with almost 80% of 180 cases surveyed consisting of male-male aggression (Fatjo et al., 2007). However, other research has suggested that aggression cases are mostly female-female aggression (Bamberger & Houpt, 2006).

This divided research suggests that inter-dog aggression can occur regardless of the sex of the puppies.

Does neutering make a difference to potential for aggression in littermates?

There is heavily conflicting information about if neutering can affect aggression. Many veterinary sites abdicate for neutering in aggression cases (Horwitz & Landsberg, 2021).

However, more recent evidence is suggesting that neutering your male dog increases the chances they show more fear related behaviours (McGreevy et al., 2018). Older papers have suggested that in fact neutering increases the chances of your dog showing aggression (Duffy & Serpell, 2006; Farhoody, 2010), which may relate to the suggested fear related behaviours observed in the McGreevy paper. This suggests that neutering your dogs may actually make your already existing behavioural issues worse!

At present, there is no research on how littermates or dogs that live together are affected by neutering. This lack of evidence, however, does not necessarily mean that littermates are less likely to become co-dependent on one another once neutered or show less aggression.

This research highlights the importance of speaking to your vet and/or vet behaviourist before deciding is neutering is the right choice for you and your dogs.

Does breed make a difference to littermate syndrome?

It seems that of the limited studies we have available, there is little evidence to suggest that one breed is less likely to develop littermate syndrome than other. There is some evidence that breed makes a difference on which sex pairing decreases your chances seeing inter-dog aggression, but this is incredibly limited.

For example, female-female Cocker Spaniel pairings were more likely to show inter-dog aggression than male-male pairings (Fatjo et al., 2007). The exact opposite can be seen in terrier breeds, who are more likely to show aggression in male-male pairings (Fatjo et al., 2007). This evidence suggests that regardless of breed, you are likely to see levels of inter-dog aggression if you bring home two puppies from the same litter.

There is also research that indicates that sibling puppies are more likely to argue over resources, regardless of sex or breed. Research indicates that aggression over resources is most likely to peak between 5 and 11 weeks, then again around one year of age (Scott & Fuller, 1974).

The same study found that littermate puppies within the home may also compete for space (both resting places and outdoor space), food, and human attention (Scott & Fuller, 1974).

This is not to say that littermate syndrome can only occur within the studied breeds. Regardless of what breed of dog you decide to bring home, it is important to make sure you do your best for the development of the puppy in front of you.

Conclusions – Should I get two puppies instead of one?

Though it can be incredibly tempting to bring home two puppies at the same time, it can lead to many practical challenges that may make it not worth it. Managing the development of two puppies is not double the work, it is triple, or even quadruple!

If you want to run a multi-dog household, it is much better to allow your puppy to develop to social maturity before bringing home a new puppy. Having a nice role model for your pup than a partner in crime! Alternatively, adopting two adult dogs who are already socially mature may be a good option.

What to do if you already have two puppies?

If you have already brought home two puppies and are worried about your dogs developing sibling syndrome, there are some preventative measures you can take.

The key measure is giving your puppies time apart every day! In order to have healthy development, puppies need time to build confidence, build bonds with their humans and be rewarded for positive decisions. To facilitate for both your puppies, it is recommended that you walk your puppies separately as often as possible, train your puppies separately whenever you can and offer them space to rest separately.

Remembering that you have two puppies is also key. They will need two separate sleeping areas, two separate feeding stations away from each other to avoid conflict and separate training sessions. As much as possible they should be walked separately and should go to the vets and/or groomers separately. This is all crucial for bonding with you, being independent of each other and promoting a healthy human-animal bond.

It may sound cruel to keep them separate as much as possible, but it is essential for healthy mental growth. The goal is to raise two, fully developed dogs who happen to exist in the same household. It is a lot of work to prevent littermate syndrome, but it is a lot more work to treat it once it’s developed!

If you have any concerns about littermate syndrome in your own puppies, or need advice on purchasing two puppies from the same litter, please get in touch with Wanderdog for tailored advice


Bamberger, M., Houpt, K.A. (2006) Signalment factors, comorbidity and trends in behavior diagnosis in dogs: 1,644 (1991-2001). Journal of the American Veterinary Medical Association. 229: 1591-1601

Duffy, D.L., Serpell, J.A. (2006) Non-reproductive effects of spaying and neutering on behavior in dogs. Proceedings of the Third International Symposium on Non-Surgical Contraceptive Methods for Pet Population Control.

Farhoody, P. (2010) Behavioural and Physical Effects of Spaying and Neutering Domestic Dogs (Canis familiaris). Masters thesis submitted to and accepted by Hunter College.

Fatjo, J., Amat, M., Mariotti, V.M., de la Tore, R., Manteca, X. (2007) Analysis of 1040 cases of canine aggression in a referral practice in Spain. Journal of Veterinary Behaviour. 2(5): 158-165

Horwitz, D., Landsberg, G. (2021) Dog Behavior and Training – Neutering and Behavior. Last Accessed: 08/02/2021. [Available from:]

McGreevy, P.D., Wilson, B., Starling, M.J., Serpell, J.A. (2018) Behavioural risks in male dogs with minimal lifetime exposure to gonadal hormones may complicate population control-benefits of desexing. PLoS One. 13(5). DOI:

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


Featured Posts
Recent Posts
Search By Tags
Follow Us
  • Facebook Basic Square
bottom of page