Raising two puppies from the same litter together creates significant behavioral problems including mutual dependence, poor human bonding, and inter-dog aggression as they mature.
Littermate syndrome causes significant long-term welfare problems for affected dogs and their owners. Dogs that develop extreme co-dependence experience severe anxiety when separated, limiting their ability to live normal independent lives. Mature littermate aggression is often severe enough to be life-threatening and frequently results in permanent separation or euthanasia of one dog. The welfare costs for dogs are substantial and entirely preventable by the owner decision not to adopt siblings simultaneously. Educating prospective adopters about this risk is an important welfare intervention.