A dog who is aggressive toward other dogs often has not be socialized properly. This happens when people get a puppy who is 4-5 weeks old, before the puppy has had a chance to imprint on other dogs. A much better solution is to let puppies imprint on other dogs at an early age, and only take them away from their mothers and littermates when they are around 8 weeks old.
Socialization means learning to be part of society. When we talk about socializing pet puppies, it means helping them learn to be comfortable as a pet within human society–a society that includes many different types of people, environments, buildings, sights, noises, smells, animals and other dogs.
Imprinting is a form of learning that happens at an early age and lasts the lifetime of the animal. The first person to thoroughly investigate the imprinting process was the Austrian ethologist, Konrad Lorenz, who won the Nobel Prize in 1973 for his contributions to animal behavior. Imprinting takes place during a relatively brief period of time called the sensitive period, after which the information learned during the imprinting process either cannot be or is very difficult to reverse.
In dogs, the sensitive period lasts roughly between a puppy’s 4th and 12th weeks of life. During that time, puppies learn who their mother is, and also learn about future mates and their social group.
If a puppy is taken away from her mother at week 4 or 5, they do not have a chance to imprint on dogs as social partners. The puppy will imprint on people, and subsequently thinks of itself as a person rather than as a dog. So when the puppy is placed in the company of other dogs, it has neither the social skills to know how to interact with them, nor even very much interest in going through dog greeting protocols. She sees herself surrounded by alien beings, and responds aggressively.
Fortunately for the dog-human bond, dogs can imprint on both dogs and people during the sensitive period of imprinting. If a dog is allowed to be with his mother and littermates from about week 4 to about week 8, he will imprint on dogs and will learn dog social skills. If the dog is then placed in the company of people during weeks 8-12, it will imprint on the people that it sees around him.
Imprinting has been shown in a number of mammals as well. The nursery rhyme, “Mary had a little lamb/Its fleece was white as snow/And everywhere that Mary went/The lamb was sure to go,” describes a lamb who was imprinted on Mary!