Dog intelligence
Dog intelligence is the ability of a dog to learn, to think, and to solve problems. Dog trainers, owners, and researchers have as much (or more) difficulty agreeing on a method for testing canine intelligence as they do for human intelligence.
Certain breeds, such as Border Collies and Golden Retrievers, are generally easier to train than others, such as some hounds and sled dogs. It is worth noting that these descriptions are relative to other dogs, not relative to the world at large. The ability to learn and obey commands, however, is not the only possible measurement of intelligence.
Dogs are pack animals, which means that by nature they understand social structure and obligations and are capable of quickly learning how to behave around other members of the pack, whether dog or human. Adult canines train their young by correcting them when they behave in an unacceptable manner (biting too hard, eating out of turn, and so on) and reward them for acceptable behavior (by playing with them, feeding them, cleaning them, and so on).
They are also den animals, so that by nature they can easily learn behavior related to keeping the den clean (such as housebreaking), relaxing in an enclosed area (such as a crate during travel or for training), and so on.
Some breeds have been selectively bred for hundreds or thousands of years for the quality of learning quickly; in other breeds, that quality has been downplayed in favor of other characteristics, such as the ability to track or hunt game or to fight other animals. However, the capacity to learn basic obedience - and even complicated behaviour - is inherent in all dogs. Owners must simply be more patient with some breeds than with others.
Some people feel that the ability to learn quickly is a sign of intelligence; others feel that it is a sign of blind subservience and that the truly intelligent dogs are breeds such as Siberian Huskies, who are not particularly interested in pleasing their owners but who are fascinated with the myriad possibilities for escaping from yards or catching and killing small animals - often figuring out on their own numerous inventive and ingenious ways of doing both.
For example, some might say that guide dogs, which are required to be obedient at all times, are not intelligent dogs because they do not spend a lot of time figuring out new things to do. However, they must learn a tremendous number of commands, understand how to act in a large variety of situations, and recognize threats or dangers to their human companion, some of which they might never before have encountered.
Some tests for intelligence involve the dog's ability to recognize and respond to a large vocabulary; other tests involve their desire or ability to respond to different situations. If you put a towel over a dog's head, is the intelligent dog the one who pulls it off or is the intelligent dog the one who sits and waits, figuring that humans do strange things from time to time and if they put the towel on the dog's head there must be a reason for it? Just as with humans, there is a wide variety of interpretations as to what makes a dog "intelligent".
Research
Various studies have attempted to confirm the intelligence of dogs in a rigorous manner. A recent example is animal psychologist Juliane Kaminski's paper in Science that demonstrated that Rico, a Border Collie, could learn over 200 words. Rico could remember items' names for four weeks after last exposure (Kaminski eliminated the Clever Hans effect using strict protocols).
Rico was also able to interpret phrases such as "fetch the sock" in terms of its component words (rather than considering the utterance to be a single word): he could give the sock to a specified person.
Psychologist Kathy Coon ranked dog breeds by intelligence in her book The Dog Intelligence Test based on a standardized intelligence test for dogs.
Stanley Coren ranked dog breeds by intelligence in his book The Intelligence of Dogs based on surveys of dog trainers.