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jThe dog or domestic dog (Canis familiaris) is a domesticated descendant of the wolf, characterized by an upturning tail. The dog derived from an ancient, extinct wolf,and the modern grey wolf is the dog's nearest living relative. The dog was the first species to be domesticated, by hunter–gatherers over 15,000 years ago, before the development of agriculture.

Due to their long association with humans, dogs have expanded to a large number of domestic individualsand gained the ability to thrive on a starch-rich diet that would be inadequate for other canids.Over the millennia, dogs became uniquely adapted to human behavior, and the human-canine bond has been a topic of frequent study.

The dog has been selectively bred over millennia for various behaviors, sensory capabilities, and physical attributes. Dog breeds vary widely in shape, size, and color. They perform many roles for humans, such as hunting, herding, pulling loads, protection, assisting police and the military, companionship, therapy, and aiding disabled people. This influence on human society has given them the sobriquet of "man's best friend."

Dog behavior is the internally coordinated responses (actions or inactions) of the domestic dog (individuals or groups) to internal and external stimuli. As the oldest domesticated species, dogs' minds inevitably have been shaped by millennia of contact with humans. As a result of this physical and social evolution, dogs have acquired the ability to understand and communicate with humans more than any other species and they are uniquely attuned to human behaviors.Behavioral scientists have uncovered a surprising set of social-cognitive abilities in domestic dogs. These abilities are not possessed by the dog's closest canine relatives or other highly intelligent mammals, such as great apes, but rather parallel to children's social-cognitive skills.

Unlike other domestic species selected for production-related traits, dogs were initially selected for their behaviors. In 2016, a study found that only 11 fixed genes showed variation between wolves and dogs. These gene variations were unlikely to have been the result of natural evolution and indicate selection on both morphology and behavior during dog domestication. These genes have been shown to affect the catecholamine synthesis pathway, with the majority of the genes affecting the fight-or-flight response (i.e., selection for tameness) and emotional processing.Dogs generally show reduced fear and aggression compared with wolves.Some of these genes have been associated with aggression in some dog breeds, indicating their importance in both the initial domestication and later in breed formation. Traits of high sociability and lack of fear in dogs may include genetic modifications related to Williams-Beuren syndrome in humans, which cause hypersociability at the expense of problem-solving ability.

Dog intelligence is the dog's ability to perceive information and retain it as knowledge for applying to solve problems. Studies of two dogs suggest that dogs can learn by inference and have advanced memory skills. A study with Rico, a Border Collie, showed that he knew the labels of over 200 different items. He inferred the names of novel things by exclusion learning and correctly retrieved those new items immediately and four weeks after the initial exposure. A study of another Border Collie, "Chaser," documented his learning and memory capabilities. He had learned the names and could associate by verbal command over 1,000 words.Dogs can read and react appropriately to human body language such as gesturing and pointing and human voice commands.

One study of canine cognitive abilities found that dogs' capabilities are no more exceptional than those of other animals, such as horses, chimpanzees, or cats. One limited study of 18 household dogs found that they lacked spatial memory, and were more focussed on the "what" of a task rather than the "where".

Dogs demonstrate a theory of mind by engaging in deception.An experimental study showed compelling evidence that Australian dingos can outperform domestic dogs in non-social problem-solving, indicating that domestic dogs may have lost much of their original problem-solving abilities once they joined up with humans. Another study revealed that after undergoing training to solve a simple manipulation task, dogs faced with an insoluble version of the same problem look at the human, while socialized wolves do not.

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