Dogs bark for many reasons, such as when perceived intruders (humans, dogs, or other animals) approach their living space, when hearing an unfamiliar or unidentified noise, when seeing something that the dog doesn't expect to be there, or when playing. Barking also expresses different emotions for a dog, such as loneliness, fear, suspicion, stress, and pleasure. Playful or excited barks are often short and sharp, such as when a dog is attempting to get a person or another dog to play. Dogs generally try to avoid conflict; their vocalizations are part of what allows other dogs to tune into their emotions, whether they're aggressive or are in a playful mood. The bark of a distressed or stressed dog is high pitched and repetitive; it tends to get higher in pitch as the dog becomes more upset. For example, a dog left home alone and who has separation anxiety might bark in such a way. Some breeds of dogs have been bred to bark when chasing, such as scent hounds whose handlers use the bark to follow the dog if it has run out of sight. Coonhounds and bloodhounds are good examples. This kind of barking is often called "singing" as the sound is longer and more tonal.
Growls can express aggression, a desire to play, or simply that the dog doesn't want to participate in what's about to happen next (being picked up for example). For this reason, most pet owners have been urged to treat growls with special attention. This includes always considering the context of a growl, and exercise caution.
Howling may provide long-range communication with other dogs or owners. Howling can be used to locate another pack member, to keep strangers away, or to call the pack for hunting. Some dogs howl when they have separation anxiety.
Whining is a high-pitched vocalization, often produced nasally with the mouth closed. A dog may whine when it wants something (such as food), wants to go outside (possibly to 'go to the bathroom'), wants to be let off the leash (possibly to greet another dog or a person), or just wants attention. A very insistent dog may add a bark at the end of a whine, in a whine-bark, whine-bark pattern.
A whimper or a yelp often indicates the dog is in pain. This is often heard when dogs play-fight if one dog bites the other dog too hard. The whimper or yelp is used only when the dog intends to communicate its distress to a pack member (or human) to whom they are submissive or friendly, and the other dog or human is expected to react positively to the communication; dogs engaged in serious fights do not whimper, as this indicates weakness. Dogs also whimper when they are physically abused or neglected by people. Whimpers are often associated with the lowering of the tail between the legs. Whimpers can also indicate strong excitement when a dog is lonely and is suddenly met with affection, such as when a dog is left alone in a house during the day and its owner comes through the door late at night. Such whimpering is often accompanied by licking, jumping, and barking. Whimpering is distinct from barking in that it is softer, higher pitched, and lower volume.
Imitation of Human Speech
Though the phenomenon is not often discussed, some dogs, followed by personal curiosities through observing the vocalization of the humans around them, may try to repeat human speech sounds, or are trained to do so. This kind of vocalization is typically achieved after lengthy training with positive reinforcement techniques. It's more likely that the dog is exhibiting these behaviors not because they "want" to communicate with humans, but is rather instead a previously reinforced behavior for a reward.