Many animals besides dogs bark, according evolutionary biologist Kathryn Lord from University of Massachusetts Amherst, but domestic dogs vocalize in this way much more than birds, deer, monkeys and other wild animals that use barks.
Most of the pet owners believe that barking is a special form of communication between dogs and humans.
However, Lord says, barking is the auditory signal associated with an evolved behaviour known as mobbing, a cooperative anti-predator response usually initiated by one individual who notices an approaching intruder.
A dog barks because she feels an internal conflict?an urge to run plus a strong urge to stand her ground and defend pups, for example. When the group joins in, the barks intimidate the intruder, who often flees.
"We think dogs bark due to this internal conflict and mobbing behaviour, but domestic dogs bark more because they are put, and put themselves into, conflicting situations more often," Lord said.
The reason traces back to the first dogs that started hanging around human food dumps about 8,000 to 10,000 years ago.
They would have experienced a serious disadvantage if they had run a mile away every time a human or other animal approached.
As Lord explains, "In evolutionary terms, dogs self-selected the behaviour of sticking around, overcoming their fear and being rewarded by getting to eat that meal before some other dog got it. The scared ones die while those less scared stay, eat, survive and reproduce. So they inherit the tendency."
During the study, the researchers identified eight different parameters in three categories, which must be met in order to classify a given vocalization as a bark.
These included tonality, noise, pitch, volume or amplitude, abrupt onset and pulse duration.
In researchers' view, barking is not self-referential communication to convey a message, but a short, loud sound characterized by combining both noise and tonal sounds, which is unusual in animal calls.
This definition widens the bark's usefulness as a functional behaviour seen in many animals, though domesticated dogs display it more often.
"Using this definition, even birds bark, and certainly many mammals besides canines, including baboons and monkeys, rodents and deer also bark," Lord said.
"In a whole bunch of mammals and birds, what they do in such conflicted situations is bark," she added.
The findings appear in journal Behavioural Processes.