So what possible functions could vocalizations that carry on for hours at a time have? Well, male songbirds sing for hours at a time to attract females and to defend their territory. Additionally, during breeding season, wolves howl for hours at a time with no detectable reply. This howling may function as a beacon to attract females from neighboring packs. Perhaps prolonged canine vocalizations have a long-term function as well?
In 2000, armed with the rich body of literature on vocal communication and the support of those already well-established in the field, I decided to pursue the question of barking in dogs. Yes, some dogs do bark incessantly and some seem to bark in any and every context, but was it possible that barks were slightly different in different contexts, so that dogs actually produced bark subtypes? If so, perhaps these vocalizations could be specific forms of communication.
With the advent of improved acoustic-analysis equipment, others had been able to test similar theories in other animals. As mentioned earlier, Owings found that squirrels emit chatters when they see mammalian predators and occasionally with avian predators; they also chatter when having aggressive interactions with another animal and immediately after copulating with a female. Though these findings might lead one to conclude that the vocalization is not functionally specific, modern sound equipment revealed that the chatter calls are structurally different in different contexts.
Similarly, Dr. Julia Fischer, a researcher at the Max–Planck Institute for Evolutionary Anthropology in Leipzig, Germany, found that Chacma baboons have different bark vocalizations for different contexts, including an alarm bark that is structurally different from a contact bark, which is uttered when mother and offspring are separated.
To test my hypothesis about bark subtypes, I found ten barking dogs and recorded them in three different situations. In situation one, the disturbance situation, the dog was recorded while barking at the sound of the doorbell. In situation two, the isolation situation, I recorded the dog when it was locked outside, isolated from its owner. And in situation three, the play situation, I recorded barks as the dog played with its owner or another dog. This sounds simple but surprisingly, even dogs dubbed excessive barkers often couldn’t be used because they only barked in two of the three contexts, which suggests that maybe dogs don’t really bark at any- and everything.
In order to ensure that I had enough barks to give a good idea of the average bark for each context for each specific dog, we set up the dogs in each situation many different times on many different days over a three-month period. Once I’d collected enough barks, more than 4,600 in all, I turned to my collaborator, Dr. Brenda McCowan, a researcher at the University of California, Davis, who specializes in acoustic communication in animals ranging from dolphins to cattle. Using a sound-analysis program that converts audio to a visual representation of pitch-over-time and amplitude- over-time and a computer macro (a set of instructions for the computer to follow) designed by McCowan, we took 60 sequential measurements over time for one bark, or along the visual representation of each bark.
The data yielded clear results. Dog barks were different in the different contexts and therefore, could be categorized into subtypes. The doorbell-disturbance barks were relatively harsh, low-pitched and with little pitch variation throughout. Dogs blurted these barks out full force and so fast that they were often fused into what I formally dubbed “superbarks.” Isolation barks, on the other hand, were higher pitched and more tonal, with more variation in both pitch and amplitude. Usually, they occurred as single barks, but some dogs definitely learned to bark more repetitively when doing so eventually reunited them with their owner. The play barks were similar to the isolation barks, except that they usually occurred in clusters rather than singly.
Not surprisingly, we also found that dogs could be identified by their barks. This basically means that as you lie in bed listening to the sound of the neighboring canid’s greatest hits, you should be able to identify exactly which neighboring canid is the offending artist.