Research explores animal sounds as a form of communication
by Dr.Sophia Yin
A sleep-deprived gentleman once asked me, “How is it that dogs can bark so long? When I yell for hours on end, I lose my voice.” Then he went on to describe the dogs in his neighborhood, whose barking bouts lasted longer than all of the Wagner operas combined.
Anyone who’s experienced an epic canine oratorio has probably wondered, “What’s going on? Do dogs just like to hear the sound of their voices?” Until recently, some researchers thought this was the case, taking the position that because dogs bark at almost anything and everything and for hours on end with no apparent reply, dog barking must not be a specific form of communication. Rather, barking is just a loud and obnoxious way for them to say, “Hey! Look at me!” More specific information, it was postulated, comes from reading body expression and olfactory messages.
Given the sparse number of studies on vocal communication in dogs, this contention seemed reasonable until consideration of the ever-expanding research on songbirds, ground squirrels and monkeys provided a very different view.
For decades, while some looked at dogs and pooh-poohed their barks as nuisance noise, others—such as Dr. Peter Marler, professor emeritus at the University of California, Davis, and a pioneer in the field of vocal communication in birds—were taking a closer look at the sounds that animals, including the common chicken, make. Says Marler, “Chickens are an obvious case … to most people, the sounds are a kind of noise, or vicarious vocalizations that have little meaning. But this view could not be more wrong. In fact, many studies show that chickens have a very rich and elaborate vocal repertoire, and that different calls mean different things.”
These studies started with the finding that roosters have specific types of predator-alarm-calls, one for aerial predators such as hawks, and another for ground predators. Play-back studies—in which hens hear taped versions of these calls in the absence of both a predator and visual signals from the rooster—show that the calls deliver specific information. Hens duck for cover upon hearing a recording of the aerial call and extend their necks and look for the danger when they hear the ground-alarm-call. These responses to the respective calls tell us the calls have specific meanings to those who hear them.
And those aren’t the only interesting chicken calls. Roosters also make a particular call when they find a morsel to eat, and this sound, part of the rooster’s courtship routine, serves to attract hens. As with the alarm calls, recorded food calls played back from behind a barrier with a hen on the other side will cause the hen to approach when she hears them. What’s more, states Marler, “If the calls were recorded from a male who had a very choice food item, like a cricket, she’ll approach faster than if the calls are given [for] a piece of grain or peanut. So the calls convey some information about food quality.” And, like the alarm calls, these differential responses indicate the calls have meaning.
But what about those calls, like barking, that go on and on with no obvious response from other animals? Dr. Don Owings, professor of psychology and animal behavior at the University of California, Davis, says, “Call communication can be organized on different time frames, so that you don’t see an immediate response to each vocalization. You have to look at the effect of signaling in a time frame that is appropriate for the signaling behavior.”
This longer time frame reveals interesting things. For instance, California ground squirrels respond to mammalian predators by uttering a chatter vocalization. Other squirrels respond by running to their burrow or standing up and looking around. If the predator lingers, the calling ground squirrels move from an erratically spaced, episodic chatter to a highly rhythmic “deet-deet-deet.” Observation reveals that individuals who hear the vocalization don’t startle or respond to each vocalization, and often return to their feeding and previous activities. However, Owings’ student, Jim Loughry, looked more closely at the overall activity and body postures over a longer time period and found that squirrels listening to this rhythmic vocalization were more vigilant overall. Even if they were eating, they would eat while sitting upright as they scanned their surroundings.
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.
So what do these findings mean? Well, we can’t tell whether dogs intentionally alter their barks to deliver a message to other dogs or people. The only way to determine this would be to teach the dog English so that he could tell us, “I am now intentionally changing my bark to deliver this message.” Most likely, the variation is a reflection of the internal motivational state the dog is in at the time that he barks.
What we can tell is that because there are specific bark subtypes, barks have the potential to play specific communicative rolls and provide specific information—intentional or unintentional—to the animals, including humans, who are listening. Not specific like, “Timmy’s stuck in the well! The one to the left of the big oak tree on the other side of the creek!” More like, “I’m separated from you! Come get me!” or “Intruder alert!” Furthermore, since bark subtypes occur in specific contexts, we can learn to tell what our dog is saying by listening to his barks and then examining the context. His “woof” for an unknown intruder may be different from his “ruff” when he alerts to a friend approaching the house. And his “huff” to come inside may be different when the desire is more urgent.
But is the barking actually communicative? Well, for a vocalization to be communication, the animal who hears the signal must respond in a specific way. As with the chicken food-call and alarm-call cases, this is usually tested through play-back studies, and such studies have not yet been performed on dogs. However, a study by David MacDonald and Geoff Carr on free-roaming dogs in Italy suggests that barks can have specific effects on other dogs, even when the “barkers” can’t be seen. The free-roaming dogs in this study lived in small groups and scavenged at local dump sites. When the largest group of dogs barked in a group prior to heading toward the dump site from up to one kilometer away, dogs in smaller groups consistently evacuated the site; they apparently knew that they were no match for the larger gang. And on a more familiar note, Marler points out that if we pay attention, it’s easy to notice that barking usually elicits a response from other dogs.
But this is only half the picture. Says Owings, “For the vocalization to be communicative, the vocalizers should be sensitive to social contexts and consequences.” That is, the animal producing the vocalization should adjust it based on the behavior of the listeners. While there is little research in this area, general observations indicate that this happens too. For instance, when one dog barks at the doorbell and another dog, or even the resident human, joins in a barky “No! No!”, the dog responds with louder and more prolonged bark behavior. Take away his back-up and suddenly, the initial barking bout abates. Or then there’s the dog who barks at you until you toss his toy, but barks harder and louder when you’re on the phone because that’s the time when you’re most likely to toss the toy quickly in order to get him to quiet down. These cases provoke the question: What exactly is the role of the human (the primary animal to whom many barks are directed) in the development of bark behavior in dogs?
Clearly, there are an infinite number of questions about barking and its communicative function for the dog, and there’s much catch-up needed to reach the same level of understanding that we have for chickens and squirrels. But it all starts with a simple study showing that dogs have different barks in different contexts, and plugs away, developing and answering one question at a time.
To test your own ability to interpret dog barks, or to read more about barks as communication or as a nuisance behavior, visit Dr. Yin’s “Nerdbook” website.