7.1: Vocal Communication and Language

As discussed in the previous chapter, primates are social animals. Living in a social structure and surviving in a complex environment requires intelligence.

The driving forces behind group-living in other species of mammals are finding and defending food resources, and avoiding predation. For example, a pack of grey wolves can bring down a bison that may weigh nearly one ton (1000 kg), prey that could not be captured by a lone wolf. For individuals that are prey for carnivores such as wolves, vigilance is vital. Herbivores such as zebra and impala herd together, so there is always at least one individual alert to danger and the chances of any one individual being singled out for attack are reduced.

Most researchers believe that overall size of a primate group is determined by food availability. Living in a group may enable members to defend their resources from other groups. However, as they forage together there may be less food available per individual compared to the amount that one individual could find by foraging alone. If food is readily available, the group can be large. For example, geladas feeding on savannah grasses live in very large communities of up to 800 individuals, but if a group has to travel to find food, it is likely that a large group has to travel further than a small group of the same species to satisfy their food requirements.

This question was investigated in two groups of leaf-eating, red colobus monkeys living in the wild in Uganda. Researchers carried out what are called focal searches, concentrating on one individual in the group for a set time and then moving to another individual and then another, monitoring each for the same length of time. This procedure allows the movement of the whole group to be determined throughout the day. The researchers calculated the following measures:

• the mean day range - the distance travelled by each group on average each day over a one-week period;
• the weekly home range - the total area explored by each group in a week, for each of six consecutive weeks;
• The total home range explored by each group during the six-week period.

Figure \(\PageIndex{1}\): Foraging in two groups of red colobus monkeys. Each feeding tree visited during a particular week is mapped by a dot within the total home range, showing how the groups moved about within their total home range during the six-week period. The large group consists of 48 individuals, total home range 37 hectares (ha), and the small group consists of 24 individuals, total home range 5 ha.

The other factor determining group size may be the need to avoid predators. In other mammals, such as savannah-living herbivores, as groups become larger, so vigilance can be shared between more individuals, reducing the risk of predation. Primate groups also tend to be larger in areas with high predation than in areas with lower predation.

Vocal Communication and Language

In order to be able to state that animals are communicating vocally with one another, scientists need to demonstrate that particular sounds made by one individual can be understood and acted upon by others.

Figure \(\PageIndex{2}\): A Diagram records the alarm calls made by infant, juvenile and adult vervets in response to various birds. Look at the data for infants at the top of the figure. Infant monkeys were observed calling in response to a bird on 24 separate occasions. The number of calls made in response to each of the species of bird shown were assigned to categories, shown by lines of different colours that are identified in the key at the bottom of the diagram. For example, the number of calls made by infants in response to a vulture is shown by a green line, indicating that between 6 and 10 calls were made.

Evidence of this occurred when different types of calls were played back to an experimental group of vervet monkeys, showing that vervet monkeys can make and interpret predator-specific calls. Infant vervet monkeys start making alarm calls when they are only a few days old. Of particular interest is how this behaviour develops over time.

How is this increase in proficiency brought about? Adult vervets can distinguish between calls made by juveniles and calls made by other adults. When juveniles call, the adults look around before reacting, presumably to check whether a predator has really been spotted; whereas adults react immediately to an adult call. Infants learn to make the same responses as their mother and they gradually learn to make calls only in response to a predator.

The fact that infants can make recognizable calls only a few days after birth suggests that infant monkeys are born with an innate ability to communicate vocally, which is refined through learning. This type of innate ability may be one of the forerunners of our own language abilities. It is thought that human infants are born with an ability to separate human speech from other sounds, but the huge advances in human speech compared with monkey and ape vocalizations have involved many other factors, such as changes in the position of the larynx (voice-box) and developments in brain regions related to hearing and speech recognition.

Vervets also call in response to the presence of a rival group of monkeys. On sighting a rival group, individuals make 'wrr' calls to communicate to the rest of the group that rivals are approaching. As the rival group comes closer, the callers start making 'chutter' calls instead of 'wrr' calls. When members of the group hear 'chutter' calls, they move closer to the callers, ready to defend their territory. Playback experiments of the two types of call show that the group is responding to the specific call, not to the presence of the rival group alone. Playback experiments have also revealed some other insightful responses. When researchers play the 'chutter' call of a particular individual repeatedly in the absence of a rival group, the rest of the group eventually ignore it and carry on feeding or grooming, etc. Changing the recording to the 'chutter' call of a different individual causes the group to respond immediately again, but if it is changed to the 'wrr' call of the first individual, the group ignore that as well. Also, it was already known that vervet mothers respond rapidly to distress calls of their own infant, but playbacks of infant distress calls found that as a mother looked towards the sound of her infant's cries, the other females in the group responded to the sound by looking towards the mother.

Social associations have also given some fascinating insights into vocal communication in monkeys. Until recently, there was no evidence of monkeys in the wild comprehending such syntactic (grammatical) rules.

Recent research, however, suggests that Diana monkeys foraging with Campbell's monkeys may be able to recognize the order of a series of calls. It was found that:

• When Diana’s monkeys hear a leopard or an eagle alarm call from a Campbell's, they give the corresponding alarm call of their own.
• Male Campbell's monkeys also make a third type of call, a low-pitched boom.
• If a Campbell's perceives a lesser threat, such as a falling tree, he utters two boom calls followed by a leopard call - a series of calls referred to as a boom-introduced alarm call.
• When Dianas hear a boom-introduced alarm call they do not give leopard calls of their own.
• Similarly, when booms were added experimentally to the eagle calls of a Campbell's and played back to Dianas, the Dianas made no eagle calls.

These responses suggest that boom calls alter the Diana monkeys' interpretation of subsequent Campbell's alarm calls, changing them from predator-specific calls to calls indicating a lesser threat. This conclusion is further supported by the observation that Dianas do respond to a played-back Diana call that is preceded by the boom calls of a Campbell's. This evidence is the first to suggest that the cognitive ability to generate and comprehend syntactic rules, albeit a very simple rule, evolved long before the emergence of human language.

Although these two factors (food availability and the avoidance of predators) influence the upper limit on group size in a particular environment, it is the rich diversity of internal relationships within anthropoid groups that is fascinating and has led to many studies of primates.

Contributors and Attributions

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