KEEPING IN TOUCH
Communication among animals is obvious in the rainforest. Even more obvious among monkeys and birds. If you actually take time to study these vocal exchanges, you would be able to discover several interesting patterns. Take for example, howler monkeys and Asian gibbons. They do much of their howling and calls in the morning so that the sound is able to bounce between the cold air trapped in the forest and the warm air above. Furthermore much later in the afternoon, the forest will be filled with sounds from birds and insects, which will drown the calls of these monkeys. For primates with low frequency calls however, they do their callings in the late morning, as their low frequency calls can be heard across the rainforest.
Howler monkeys usually make up the highest percentage of the local primate biomass in huge areas, ranging from the rainforest of southern Mexico down to northern Argentina. The six species of these New World Monkey are capable of producing loud calls that are in fact, among the loudest made by animals. According to recent studies, there is a relationship between the development of their loud howling behaviour and their feeding patterns.
The distinctive calls of howlers can be heard for well over a kilometer, especially at dawn, late afternoon, and during rainstorm. These loud choruses of roaring or howling usually last many minutes. Starts with an accelerating series of deep grunts by male, it grades into long roars; the females join in with roars of higher pitch. In alarm, another howler in the group gives a few short grunts. A troop does not maintain an exclusive territory but shares part of the home area with other groups of howlers. By doing this each morning and whenever it moves on during the day, the other troops are informed of its specific location.
As howlers are relatively slow moving, all potential food sources should lie within a howler’s home range or lie within an average day’s travel time. To ensure this, the male, which is capable of producing the deepest calls, help settle disputes in the troop and protect certain important food trees from neighbouring howler troops. By listening to the various howls and roars, weaker troops are made aware of the locations of stronger ones; they can avoid meeting them during the day. Accordingly, there is a slimmer chance of them getting involved in long intertroop squabbles over food trees. Hence, the troops are able to space themselves more efficiently in terms of retrieving food sources.
The tree frog has a unique way of communicating and it varies in different species. For instance, the South American blacksmith tree frog has a call like chiming metal; the barking tree frog of North America has a deep bark, while the green and gold bell frog of Western Australia makes a sound like a saw going through wood. As a male frog sings, the vocal sac beneath its chin inflates. This happens when it inhales air through its nostrils, then close nostrils and mouth to trap the air inside. The air is passed back and forth over the vocal chord. This accounts for the sound produced. Often, these frogs sing in groups. When a frog begins to sing, the others join in. In many species, it is only the male that sings. But when the female of the same species hears, she joins in the singing. This is especially common during the mating season. The male tree frogs will make various noises to attract the females.
Besides sound, animals communicate using various methods. Such includes sight and scent. A prosimian that uses all three senses to communicate is the bush baby. Although postures and facial expressions are relatively unimportant, ear and tail position are most expressive and are mostly linked with particular calls. Nevertheless, bush babies communicate by loud cries. The lesser bush baby is able to produce at least eighteen different sounds. These sounds can be correlated with modes of behaviour. They belong to three functional groups. They are ‘social contact’, ‘aggressive and defensive behaviour’ and ‘annunciatory’. Loud cries are made as alarm calls alerting conspecifics to danger and also, to identify their territory. These sounds are the reason why these mammals are called ‘bush babies’. Communications are also made by means of scent. Bush babies mark urine throughout their journey by an unusual method. This is done by balancing on one foot, depositing urine in the hollow of the other foot and of the hand on the same side. Then it rubs one against the other. The same operation is carried out on the other foot. Using urine and a variety of glands, bush babies are able to scent-mark their pathways, sleeping trees and one another. Even the reproductive status of females is monitored through urine testing! Through cries and urine marking, bush babies can maintain social relations at a distance or ‘deferred communication’. Being nocturnal, not until next morning do they gather again. A special rallying call is made before going to sleep as a group in a hole in a tree, or in nests.