Non-vocal communication

Non-vocal communication includes behaviours that are also used to assert rank, including breaching, water slapping and jaw clapping. Breaching is probably the most commonly observed non-vocal signal. When it is a dolphins intention to communicate its presence, it will deliberately twist it's body to effectively 'belly-flop' when it leaps out of the water. This 'belly-flop' is load enough to be heard up to a kilometre away.

Vocal communication & sound production

Toothed whales can produce sounds for two overlapping functions: communicating and navigating. A bottlenose dolphin can communicate and navigate at the same time. Exactly how they are produced is a mystery as dolphins do not possess vocal chords. They do however possess a complex arrangement of air passages and these together with their muscular larynx are probably used to create the necessary vibrations that produce sound.

Researchers have theorized that at least some sound production originates in the larynx. Early studies suggested that "whistles" were generated in the larynx and "clicks" were produced in the nasal sac region.

Technological advances in bioacoustic research enable scientists to better explore the nasal region. Studies suggest that a tissue complex in the nasal region is probably the most likely site of all sound production. Sounds are probably produced by movements of air in the trachea and nasal sacs. During some vocalizations, bottlenose dolphins actually release air from the blowhole, but scientists believe that these bubble trails and clouds are a visual display and not necessary for producing sound.

Unlike land mammals dolphins can not afford to 'waste' air by expelling it to produce sound, instead they recycle it, repeatedly passing the same air through their larynx. Variously described as whistles, moans, grunts, trills, squeaks, creaking doors, rumbles and whistles. They make these sounds at any time and at considerable depths. Vocal communication probably contains more information that non-vocal water slaps, but exactly how much is debatable.

Clicks

Squeals

The sounds vary in volume, wavelength, frequency, and pattern. The frequency of the sounds produced by a bottlenose dolphin ranges from 0.25 to 150 kHz. The lower frequency vocalizations (about 0.25 to 50 kHz) probably function mainly in social communication. Social signals have their most energy at frequencies less than 40 kHz. Higher frequency clicks (40 to 150 kHz) probably are used primarily in echolocation. Peak frequency of typical echolocation clicks is about 100 kHz, but frequency varies considerably with specific echolocation tasks. (For more information on echolocation click here and go to the echolocation section). The dolphin 'whistles' are made up of many short pulses, approximately 300 per second

Direct observation of wild dolphins together with studies of those kept in captivity suggest that they also use these vocalisations in a similar way to other mammals: as signs of recognition, alarm and threat.

Bottlenose dolphins identify themselves with a signature whistle. However, scientists have found no evidence of a dolphin language. A mother dolphin may whistle to her calf almost continuously for several days after giving birth. This acoustic imprinting helps the calf learn to identify its mother.

In one experiment up to 30 distinct calls of the bottlenose dolphin were recorded and then played back individually to assess the reaction. Many of the calls simply prompted a reflection of the same call back, but some resulted in the dolphins responding in a specific way such as by swimming in a highly agitated manner, producing echolocation sounds, or showing heightened curiosity about their immediate surroundings. From day or night there will always be dolphins thus it might be concluded that different calls do impart different meanings although it can be dangerous to assume too much. For while the response of a dolphin and other cetaceans a specific call can be recorded, it does not mean that the caller intended to communicate a particular message.