Special Communication Systems: Marine Mammals
Long-term scientific observation of animals’ behavior has revealed the complex systems that have been developed for inter and intraspecies communication. Indeed, many of those signals have already been decoded. In that way, humans have been given the opportunity to understand better the complexity of other organisms in nature, and also to conceive the information that is transmitted from an animal sender to an animal receiver. Communication usually occurs for vital and socialization reasons within an animal community. Basic senses, like vision, olfaction, tactile sensation, or hearing can be stimulated by external triggers and transmit the message to the central nervous system where the information will be translated. However, due to physical restrictions, marine animals are not able to efficiently use all of the above abilities for their communication. In reality, light is differentially scattered in the water than on the surface of the earth and also molecules are more slowly diffused in the water than in the air. That means that vision and olfaction are not always favorable ways of communication under the sea. On the other hand, sound travels four times faster underwater than in the air. This led marine animals to develop a unique sonar communication system.
In the 1940s, both Dr. Maurice Ewingand and Leonid Brekhovskih described the Sound Fixing and Ranging channel (SOFAR), or also named Deep Sound Channel (DSC), as a horizontal oceanic water layer in which low frequency sounds may travel for very long distances before dissipating. Later, in 1970, it was found that cetaceans take advantage of this channel for their communication. Whales and dolphins produce sounds in a specific frequency gradient that serves to better their communication. These sounds also help them find food, navigate, travel in groups, find a mate, warn other animals about predators and maintain their cohesion. Whales are separated into two large groups, the odontoceti (toothed whales) and mysticeti (baleen whales). In contrast to humans that produce sound by exhaling air, cetaceans produce the sonar by circulating air inside their body. In odontoceti, including dolphins, air moves between the spaces and sinuses of the head. The phonic lips is a structure similar to the human vocal cords located in a cavity, which is similarly related to the human larynx. When air passes through the narrow passage of phonic lips, the subsequent membranes are sucked together causing the surrounding tissue to vibrate. It is a system controlled by the animal with great sensitivity. The remaining air can be recycled back and reused for sound creation. Baleen whales on the other hand, do not have a structure that resembles the vocal cords. Scientists found an organ similar to a larynx that should contribute to sound production, but they are not certain yet about the complete mechanism of their vocalization.
Marine animals, especially marine mammals, use sound signals for their communication. However, there is not an external physical characteristic like ears, that could be thought to receive the message. The truth is that cetaceans do have ears but they are not visible on their body. They receive sounds and process them with internal organs, specifically in cavities in their lower jaw within their skull. Additionally, their whole body is covered with sensors that can be triggered electromagnetically, mechanically and chemically. The information is forwarded to the internal part of the body where the message is translated. It is said that the sound waves that trigger the sensors of cetaceans enables them to perceive their environment better. For example, especially in cases of darkness or blurred water, the echoed sounds are giving information about the depth of the ocean, the seafloor obstacles, the distance from other fish or the threat of predators. In other words, they can create a 3D map of their environment.
The U.S. Navy was the first to record whales’ sounds by using hydrophones in 1950. Until now, scientists and cetacean observers use bioacoustics software to create spectrograms to understand underwater vocalization. Unfortunately, the military has started sending out underwater sonar waves in order to locate hidden submarines, but in frequencies similar to the ones that dolphins and whales use for their communication. This fact has raised concerns about the level of underwater noise pollution that is caused, and the extent to which marine animals may be affected. It is thought that military sound waves may disorient the marine mammals, and therefore threaten their communication activity and survival. The same results may also come from other vessels such as warships, cruise liners or exploration boats.
The exploration of nature’s systems usually highlight the admirable ways that organisms develop to survive and socialize. The underwater world remained, for ages, a mystery for terrestrials, but contemporary observations reveal that marine animals are equally able to communicate and interact with other animals and their environment. Whales and dolphins take advantage of the sea water’s properties and have developed systems that enable them to feel their environment, chase their prey, be protected from predators, and maintain cohesion of their groups. Sounds that are perceived as whistles and clicks by the human ear can include messages that travel on the bottom of the sea for hundreds of kilometers before being dissipated. It is important that military and civil marine actions are kept at limited levels, because the underwater equilibrium is very easily destabilized and this can negatively affect marine mammals.
Sources: St. Petersburg Polytechnical University Journal: Physics and Mathematics, Sea-Earth-Atmosphere (SEA), Whale and Dolphin Conservation