Whales and dolphins rely heavily on sound communication underwater along with most other marine mammals, and actually other marine organisms such as fish and shrimps.
Whales and dolphins have the ability to produce variety of sounds in different behavioural context and to receive these signals, evaluate their meaning and even to locate the sound source, these animals have an exceptionally acute sense of hearing.
Different species have different sensitivity to different sound frequency levels. Toothed whales, such as sperm whales, dolphins and beaked whales have adapted to producing high frequency sounds (> 20 kHz) for short range communication and directional broadband pulses for navigation. Since toothed whales are generally highly social animals they do not need to communicate over hundreds of kilometers such as the more solitary baleen whales. Therefore, more diverse, high frequency signals can be used for complex communication without having to worry about attenuation of the sounds over long distances.
Low frequency sounds take longer time to attenuate in the ocean and can be carried longer distances than the high frequency sounds. That is why killer whales, white-beaked dolphins and other dolphin species lower their "voice" while increasing the sound intensity when they need to communicate over long distances (e.g. few kilometers).
Baleen whales, however, produce lower frequency sound signals which are more suitable for long-range communication. These signals are generally simpler and less diverse such as the complex whistles of the dolphins, but with increased distance the signals change due to attenuation. So, a complex signal changes drastically over a long distance and might sound very differently to the receiver. The intended receiver of the signal needs to understand its meaning though it changes significantly. Therefore, baleen whales rely on low frequency (~10-5000 Hz) sounds with relatively simple contour for long-range communication.
The problem facing these animals, particularly those that need to communicate over long distances, is the ever increasing noise in the ocean. The ocean environment is certainly rich of various natural sounds, such as wave sounds, wind across the sea surface, tidal or storm surges, seismic events and sound producing animals, to name just few. However, man-made sounds are becoming an increasing part of the marine soundscape. These man-made sounds are, for example, generated by ship traffic, construction activities, airguns during seismic surveys, low-flying aircrafts, among other sources.
The whale calls are cancelled out as a ship passes by, even tens of kilometers away. With constant ship traffic into important habitats of the whales, their social network is constantly being ripped and reformed. When the whales are unable to communicate they will have trouble finding each other and are thus forced to spend more time alone. Studies have even shown that some species need to increase or lower their pitch and increase the amplitude of their sounds to get the message across. Such development is very stressful for these animals and will certainly reduce their ability to survive in the nearest future.
This topic is particularly relevant for the Arctic areas right now where shipping traffic, seismic surveys and oil drilling has been and will be increasing in the next years
Photo credit: Stefan Fichtel. Sources: C. W. Clark, Cornell Lab of Ornithology; Brandon Southall , University of California, Santa Cruz; Kathleen Vigness-Raposa , Marine Acoustics, Inc
Edda Elísabet Magnúsdóttir
Marine biologist, University of Iceland