Marine Mammal Acoustics
Light is limited in the marine environment. Marine mammals have adapted to overcome sensory limitations by developing complex vocalisations to assist in communication, foraging and navigation in the marine environment.
Cetacean vocalisations cover a large range of frequencies from the very low infrasonic range, to the mid level frequency range audible by humans, to the extreme high frequency ultrasonic range. The frequency range at which marine mammals vocalise is based upon their hearing sensitivities and thresholds.
How do whales produce sounds?
Most cetaceans make sound of some sort. The whistles, screams, groans and clicks of cetaceans echo throughout the oceans. Toothed whales mid-frequency tonal sounds to communicate and produce biosonar clicks to detect and locate objects around them, including prey. Baleen whales tend to make low frequency tonal sounds. Some moans made by the blue whale are so low that we can’t hear them, but they travel underwater for hundreds, maybe thousands, of kilometers.
Cetaceans have no vocal chords like we do. They make sounds with their blowholes, not their mouths. The exact method is not clearly understood, but it involves air trapped behind the nasal plug in the blowhole. Sometimes this air is released through a partly open blowhole, and small bubbles can be seen as the whale vocalizes. More often there are no bubbles, as air is pushed back and forth inside the nasal cavity in the head.
It is not known how the sound ‘radiates’ out of baleen whales. In toothed whales, sounds are channeled through fatty tissue in the whale’s rounded forehead, which is called the melon.
How do we study marine mammal vocalisations?
Sounds produced by marine mammals are detected using specially calibrated acoustic transducers called hydrophones made of made piezo-ceramic material which converts acoustic pressure waves to volts. These analogue signals are then digitised through a processing unit and the resulting signals are stored or displayed for real-time monitoring by a bioacoustician.
Arrays of hydrophones can be arranged and towed from a vessel, or fixed by mooring to the seabed. Information processed from the hydrophone array can provide a measure of bearing and range of the received signal. Tonal sounds are displayed on a scrolling spectrogram while short pulse echolocation clicks are displayed in a click detector.
What is echolocation?
Echolocation is a type of ‘sonar’ used by toothed whales to find their food and to ‘see’ where they are heading. They are very high-frequency sounds called clicks, which the whales produce and direct through their melon and then listen for the returning echoes.
The echoes then carry information about the objects they bounce off, such as size, texture, speed, location and whatever other information is needed to get a three-dimensional picture of the surroundings. Some echolocation clicks are ultrasonic, meaning they are so high in frequency we can’t hear them. Those we can hear sound like a ticking clock, or in the case of the sperm whale, like horse hoof beats.
Do cetaceans talk to each other?
Many species of both baleen and toothed whales communicate by using sounds that travel great distances underwater.
The communication that has been shown to occur in whales is relatively simple, similar to the communication that many other animals engage in; identifying individuals and communicating breeding status. Whether or not whales and dolphins are able to communicate more complex ideas and thoughts has never been adequately shown.
Humpback Whales (Megaptera novaeangliae), have elaborate ‘songs’ that last from 6 to 35 minutes before being repeated. The whales may sing continuously for up to 22 hours!
Each individual Humpback has its own vocal signature, and therefore may be recognized by its song. In addition, each distinct population of Humpbacks (e.g. North Pacific, North Atlantic) sings its own particular type of song. These populations subtly change their songs from year to year, all individuals within the population changing their songs in the same way. It seems likely that the humpback song functions for mate attraction, since the songs are sung on the breeding grounds.
Killer Whales (Orcinus orca) the largest of dolphin family also communicate by making noises. Again, individual whales have distinct calls, and can be recognized by them. An important function of the sounds is probably one of identification.
Through the whales sounds, the identity, location, sex, and status may be recognized by other whales. Since different pods of Killer Whales may have very different ‘dialects’, the sounds may also identify which pod a whale is from. This may be very important in these whales, which have very distinct matriarchal groupings, and they do not usually move from pod to pod.
Many other species of dolphin for example the Common Dolphin (Delphinus delphis) have also unique whistles for each individual, called signature whistles, through which they name themselves so that others can recognize them.
Some images courtesy of Seiche Ltd