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Discovery of Sound in the Sea
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How do marine mammals produce sounds?

All species of marine mammals are known to make sound. Marine mammals produce sounds that are used for communication, feeding, and navigation. Almost all vocalizations are produced through the movement of air from one area of the head to another. In humans, air is moved from the lungs and across the vocal folds (also known as vocal cords). The vocal folds vibrate which produce sounds that we form into words and other communication signals (vocalizations). Marine mammals such as seals and sea lions can produce vocalizations using the same mechanisms as humans do. Other marine mammals such as dolphins and whales pass air through air sacs in their heads to produce vocalizations. These animals are not thought to produce sounds using their vocal folds. Marine mammals also have the capability to produce and use other sounds for acoustic communication. The animals may intentionally slap their bodies on the water or slap body parts together to make distinct sounds, like the sounds produced by a whale breaching or slapping its flippers.

Diagram detailing the air sac system dolphins use to produce sounds.
Dolphins produce sounds by passing air through air sacs in their head. Adapted from Dolphin Acoustical Structure (1991) Scheifele, P.M. NUSC TR3080.

Pinnipeds (seals, sea lions, and walruses), polar bears, and sea otters produce a variety of vocalizations which include barks, cries, howls, roars, and snorts. It is generally believed that these sounds, which are produced in air, are created by the vibration of vocal folds in the larynx. Air passed from the lungs across the vocal fold ligaments creates sound by vibration. The sound produced by vibration can then be modified by the placement of teeth and tongue, or by the shape of the mouth. The final sound is also influenced by sound passing through resonating chambers such as sinuses and other air sacs found in the head. Some pinnipeds also produce vocalizations underwater, but these sounds are not thought to be associated with vibrating vocal folds. Instead, the sound is generated by cycling air through air pouches in the animal's head. This mechanism is thought to be the generating source for underwater sounds of some seals, seal lions, and male walruses. Underwater vocalizations can include clicks, trills, warbles, whistles, and bell-like sounds.

Photo of a male California sea lion.
Male California sea lions, like the one pictured above, bark profusely when defending their territories. In air, these sounds are produced using the same mechanisms that humans use. Photo ©Tom Kieckhefer.

The vocalizations emitted by odontocetes (toothed whales and dolphins) are all produced underwater. Odontocetes produce a wide variety of sounds which include clicks, whistles, and pulsed sounds. These sounds were once thought to be produced in association with a vibrating larynx, but studies with live dolphins showed that the vocal folds did not move during high frequency vocalizations. Where is the sound coming from? The answer lies in the nasal system. The nasal system is made up of a number of nasal air sacs and plugs that open and close when air is moved from one sac to another. The details of sound production in toothed whales are complex, and the exact site of sound generation is still a controversial topic. Movement of air stimulates vibrations, which may be amplified by air sacs that act as resonators. The sound is then channeled through fats in the melon to the water in front of the animal. The beam or sound generated by toothed whales is very precise due to the channeling inside the head and melon.

Photo of a Beluga whale.
The large forehead of this Beluga whale is called the melon. It helps focus outgoing sound waves. Photo ©Tom Kieckhefer.

The details of sound production in mysticetes (baleen whales) and sirenians (manatees and dugongs) are not well known. Both groups of animals produce vocalizations and possess a larynx and vocal folds. Manatees make high pitched squeaks, while baleen whales produce lower frequency thumps, moans, groans, tones, and pulses.

All the major groups of marine mammals also produce other sounds that are used for acoustic communication. These sounds are typically made by slapping a body part against the surface of the water. This action makes both a sound and splash. Tail or fluke slapping is common in the cetaceans and sirenians. Kerplunks are also made by the tail. Other parts of the body that are typically used in producing noise in a slapping manner are pectoral fins and the body as a whole during lunges and breaches. Jaw claps are made either above or underwater by forcefully clapping together the upper and lower jaws. These types of sound often signal aggression in toothed whales.

Photo of a humpback whale breaching.
The sound of humpback whales crashing onto the surface of the water can be heard underwater. Photo ©Tom Kieckhefer.
 
Photo of a dolphin jaw clapping.
Marine mammals make a variety of sounds that may be used for acoustic communication. Bottlenose dolphins make a "bang" sound when they clap their jaw. Photo ©Tom Kieckhefer.