Exploring Our Fluid Earth

Communication and Echolocation

Fig. 6.30. Pod of spinner dolphins (Stenella longirostris) communicating during play, Lana‘i, Hawai‘i

Image courtesy of Steve Jurvetson, Flickr

Marine mammals display a wide range of social behaviors. At the center of these behaviors is the need to communicate. Communication is the process of sending a signal to an individual or receiver and then having the receiver use that information to respond. Communication can occur through visual, touch, smell, or sound signals but there are important considerations for each of these as outlined in Table 6.5. Otters and polar bears live primarily out of the water or very close to the surface and communicate by sound through the air, touch, and smell. Pinnipeds live both in and out of the water and communicate by sound both in and out of the water and by visual cues on land. Cetaceans and sirenians live their entire lives in the ocean, thus sound is an important communication pathway, however visual and touch can also be used. Fig. 6.30 shows two dolphins communicating during play.

The most effective means of communication in water is sound. Sound travels over long distances and is 4.5 times faster in water than in air. Many marine mammals have adaptations for producing and receiving sounds underwater. Sounds are generated when pressure waves travel through air or water. In humans, sound is generated when air is expelled from the lungs and moved through the larynx. The vocal cords in the larynx, along with the throat, tongue, lips, and teeth, change the sound into different vocalizations (Fig. 6.31 A).

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Fig. 6.31. (A) Anatomy of sound production in humans

Image courtesy of Tavin, Wikimedia Commons

Fig. 6.31. (B) Anatomy of underwater sound production in an odontocete whale

Image courtesy of Emoscopes, Wikimedia Commons

For marine mammals that spend time out of the water, the mechanism of sound production is relatively similar to that of humans. Pinnipeds, otters, and polar bears produce a wide variety of sounds on land, including barks, cries, howls, and roars, all of which can be used to communicate. In pinnipeds, sound can also be modified by resonating through the sinuses and air sacs in the head. Underwater, sound production has been noted in several pinnipeds and includes things like clicks, whistles, and bell-like sounds.

The mechanism of sound production in cetaceans is complex and still being studied. Unlike with humans and other marine mammals, cetaceans do not need to exhale air in order to produce sound. Odontocete whales generate clicks, whistles, and pulses in the nasal system. These sounds are then amplified by air sacks and directed through the melon, a fatty organ in the head (Fig. 6.31 B). Mysticete whales produce very low frequency sounds similar to groans, thumps, moans, and pulses. Scientists believe that mysticete whales likely use a larynx structure to generate these sounds, however further research in this subject is needed. Cetaceans also make sounds by slapping a body part like a tail or flipper against the surface of the water.

Fig. 6.32. Anatomy of the human ear

Image courtesy of Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

For virtually all mammals, sound reception, or hearing, occurs through two ears on the head. Mammal ears can generally be split into three sections: the outer, middle, and inner ears (Fig. 6.32). The outer ear directs sound vibrations into the ear canal, where it is amplified by the middle ear and causes the tympanic membrane, or eardrum, to vibrate. Three tiny bones in the middle ear called auditory ossicles transfer that signal to the inner ear. The inner ear then converts the sound vibrations to electrical signals that are sent to the brain. These hearing organs differ in some marine mammals. The external ear flaps are largely reduced or non-existent in most pinnipeds, though there is a visible opening to the ear canal. Cetaceans do not have an external ear structure to receive sounds and no opening to the ear canal. Scientists have evidence that sound vibrations pass through the skin and then are focused through the bones and fats in the skull to the inner ear (Fig. 6.35 A).

Hearing marine mammal sounds underwater is a special treat for swimmers and scuba divers. Since sound travels so efficiently underwater, the animals do not have to be nearby for humans to hear their calls. Scientists deploy hydrophones from boats to measure the sound signals they make (Fig. 6.33). Humpback whales produce the most familiar whale songs. A whale song is a long pattern of regular sounds made by the whale (Fig. 6.34). The male humpback whale will produce songs during mating season, which may aid in attracting a mate or identifying territory. Scientists propose that mysticete whales also produce songs for greeting and individual identification and to communicate a threat.

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Fig. 6.33. Hydrophone being lowered into the ocean

Image courtesy of National Oceanic and Atmospheric Administration (NOAA)

Fig. 6.34. Sound spectrogram illustrating the range of frequencies in a humpback whale song

Image courtesy of Spyrogumas, Wikimedia Commons

Sound is also useful for navigation and locating prey. Echolocation is the process of emitting sound and listening to the sound echoes returning from nearby objects. Odontocete whales, as well as many bat species, use echolocation (Fig. 6.35). A scientist or swimmer listening to a recording would hear a series of rapid pulses or clicks. The premise is similar to the sonar that is used for navigation in many ocean-going ships.

Fig. 6.35. (A) Diagram showing principle of echolocation in an odontocete whale. Click the image to see the animation.

Image courtesy of Malene Thyssen, Wikimedia Commons

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Fig. 6.35. (B) Diagram illustrating how mammals like cetaceans and bats use echolocation to find prey

Image courtesy of Petteri Aimonen, Wikimedia Commons

Migration

Fig. 6.36. Migration route of the humpback whale (Megaptera novaeangliae)

Image courtesy of National Park Service

Many marine mammals migrate seasonally. Migration is the movement of individual organisms over long distances, often in response to seasonal changes in the availability of food and other resources. Migration can also allow individuals to mate or give birth to offspring in favorable environmental conditions. For example, humpback whales (Megaptera novaeangliae) typically migrate up to 25,000 kilometers (km) each year, moving between polar waters in the summer and warmer waters in the winter (Fig. 6.36). Humpback whales feed heavily on small fish and plankton in colder waters during the summer months and build up fat reserves. They live primarily off these fat reserves during the winter months as they perform courtship and breeding behaviors in tropical and subtropical regions. Northern elephant seals (Mirounga angustirostris) have been recorded migrating over 20,000 km roundtrip between summer feeding areas off southern Alaska and winter breeding grounds from along the California coast.