Superpowers in Nature: The Dolphin’s Sonar


This is my most recent process essay. I thought others may be interested in reading it:

      Dolphins are acoustic creatures who live in a world of sound. In the ocean, visibility is oftentimes limited, so dolphins rely on a complicated system that aids them in their underwater lifestyle where they are surrounded by the dark depths of the sea. In their challenging aquatic environment, dolphins must be able to detect topographical changes, pressure changes, and variations in their dynamic world. The system is called echolocation, and it allows the dolphins the amazing ability of sensing sound waves. It sounds like something right out of a superhero comic book, but in truth, the power to see sound waves exists in the natural realm, although it involves a fairly complicated process.

     Dolphins must rely on echolocation to navigate through the murky depths, to hunt for food, or simply to explore their surroundings. This process is especially valuable to freshwater dolphins who often have poor eyesight and live their entire lives in muddy rivers. These dolphins have no lenses on their eyes and can only see light and darkness. Therefore, they must find their way around almost exclusively by this complex sonar system. Dolphins who live in clear waters don’t have as much of a need to communicate through echolocation.

     The auditory nerve is thicker in dolphins than in humans, and is the largest nerve in the dolphin brain. (G. Carleton Ray, Jerry McCormick-Ray 1999) Sound is important to the natural lifestyle of dolphins, and acts as a vital survival tool, not just in echolocation, but in group communication over long distances. The large size of the auditory nerve is probably due to the large amount of information that must be transmitted and received in the process of echolocation. Several of the dolphin’s organs must work together in four distinct steps in order for the echolocation to work: Creating sound, focusing the sound, receiving sound, and translating the information that was received. The dolphin’s head operates as a complex array of sophisticated sonar equipment that is very effective in a watery world where sound travels 4.5 times faster than it does in air.

     The first step of echolocation is the creation of the sound. We create sounds with our vocal chords and form them with our mouths, but dolphins do not create sound this way. In fact, dolphins have no vocal chords at all. Instead, this process of sound creation begins in a very complex group of nasal tissue called the dorsal bursa. The dorsal bursa is located beneath the dolphin‘s nostril which is at the top of its head. Many people know it as a blowhole. Most mammals have two nostrils, and many large whales have two blowholes, but dolphins only have one. The dorsal bursa is the second opening in the dolphin’s skull that was once the animal’s second nostril. Over time, this second nostril formed the nasal and sound generating organs that are found beneath and behind the animal’s single functioning blowhole.

     If you have ever heard dolphins communicate, you know they make a wide variety of sounds; from clicks, to chirps, to whistles. They even make high-frequency sounds that our human ears are unable to hear. All of these noises are produced by the sound creating organs here in the nasal region. The exact location where the sound is produced in this region is unknown to researchers, and much debated in the scientific community.

     We cannot see into the dolphin’s nasal passages while it is emitting sounds. The technology we use today to see into the heads of human beings is difficult to use on dolphins. Until medical technology progresses, scientists are limited in their ability to gain insight into the dolphin’s sound mechanism. Because we cannot directly observe all of the inner workings of how echolocation is done, scientists are left to hypothesize.

    There are two plausible hypotheses that may explain where exactly the dolphin sound is produced in the dorsal bursa. Many scientists say that echolocation begins when the dolphin passes air between the phonetic lips located within the dorsal bursa. The passing air causes a vibration and can produce high frequency sound that is undetectable by the human ear. The frequency range of echolocation may vary from .2 to 150 kHz. (Reynolds & Rommel 1999)

     Other scientists believe that the sound is produced in three pairs of nasal sacs. When the dolphin holds its breath, air from its lungs moves into the channels that lead to the nasal sacs, which in turn inflates them. The dolphin may force the air out of the air sacs to create sounds; not unlike filling a balloon and pinching the end while releasing the air. This action emits sounds similar to a dolphin’s whistle. The passing of the air back and forth in the nasal cavity means that the dolphin has an air re-use system that allows it to create underwater sound without losing air in its lungs. After all, dolphins are mammals like us and breathe air.

     The sounds dolphins produce may be in the form of burst pulses, individual clicks, or many short clicks called a “click train.” The sounds may be audible to the human ear, or too high in frequency for us to hear.

     Dolphins have a great amount of control over the muscles in their blowholes which allows them to manipulate the kind of sound they are producing. Of course dolphins have the most control over these sounds above water when they can use their blowhole as an orifice like you and I use our mouths in order to form different sounds. These noises may seem like mere whistles or clicks to us, but they actually contain a lot of information; just like a dolphin language. Dolphins even name each other with “signature whistles” and calls that are unique to each individual of the school. The types of sounds emitted by dolphins probably vary depending on depth and pressure in the water. Because dolphins are designed to live underwater, some scientists believe that gravity could have an effect on the dolphin‘s sound, so they choose to focus their research on the sound dolphins emit underwater.

     After the sound is created, the dolphin continues to the second step: Focusing the sound into a beam. This part of the process takes place in the dolphin’s forehead called the “melon.” The melon is a lens-shaped organ that consists of fat in different densities. It acts as a sort of lens that focuses and steers the beamed sound toward an object. The dolphin will direct its head and aim the sound, scanning its surroundings. It may sweep its head back and forth and use its sonar as though it is a spotlight, searching the deep, dark depths of the sea.

     Once the sound has passed through the melon it will travel quickly through the saltwater environment until it hits a solid object. When the sound is deflected, the echoing sound waves will bounce back to the dolphin. The sound waves are received during the third step of the process. Through fatty channels in its lower jaw called “the acoustic window,” the dolphin is able to catch the incoming sound waves. These channels are critically important for receiving the high frequency signals of echolocation, because they are connected to the ears.

     Some scientists speculate that the dolphin’s teeth play a large role in receiving echolocation clicks. The teeth on one side of the jaw are spaced exactly one tooth space apart. The teeth on the other side of the jaw are spaced exactly one half a tooth space forward from the teeth on the other side. This pattern could act as an antenna, or help the dolphin gain more information from the echoes. (Goodson & Kilnowska 1990) The mandibular nerve (a nerve located in the dolphin’s jaw) may also play a part in the reception of information from echoes.

    The sound travels through the jaw channels, into the middle ear and ultimately is received by the dolphin’s brain where the information is translated and analyzed in the fourth stage of the process. The time lapse between click and echo, as well as the strength of the incoming echo is analyzed to determine nearly everything about the object that deflected the sound. From the density and distance of the object, to the shape and size, the speed at which the object is swimming, and the distance between it and the dolphin. Both of the dolphin’s inner ears are located on either side of its head, and act independently of each other. This allows them to determine which direction the object is in.

  The dolphins’ brain is capable of forming a mental picture of the object that the echoes bounced off of. The information received by the dolphin through this sonar is truly remarkable and very detailed. Dolphins aren’t just able to create a picture of a sound, but they are able to sense objects that are hidden. Fish that live beneath the sand are not safe from the jaws of a hungry predator. The dolphins sometimes echolocate at close range over the sandy bottom and detect fish that are hidden in the sea bed. They can even tell if a creature is pregnant. Some scientists who have studied captive dolphins have found that they can see objects that are hidden within wooden boxes by using their sonar. It seems as though the sense of vision and hearing are highly connected.

     Scientists speculate that dolphins are able to see these patterns of sound just like we are able to see images with our eyes. Yet they have neurons in their brain that give them the ability to hear sounds in very high frequency. This makes the dolphin sonar as good as vision. Maybe even better. Dolphins have learned to use this high frequency sound to their advantage when it comes to hunting and foraging. They have learned that blasting schools of fish with sound to corral or stun them makes them easy prey.

     The dolphin’s sonar is truly an amazing device that leaves us humans shaking our heads and wondering what else it is capable of. We have been studying the system for years and still are in awe of its accomplishments and its mysteries. United States Navy specialists have become envious of dolphin sonar. Because of dolphins’ great intelligence and amazing abilities, the United States Navy has actually trained dolphins to detect underwater mines, and to take part in underwater missions. Nearly every species of cetacea (dolphins, whales, and porpoises) has echolocation abilities to some degree, and all of them possess very complex communication skills. The technologies in nature often surpass the technologies that our human hands and minds are able to create. These superheroes of the sea, dolphins, with their amazing abilities and wondrous mysteries, continue to amaze us.

Citations:

Fulton, James T. Dolphin Biosnoar Echolocation: A Case Study

Goodson & Kilnowska. Sensory Abilities of Cetaceans. New York: Plenum Press (1990)     p.255-268

Ray, G. Carleton and Jerry McCormick-Ray. Coastal-marine conservation: science and policy. Washington DC: Smithsonian Institution Press (1999) p. 19

Reynolds, John E and Sentiel Rommel. Biology of Marine Mammals. Melbourne University Publishing. (1999) p. 287-323

 

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