Biology

Abstract
This paper tackles in detail the mechanism of the dolphin sonar  the transmission, reception and signal interpretation process. It also discusses the nature of echolocation signals of dolphins that can whistle and of those which cannot. Moreover, the paper gives some insight on how different bat echolocation is from that of the dolphin, as well as the differences between the inner ear structures of humans and dolphins and its significance.

Introduction
It is said that even in the clearest tropical water, one cannot see farther than a few hundred feet (Jewell, 2005). Even in some bodies of water, visibility does not go beyond 30 feet. This simple fact means that whales, dolphins, and porpoises, collectively called cetaceans, cannot rely much on their vision for communication or foraging. Somehow the only way they can explore the sea is through the use of sound.

Dolphins use echolocation, or dolphin sonar, for navigation and for the accurate tracking, detection and localization of prey. What the dolphins have is similar to other biological sonar systems like those of bats and sperm whales. The dolphin sonar consists of a transmitter, a receiver and a processor. This system is efficient in targeting animals as small as a sardine which is but 9-18 cm long and even at distances of zero to over 100 meters (Mo, 2007). It is believed that the dolphin sonar can even outperform man-made sonar systems (Dobbins, 2007) because unlike man-made sonar systems, a dolphin sonar system can work efficiently in shallow waters and produce signals that are not confounded by water turbulence, increased sound wave reverberations, and suspended sediment (Mo, 2007).

A better understanding therefore of the dolphin sonar can lead to the development of better and improved man-made sonar systems. A clear and thorough understanding of even just the basic principles that govern the mechanism of dolphin sonar is essential to understanding not only how animals react to their environment but also how man-made sonar systems can be improved through this knowledge. Most of all, such an understanding can lead us to think of ways on how to apply such principles in the fields of human psychology and brain physiology.

The Sonar
SONAR stands for Sound Navigation Ranging. It is a technology that involves the use and interpretation of sounds in order to detect the location of something underwater. There are two reasons that somehow make the sonar an ideal type of equipment first, bodies of water are often deep and its murkiness impedes visibility, and second, sound travels more quickly in water than in air (McGrath, 2010).

The sonar is an apparatus particularly used by certain ships and submarines for industrial exploration, that is, the detection of oil and gas, as well as for seismic exploration, which refers to the search for fossil fuel sources on the ocean floor. One particular type of sonar, called the military sonar, is normally used for military exercises and for detection of vessels, mines and torpedoes (Sound and Sonar Issues, 2010).

Sonar operates by producing sound to be able to detect objects from afar especially underwater. Passive sonar involves listening in order to locate and study marine life as well as threats in the ocean. On the other hand, active sonar involves both transmission and reception of sound waves, that is, sending a sound pulse and measures how long the sound will be reflected back (Nicholson, 2010). The dolphin sonar is an example of active sonar.

The Dolphin
Dolphins are marine mammals that belong to the family Delphinidae of the order Cetacea and are closely related to whales and porpoises. With almost 40 species in 17 genera, dolphins are found worldwide and are said to have evolved about 10 million years ago but have somehow retained a primitive mammalian brain characteristic of that of its ancestors 50 million years ago during the Eocene epoch (Bekman, 2009).

Although dolphin intelligence has not been accurately measured to date, this particular cetacean has several otherwise intelligent characteristics a primitive yet highly elaborate archetypal brain, complex vocalizations that somehow prove the ability to communicate with members of their species, and a sonar system that works hand in hand with their vision (Bekman, 2009). There have also been claims that dolphins can alter the mental make-up and overall physical health of autistic and disabled children as well as patients with diseases that include even cerebral palsy and mental retardation (Healing through dolphins, 2009). Moreover, there is considerable scientific proof that the reasons for these sophisticated features of the dolphin are due to particular characteristics of its brain, particularly the neocortex.

Morgane and Glezer theorized that the neocortex of the cetacean brain has retained during evolution the capability to adapt of modify neuronal types to whatever form is most opportune for its specific function. The implication is that the neocortex is not phylogenetically preordained and it is this very feature that enables animals to continually modify their behavior in order to adapt themselves to novel events in their environment (as cited in Thomas  Kastelein, 1990). These findings somehow imply that dolphins may have retained the otherwise sophisticated abilities of the ancestral mammalian brain to adopt to environmental changes hence their elaborate sonar systems and other specialized characteristics.

Evolution of the Dolphin Brain and Sonar
Cetaceans are believed to have descended back to the sea around 50-70 million years ago. They may have adapted themselves to the new conditions of their aquatic environment but they certainly appear to have preserved unique features of the original structure of the brain of primitive mammals extant during that period in a much greater measure compared to land animals. One more factor in the advancement of the cetacean brain is that the new aquatic environment was conducive to develop novel and specific features of brain adaptation of which land mammals were incapable. Morgane and Glezer, in their studies of the cetacean brain for over 20 years, found out that the cortex of the dolphin has remained unchanged for the past 50-70 million years. The cetacean brain, with its extremely vigorous quantitative expansion of neocortex, may then perhaps represent a prototypic mammalian brain which is considered to be the most ancient among present-day mammals. Morgane and Glezer also claim that such a primitive yet highly more specialized neocortex among cetaceans compared to land mammals may have existed for one reason the assumption that the return to water somehow retained such primitive brain characteristics. The return of the cetaceans to water took place before land animals developed pyramidal cells in their neocortex, which was the beginning of the highest degree of sensory and motor cortical specialization in land mammals (as cited in Thomas  Kastelein, 1990).

According to Professor David Lindberg, an evolutionary biologist at University of California-Berkeley and co-author of a study on the evolution of echolocation among toothed whales, as to the evolution of the dolphin sonar, scientists believe that just like bats which developed sonar while chasing flying insects, the cetaceans developed sonar to chase squid at night. Lindberg added that as soon as the first cetaceans transferred to the ocean, they found this incredibly rich source of food surfacing around them every night and bumping into them. Thus, in order to adapt to this otherwise difficult yet essential lifestyle, the biosonar system evolved   (Whale and dolphin, 2007).

Mechanism of the Dolphin Sonar
Dolphins use a sensory sonar system for locating things in their environment and for communication. The basic mechanism is that dolphins release a concentrated beam of sound waves in the form of clicking sounds and then listen to the corresponding echo. From this, they are able to determine certain key aspects of an object that include the size, shape, distance, speed, direction as well as internal structure depending on the object (Jewell, 2005).

The dolphin sonar system is actually made up of three subsystems transmission, reception, and decision makingsignal processing subsystems. The transmission subsystem involves the mechanism of producing sounds, acoustic propagation coming from inside the head of the dolphin out into the water, and the nature of the signals traveling in the surrounding environment. Next, the receiving subsystem involves the dolphins auditory system. Lastly, the signal processing or decision making subsystem consists of the ability of the dolphins auditory system to extract useful information from the echoes as well as the cognitive capabilities of the animal. Optimal use of acoustical information usually requires an auditory system that can cover a wide frequency range (Au et al., 2000).

Basic Mechanism of the Transmission of Sound Waves. Mo (2007) presents a step-by-step process to illustrate the mechanism of a dolphin sonar first, bursts of clicks of varying frequencies are produced in a series of air sacs contained in the nasal cavities. Second, the valves called bursae or monkey lips, or what Au et al. (2000) refers to as the monkey lips-dorsal bursae complex, open into the blowhole passage. Third, an air-filled cavity emits the dolphins echolocation signals into water, causing a mismatch in the propagation of the sound waves from air to water. Fourth, such an imbalance is overcome by the melon, which is a large deposit of fatty tissues extending into the nasal sac muscles. The melon acts to slow down sound waves as they are emitted from the nasal air sacs in order to make the transfer of sound waves from air to water smoother.

Dobbins (2007) states that it is now a widely accepted fact that the lower jaw is a major component of the echo-receptor in dolphins. The lower jaw and the surrounding structures is the part of the dolphins head through which many of the acoustic signals to which dolphins are sensitive are brought to the middle and inner ear.

In addition to the lower jaw and the parts that originally facilitate the transmission of the sound waves, the dolphins teeth also play a major role in the mechanism of the dolphin sonar and the role is that of a sonar array. Dolphins, first of all, are homodonts, or animals with uniformly-shaped teeth evenly-spaced along the jaw in two straight lines. The teeth actually serve as resonant pressure transducers, which means that they act as an array of receivers which produce vibrations according to changes in pressure caused by sound waves while they travel through the water. Moreover, there is some experimental evidence pointing to the role of dolphins teeth in receiving acoustic signals. The teeth of dolphins have actually been found to resonate at frequencies ranging from 115,000-135,000 Hz in response to sounds. However, this theory on the role of the dolphins teeth in echolocation was opposed by Whitlow Au, the chief scientist at Marine Mammal Research Program of the Hawaii Institute of Marine Biology as chief scientist, who contended that several dolphins who have lost their teeth were still able to activate their sonar systems (Dobbins, 2007).

Characteristics of Echolocation Signals. There are basically two kinds of dolphins the ones capable of whistling and the ones that cannot whistle (Au et al., 2000).

Dolphins that are capable of producing whistling signals include the bottlenose dolphin (Tursiops sp.), Pacific whitesided dolphin (Lagenorhynchus obliguidens), Amazon River dolphin (Inia geoffrensis), Atlantic spotted dolphin (Stenella  frontalis), Rissos dolphin (Grampus griseus), Pacific spotted dolphin (Stenella attenuate), rough tooth dolphin (Steno bredanensis), spinner dolphin (Stenella longirostris), and Chinese river dolphin (Lipotes vexillifer). However, most of what is known about dolphin sonar systems was obtained from the bottlenose dolphin as well as two other cetaceans beluga whale and false killer whale. Peak frequencies usually reach between 120,000 and 130,000 Hz (Au et al., 2000).

The second type of echolocation signals are produced by dolphins and porpoises that gave no evidence of being able to emit any whistle signals. The production of their sounds is restricted to high frequency, low intensity click signals. These species include Commersons dolphin (Cephalorhynchus commersonii) and the Hectors dolphin (Cephalorhynchus hectori) among the dolphins. The three other species that have the same characteristics are porpoises. Compared to that of whistling signals, the duration of click signals is much longer. The signals also exhibit many oscillations and the spectra are much narrower. The peak frequencies reach up to 140,000 Hz (Au et al., 2000).

Comparison with Bat Echolocation. Bats are the first animals found to possess the ability of echolocation. Both bats and dolphins possess biological sonar systems. However, the tonal airborne signals of bats are normally much longer in duration compared to the click-like calls of dolphins. The shorter calls of the dolphin somehow enable them to produce a good temporal resolution in water since water is a type of medium where the speed of sound is five times as fast as in air (Troitino, 2010).

Another difference between bats and dolphins when it comes to their sonar systems is that dolphins possess a greater control of its sonar system and can alter the sonar transmitter, whereas bats can alter the gain of the sonar receiver. This difference somehow is of advantage to dolphins because their middle ear muscles are stiffer and denser than in bats which make the latter less adaptable (Troitino, 2010).

One more difference is in the perception of acoustic images. Those perceived by dolphins are most likely not comparable with those perceived by bats. The reason for such difference is the greater density of water and its relatively lower compressibility compared to air thus accounting for greater acoustic impedance in water. It is believed that dolphins can use this difference to their advantage by detecting their prey with it even if the prey is under sediment (Troitino, 2010).

One last difference between bats and dolphins is that the latter produce their echolocating clicks from the inside of the phonic lips located in the nasal passages, specifically in a series of air sacs (Moh, 2007), transmit this sound through a special structure called a melon, or a sac-like pouch of fatty tissues located on the forehead, and receive the echoes through their lower jaws (Troitino, 2010).
Dolphin Sonar Frequency Range. A majority of marine dolphins have a large repertoire of sounds including two general types of pulsed sounds, the first of which is used for its sonar or echolocation ability and the second type is emitted when the animal is in a state of emotion. Aside from the echolocating clicks, dolphins also emit whistles and chirps which are in fact pure tone sounds. The whistles and chirps have varying loudness and duration (Erber, 1996).

When it comes to frequency of the dolphin sonar, various studies have found out that the range of the low end is from 100 Hz to 8,000 Hz while the higher end of the range shows a variation of 120,000 Hz up to 200,000 Hz. Mo (2007) specifies an average dolphin sonar frequency of a maximum of 160,000 Hz. One factor that accounts for the differences is the relative accuracy and sensitivity of the tools used to measure the frequencies through the years. The frequency ranges are also still subject to greater scrutiny since there is a need for scientists to consider the size, gender and age of the dolphins, the location of the measurements, the time of the year the measuring is done, plus many other factors that may have caused the present discrepancies in the current data in the first place (Erber, 1996).

High frequency sonar systems for dolphins may share the same advantage as their mechanical counterparts. The use of high frequency sonar systems such as 1.8 and 2.4 MHz is said to provide a sufficient resolution for target recognition and identification (Wilcox  Fletcher, 2004).

Basic Mechanism of the Reception of Echoes. Au (1993) states that the receiving system of a dolphins sonar is its auditory system which consists of the outer, middle and inner ears. Even until now, it is not exactly clear how and where exactly sound is conducted into the middle and inner ears of the dolphin considering the narrow structure and fibrous structure of the external meatus. These two characteristics make the outer ear hardly capable of being an acoustic pathway to the middle and inner ears. Although some scientists argue that the external meatus is indeed the primary pathway for sound transmission, there is an alternate theory suggesting that the external meatus is nonfunctional and that it is through the thinned posterior portion of the mandible that the sound enters the dolphins head. The sound is then transmitted across a fat-filled canal to the tympano-periotic bone where the middle and inner ears are located. At this point, the sound is basically interpreted in the same way as in any other mammalian auditory system.

However, Wever et al. made an estimate of the ganglion cell population associated with the cochlear hair cells in the inner ear. They were able to estimate a population of around 70,000 for the Lagenorhynchus species and 95,000 for Tursiops. These numbers were found to be considerably greater than those found in man which number only 30,500. The ratios of the number of ganglion cells to hair cells in the two previously mentioned species of dolphin are four and five times respectively, surpassing the human ratio of only two times. Wever et al. said that this high ratio of ganglion cells to hair cells in dolphins may explain why these animals are capable of interpreting high frequency acoustic information and finer details of cochlear events in the more highly activated centers of their auditory nervous system (as cited in Au, 1993).

Actually, the dolphin sonar enables the dolphin to perceive objects around it in a much more complex way than it seems since the information available from the dolphin sonar may at times include things that cannot be seen with the naked eye. Depending on the object, sound waves can penetrate the surface before bouncing off thus giving feedback and information about the internal structure of the object (Jewell, 2005).

The moment an echolocation click hits a particular target such as a fish, a proportion of the click bounces back as an echo and is physically detected by the dolphin. The dolphin can determine how far away the fish is from the moment the echolocation click was sent out to the time it returns. The dolphin, however, has to make several more clicks and hear more echoes before it can precisely determine how fast and in what direction the fish is moving. The dolphin sonar can detect an object with the length of as little as 2.5 cm from an unusually long distance of 72 meters, while Moh (2007) claims only an object 9-18 cm in length but at a distance of up to 100 meters. Nevertheless, what is important is that the closer the dolphin is to its target, the more clicks it sends out and the more echoes bounce back (Jewell, 2005).

Conclusion
Dolphins are certainly not only animals one enjoys watching during shows. The animal has, through the millennia, evolved into a species that possesses not only healing abilities but well-developed sonar systems. The dolphin sonar is indeed one of the most sophisticated characteristics of the aquatic mammal. This characteristic basically evolved from the hunting habits of the aquatic ancestors of cetaceans while there are claims that the brain of the dolphin, particularly the neocortex, has remained unchanged for millions of years thus accounting for a sharply primitive way of dealing with their environment. However, no matter what the evolutionary origin of the dolphin sonar is, this specialized mechanism currently involves an extremely complicated process of transmission and reception of sounds, followed by signal interpretation. Furthermore, recent studies into the nature, origin and significance of the dolphin sonar are leading to more and more questions on the nature and inherent intelligence of cetaceans and of what significance it particularly has on human life and psychology.