Animal Bioacoustics: Communication and echolocation among aquatic and terrestrial animals
The study of bioacoustics emerged as recording technologies became more available throughout the early 20th century. The study of animals and the sounds they make have assisted us in learning more about how our environment works around us. Echolocation is a particularly interesting field of bioacoustics and this paper examines the role echolocation plays for dolphins and bats. This paper examines what happens in the ultra and infrasonic ranges to see how they affect these areas of communication. I will be sure to mention the differences in qualities, ranges and speeds of aquatic and aerial sound. This study will examine Animal Bioacoustics and the use of echolocation, communication, and defense among aquatic and terrestrial animals.
Animal bioacoustics, often called simply bioacoustics, is the study of sound in non-human animals. It includes within its scope: animal communication and associated behavior, sound production anatomy and neurophysiology of animals, auditory capacities and auditory mechanisms of animals and instrumental sonar: use in population assessment, identification, and behavior effects of human-made noise on animals 3. In other words, it is the study of sound in animals and the way they use sound to communicate. By recording and reviewing audio tapes, researchers are able to not only to identify a species but also to determine its movements, population densities, and evolutionary relationships. Such knowledge aids ecologists and conservationists as it furthers the understanding of how animals communicate, survive, and respond to human encroachment7. Several universities and governments have programs and projects that study Bioacoustics, but most deal with the study of underwater sound. As man has expanded his grasp on the way the world works, it has become increasingly harder to study animals in their natural habitat with out the interference of a human produced noise. Given the vast amount of water on earth, and the availability of undisturbed habitat, it is much more practical to research ocean life and the sounds they make. It is also convenient for university and government projects to work together in joint studies to benefit the range of knowledge for Bioacoustics, but to also test and develop new technologies for civilian and military use. Despite the fact that most of these studies deal with underwater projects, a great number of studies that involve the use of echolocation in bats, insects and a few terrestrial animals like the shrew.
The study of Bioacoustics began in the early to mid 20th century when recording technology became more broadly available to scientists and the archiving of animal sounds began. The Acoustical Society of America (ASA) was founded in 1929 to increase and diffuse the knowledge of acoustics and promote its practical applications. A wide variety of organizations including public and private, have initiated archives of animal sounds that continue to help us understand animals and their means of communicating. The sound archives have actually helped to locate species that were thought to be extinct. In 1935, Cornell University ornithologists Arthur A. Allen and Peter Paul Kellogg made the only known recording of the ivory-billed woodpecker. Four decades later, the bird, which is now thought to be extinct in North America, was rediscovered in Cuba with the aid of that recording7.
Some current programs that are underway include the Cornell Bioacoustics Research Program and The Center for Bioacoustics at the Texas A&M at Chorpus Christi. Both have extensive, ongoing research facilities and maintain large collections of bioacoustical recordings. At Cornell, Allen and Kellogg's pioneering work recording and documenting sounds has grown into the world's largest collection of wildlife noise, the Library of Natural Sounds. Recordings include the utterances of Rwandan Mountain gorillas, a newly discovered Peruvian parrot, and more than half the world's estimated 9,000 bird species. The collection contains over 65,000 recordings, with 3,000 to 4,000 added every year7. Other institutes include the Centro Interdisciplinare di Bioacustica e Ricerche Ambientali (Interdisciplinary Center for Bioacoustics and Environmental Research) at Italy’s Università degli Studi di Pavia, another extensive laboratory involved in bioacoustc research. Both Cornell and C.I.B.R.A. have programs that run with the assistance of their respective government navies. Cornell has investigated whale sounds using the US Navy's Integrated Undersea Surveillance System (IUSS). The IUSS is a vast network of bottom-mounted and surface-towed hydrophone (underwater microphone) arrays originally deployed to track Soviet submarines1. The C.I.B.R.A. began similar research to study cetaceans in the Mediterranean Sea with the Italian Navy in 19952. These studies have helped us to learn a great deal of information on underwater sound and the way marine life communicates and navigates using sound.
One such topic of extensive research is the use of echolocation among marine and terrestrial animals. The term "echolocation" refers to the ability that some marine mammals and bats possess that enables them essentially to "see" with their ears by listening for echoes. This process allows animals to echolocate by producing clicking sounds and then receiving and interpreting the resulting echo5. Echolocation, high-pitched sounds are emitted by certain animals to locate their prey or to avoid obstacles. The sound waves are reflected back to the animal indicating such specialized information as distance to an object or the size and direction of prey4.
Marine mammals and fish use echolocation to help them navigate, communicate and search for food when light levels are low in water. However land animals such as bats and the oilbird, or guacharo, use echolocation extensively to find prey and navigate in the dark. It has even been suggested that some land mammals such as the shrew use echolocation to find prey. Instead of navigating visually, these animals audibly "see" by receiving information about the location of nearby objects and conditions from the clicks that they constantly emit. When the reflection of the clicks are received by the animal, it is can understand its surroundings by analyzing these clues.
Dolphins emit pulses and clicks in directional clicks in trains at a rate of over 300 per second and each click lasts about 50 to 128 microseconds. The click trains pass through the melon (the rounded region of a dolphin's forehead), which consists of lipids (fats). The melon acts as an acoustical lens to focus these sound waves into a beam, which is projected forward into water in front of the animal. Sound waves travel through water at a speed of about 1.5 km/sec (0.9 mi./sec), which is 4.5 times faster than sound traveling through air. These sound waves bounce off objects in the water and return to the dolphin in the form of an echo5.
This method of understanding its environment is very accurate and has been extensively studied since World War II when the heavy use of submarine warfare began. Using this method of echolocation, a dolphin can find a single BB dropped into the opposite end of a 70-foot pool! Along with this great ability comes an intelligence that is comparable to our own. They have shown stages of vocal learning seen only in humans and some bird species. Calves even have a "babbling" stage much like that of a human baby. Dolphins are able to spontaneously mimic sounds, particularly a variety of whistles6. They even have a particular whistle that is unique to each dolphin that can be thought of as an individual’s name in "dolphin-speak." Evidence has even proven that an 8-yr.-old female bottle-nosed dolphin at the University of Hawaii can immediately recognize by sight complex shapes she had only "heard" before through echolocation. Conversely, she can also immediately identify "blindly," using her sonar alone, objects she had seen before only with her eyes8.
All microbats navigate—and most insectivorous species also target their prey—by echolocation. This is the pulsed emission of high-frequency sounds that are reflected back as echoes to a bat's ears from surrounding surfaces, indicating the position, relative distance, and even the character of objects in its environment. In this sense microbats "see" acoustically. This is the basis for their ability to navigate in total darkness10. Unlike the complex underwater abilities of echolocation in dolphins, the bat family seems to have less developed capabilities in the air. Analysis of the ability of free-flying echolocating bats to distinguish between prey under natural foraging conditions reveals that insectivorous bats attack any moving target and fail to discriminate on the basis of target texture and shape9. Studies have revealed that bats use one of two types of echolocation: constant frequency (CF) sonar and modulated frequency (MF) sonar. The constant frequency biosonar allows precise determination of relative speed between bat and insect while the most important advantage in frequency modulated sonar is that the object size and distribution can be determined11. The physical properties of the emitted sounds vary in characteristic ways among species. The sound pulses are generated in the larynx, and in different species are emitted either from the mouth or nostrils10. Short prey identification range intrinsic in echolocation and the rapid flight of bats contribute to their inability to distinguish among prey. Bats attack stationary prey less than moving prey9. Their ability to use sonar to actively "ping" for insects lies in the ultrasonic range. These high frequency tones allow bats to home in on small targets like insects with relative ease.
Sound through Air and Water
The speed of propagation of sound in dry air at a temperature of 0° C (32° F) is 331.6 m/sec (1088 ft/sec). If the temperature is increased, the speed of sound increases; thus, at 20° C (68° F), the velocity of sound is 344 m/sec (1129 ft/sec). Sound generally moves much faster in liquids and solids than in gases. In both liquids and solids, density has the same effect as in gases; that is, velocity varies inversely as the square root of the density. The velocity of sound in many other gases depends only on their density. If the molecules are heavy, they move less readily, and sound progresses through such a medium more slowly. Thus, sound travels slightly faster in moist air than in dry air, because moist air contains a greater number of lighter molecules. The speed of sound in water, for example, is slightly less than 1525 m/sec (5000 ft/sec) at ordinary temperatures but increases greatly with an increase in temperature4. Sound is also governed by reflection, obeying the fundamental law that the angle of incidence equals the angle of reflection. An echo is the result of reflection of sound. Sonar and echolocation both depend on the reflection of sounds propagated in water4. Water is one of the most efficient mediums for carrying sound over distances. Although sound in shallow water (40-50 feet) carries just 5-10 miles, sound in very deep water (thousands of feet) can carry for astonishing distances. Clark said he once heard a blue whale singing off the cost of Newfoundland while he listened off the coast of Puerto Rico23.
Some sound is not audible to humans and lies above or below our natural range of hearing. Many sea creatures emit sounds that are inaudible to the human ear. The humpback whale has an extensive and varied sound repertoire, with both chirp and moan-like social sounds, and trains of repeating sounds termed song, reaching to 5000 Hz in frequency22. It has been discovered that most of the energy in the calls of both Asian and African elephants is concentrated at 14-35 Hz - frequencies near or below the lower limit for human hearing, which is around 20 Hz. The higher-frequency components of some of these calls are audible to humans as low, soft rumbles. Therefore, the ability to use infrasound gives elephants a distinct advantage when it comes to long-range communication18. Low frequency waves travel through air much farther than high frequency waves. Efforts are being made to see how the environment affects the distance traveled for such low frequencies. Such is the case in the recordings to be made by Cornell University in Africa over 100km2. By studying the recordings and changes in environment, Cornell hopes to see how long-distance-calling animals respond to daily changes in sound propagation. These recordings may also make it possible to keep an accurate number on the elephants and assess their health on acoustical info alone19.
Communication among animals
Animals communicate in a variety of ways but most notably they communicate audibly using sounds. In the vast area of animal communication, several species of animals compete for "air time." Some species of frog strategies allow them to identify and send messages amidst noise, while others involve making use of available media in the environment, such as leaves and muddy ground13. Other frogs avoid competition altogether and channels its mating calls underwater12. Other animals that aren’t traditionally thought of as auditory communicators have been discovered to "speak" to each other in communicative gestures. According to William F. Towne of Kutztown Pennsylvania University and German colleague Wolfgang H. Kirchner bees "hear" airborne noises in their communicative dances which take place in the dark15. Fish also seem to make audible noise when communicating. Since it was discovered in 1988 hamlets (fish) were singing songs of love during mating, that translated into terms of human technology, were beeps and pulses ranging from 350 to 650 hertz, or cycles per second. Sounds regularly heard include such frequencies as "middle C" that is 256 hertz.21. In fact the singing of animals in the wild isn’t uncommon. Research shows that animals in a habitat find available niches in the sound spectrum and occupy specific tonal ranges similar to specific instruments in a symphony orchestra. Field recording of ambient forest sounds in the Amazon and Africa reveals that animals pick sonic niches unoccupied by other animals, and the niche is maintained by members of the same species picking up the sounds if one member stops vocalizing20.
Defense and Offense
On the offensive and defensive end of the audio spectrum it’s hard to believe that animals can in fact use auditory signals to harm other animals. However, in the case of the bottlenose dolphin, echolocation frequencies can be over ten times our upper hearing of 20 kHz. Some high-intensity click sounds (230 dB) by bottlenose dolphins, beaked whales, and sperm whales may serve to debilitate prey by overloading fish lateral lines, ears, or shattering bony ossicles and other tissue22. Other studies prove that sounds are often used in defensive modes. Many species, such as groupers that can alarm even skin divers with booming noises, make sounds - whether of fear or warning - when approached21.
Some even avoid being preyed upon by emitting false signals to prey such as bats who are echolocating for food. The ultrasound acoustic startle response (ASR) of crickets (Teleogryllus oceanicus) is a defense against echolocating bats. The ASR to a test pulse can be habituated by a train of ultrasound prepulses17. Other research has been done to show how birds and other animal use sound to understand how far away others of their species are so tat they can maintain their territory. Studies of auditory distance perception in songbirds have shown that the overall degradation of songs during atmospheric propagation can be used to estimate the distance of the singer, called ranging 16.
This paper examines animal bioacoustics and the use of echolocation, communication, and defense among aquatic and terrestrial animals. It also explores audio in the ultra and infrasonic ranges to see how they affect these areas of communication. I will be sure to mention the differences in qualities, ranges and speeds of aquatic and aerial sound. This has become possible through recording technologies that became more available throughout the early 20th century. As a result, the study of bioacoustics emerged. By recording and analyzing the sounds that animals make have assisted us in learning more about how our environment works around us. Echolocation is one field of bioacoustics and this paper examines the role echolocation plays for dolphins and bats.