Running Head:
Name:
Number:
University:
Instructor:
Date:
DOLPHIN ECHOLOCATION
Echolocation, also referred to as dolphin sonar, is a tool that is used by dolphins to communicate, locate predators or prey. It enables the dolphin to locate its environment with great accuracy using ultrasonic sounds that bounces off objects. Experiments done on dolphins have shown that even without eyesight, dolphins can still locate objects of their environment accurately. A dolphin will emit whistles and clicks then listen to the environment for the echoes returning from various objects which the animal uses to locate, range and identify environmental objects. The paper focuses on various scientific researches done to understand more on the dolphin echolocation.
Other animals with echolocation include bats. Au and Simmons (2007) investigated echolocation in bats and dolphins and how physical constraints have influenced development of their echolocation systems. Discovery of animal echolocation was made in bats in 1773 but it was not until 1938 when it was confirmed. This began research of animal sonar which paved way for discovery of echolocation in dolphins. Dolphins produce these ultrasonic sounds through their nasal system using whistle and clicks while bats produce biosonar through their mouth.
Ivanov (2004) investigated the echolocation signals of dolphins while in an auditory complex environment using targets of different strengths as test objects. This experiment was conducted in an open cage to see how the dolphin responded to targeting with accuracy when an environment is full of other sounds. The results showed that a dolphin is able to detect objects as far as 600 meters. The possibility of results being recorded in open cage indicates that laboratory settings can also be used. The dolphin is able to detect objects even when there were interferences but the dolphin needs to be psychologically stable during the experiment through giving food items as targets. However, a greater number of pulses to detect a target in water are required for smaller volume of space of an acoustic pulse.
As Au (2004) puts across, the dolphin sonar system is the most sophisticated as dolphins are able to effectively detect, discriminate and recognize underwater targets. The dolphin does this through possessing short range echolocation system especially in shallow water where cluster echoes and reverberations are high and it is able to discriminate even when the environment is filled with other noises. The threshold ability of dolphins to detect echoes occurs at echo-to-echo ratio of 4dB with an ability to differentiate thickness of targets and material differences. The high resolution ability of the dolphin echolocation system combined with a dynamic range of its auditory system is essential factors in the ability of the animal to discriminate between targets. Despite having good characteristics of a good sonar receiver that includes the ability to localize sounds and spatially resolve sounds while rejecting any external interference and ability to recognize sound patterns, a dolphin has a mediocre echolocating system when compared to technological sonar. The technological sonar has high energy content transmission and receiving beam widths are small and have wide auditory filters, contrary to the features of dolphin sonar. According to Au (2004) the sonar system of a dolphin has several properties that enable it to have high performance. The dolphin uses broadband echolocation signals enabling a dolphin to have fine temporal resolution making the dolphin able to process echoes in time domain. This makes it possible for a dolphin to identify targets buried in ocean sediment. The range of its system is dynamic because a dolphin seeks orientations that are specific and will produce highest echo and also most information. Though in most cases, a dolphin is concerned with information more that the echo level. The sonar search done by dolphins is in an adaptive way making it possible for it to search a target at different orientations. The final feature is that the sonar system of a dolphin is controlled by a mammalian brain that has the capability of learning continuously where a dolphin can adjust to different conditions given on previous encounters.
Dubrovsky (2003) sought to test the hypothesis that was stated by Velmin and Dubrovsky which suggests that bottlenose dolphins have two auditory systems that are different in functions where one of them analyzes extraneous sounds similar to the one on terrestrial animals that do not echolocate and the other analyzes echoes produces by the animal itself. The first subsystem is referred to as passive hearing and the second one active hearing. The author had to consider an analyzer that includes source of sound, receiver and system of their interaction. The echolocation system used is where an acoustic sound is produced by the source focused by the field-forming system before transmission through water. This echo signal which is produced by the objects under influence of the echolocation click is transmitted through the water and received by the auditory system where auditory analysis takes place in development of images. The experiment was designed to include two situations where the first situation the animal did not know the direction of the arriving sound, intensity or even the distance to the source making sure that in order for the animal to make an efficient perception of the sound, the organ must have panoramic properties. In the second situation, the direction of the echo is known and the distance and intensity are also known. The results of the experiment indicated that activation of the active system must take place when a dolphin emits echolocation clicks and also when receiving external acoustic sounds even when the dolphin is not emitting echolocation clicks. The experiment confirmed the hypothesis showing that the auditory system in a dolphin is adjusted to the analyzing stimuli from objects under the water. It portrays a high sensitivity to hearing pulsed signals with a weak threshold summation in case of repetitive echolocation clicks. The auditory system also has high differential sensitivity to differentiate the short pulse by intensity and a difference in the mechanisms of auditory processing which are sued in echo components inside or outside the critical environment showing that a dolphin has two separated auditory subsystems, active and passive.
Most of the knowledge on echolocation in dolphins is from studies done using dolphins that have been held captive and they perform echolocation. Au (2004) investigated the echolocation of wild dolphins in a natural setting which would increase understanding of echolocation in nature. The items used were a four hydrophone array in a symmetrical arrangement in a star pattern to measure echolocation of four species of wild dolphins. Results indicated that the dolphin echolocation system has a time-varying gain showing that as the dolphin gets closer to the target, the echolocation signals decrease. The echolocation signals are predominated by bimodal spectra. The overall indication showed that there was a difference in signal waveform and spectrum between captive dolphins and wild dolphins questioning whether results obtained from the captured dolphins portray the true situation or the dolphins use echolocation because they are required to.
An echoic eavesdropping hypothesis in dolphins refers to a scenario where one dolphin does not transmit echolocation clicks but listens to echolocation clicks produced by others so that it can gain information about its environment. This was the basis of research done by Smith (2007). As per the hypothesis, the dolphin relies on other dolphins to locate food through monitoring echolocation systems of others. This shows that a dolphin is also able to locate signal that are not own biosonar to get useful information about the target resulting to dolphins swimming in close proximity. Experimental research done showed that in a synchronized swimming of dolphins, only one dolphin was producing echolocation signals whereas if the swimming was unsynchronized, several dolphins produced echolocation signals. However, the hypothesis remains untested due to complications arising from listening position and also the problem on determining whether lower frequency signals could be used by an eavesdropping dolphin. The listening position of the investigator that would produce performance without discrimination is important to determine the echo process used by an eavesdropping dolphin. However such research would also require use of multi-source eavesdropping hypothesis to learn how important an on-axis echo is to the dolphin and whether the dolphin can relate to it.
Rimskaya-Korsakova and Dubrovsky (2006) explain the ability of the dolphin’s auditory system to analyze short high frequency echo signals by burst principle. According to the principle, encoding of signals by the auditory periphery is determined by a combination of effect on many excited auditory nerves that are synchronized. Sensitivity is determined by the proximity of fibres’ sensitivity to that of the synaptic potential stirred by the stimulus. In case of long signals, such and effect is only achieved due to adaptation of the auditory system which causes the fibre sensitivity to be adjusted to the potential of the synaptic stimulus. Dolphins identify targets through analyzing a single short echo signal whereby during echolocation, the intensity of the probing pulse is varied over a wide range. The researchers made use of a model of high frequency auditory periphery to test the hypothesis that suggests that variation of intensity is an adaptive mechanism to adjust the intensity of the echo signals to correspond with fibre sensitivity which develops due to effects of lot of noise. For the burst principle to be realized, it is necessary that differentiating innate properties in response of many auditory nerves is present.
In conclusion, a dolphin has a very sophisticated echolocation system that enables even blind dolphins to locate and discriminate objects through analyzing the depth of the echoes. A dolphin can also analyze other echolocation systems of other dolphins which also helps it identify predators or food and any other obstructions on its path while under water. It is possible for a dolphin to differentiate echoes initiated from it even when there are other noises present in the environment. The echolocating system of a bat is similar to the dolphin only that a dolphin uses nasal system while a bat uses the mouth to produce echolocation signals. Many experiments have been done on dolphin echolocation and many more will be done because there are still some unanswered questions like where is the exact location of the origin of echolocating signals in a dolphin.
REFERENCES
Au, W.W. L. (2004) The dolphin sonar: excellent capabilities in spite of some mediocre properties. AIP Conference Proceedings, 728(1), 247-259, DOI: 10.1063/1.1843019
Au, W. W. L. (Jul 2004) Echolocation signals of wild dolphins. Acoustical Physics, 50(4), 454-462, DOI: 10.1134/1.1776224
Au, W. W. L. and Simmons, J. A. (Sep2007) Echolocation in dolphins and bats. Physics Today, 60(9), 40-45.
Dubrovsky, N. A. (May2004) Echolocation system of the bottlenose dolphin. Acoustical Physics, 50(3), 305-317, = DOI: 10.1134/1.1739499
Gregg, J. D., Dudzinski, K. M. and Smith, H. V. (2007). Do dolphins eavesdrop on the echolocation signals of cospecifics? International Journal of Comparative Psychology, 20(1), 65-88.
Ivanov, M. P. (Jul 2004) Dolphin echolocation signals in a complicated acoustic environment. Acoustical Physics, 50 (4), 469-479, DOI: 10.1134/1.1776226
Rimskaya-Korsakova, L and Dubrovsky, N. (Jul2006) Dolphin’s echolocation strategy of target identification: is it determined by the peripheral auditory encoding? Acoustical Physics, 52(4), 446-454, DOI: 10.1134/S1063771006040129
