hot topics in animal bioacoustics

Some hot topics in animal bioacousticsWhitlow W. L. AuMarine Mammal Research Program, Hawaii Institute of Marine Biology, P.O. Box 1106, Kailua,Hawaii 96734

~Received 16 November 1996; accepted for publication 19 February 1997!

This paper is derived from a ‘‘Hot Topics in Animal Bioacoustics’’ presentation at the 130thmeeting of the Acoustical Society of America in St. Louis, Missouri. Six bioacoustics studies on awide variety of species are discussed. Two of the studies are concerned with insects, the parasitoidfly, and cotton bollworms. The remaining bioacoustics studies are on aquatic animals including theWest Indian manatee, elephant seals, and dolphins. ©1997 Acoustical Society of America.@S0001-4966~97!07005-7#

PACS numbers: 43.10.Ln, 43.80.Ka, 43.80.Lb@DWM#

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INTRODUCTION

The field of animal bioacoustics is diverse, involvingwide variety of species varying from insects and birdsterrestrial and aquatic animals. Interest in this area of acotics has been growing, and the Animal Bioacoustics SpeTechnical Group will become a full committee of the Acoutical Society of America following the fall 1996 meeting iHonolulu. This paper is derived from a ‘‘Hot Topics in Anmal Bioacoustics’’ presentation at the 130th meeting ofASA in St. Louis, Missouri, in which interesting researcbeing performed with different species was highlighted. Rsults of some of the studies that will be discussed havebeen published so they can be discussed only in genterms without compromising the publication plans of tvarious investigators. The objective of this paper ispresent the reader with a general flavor of the type of anibioacoustics research that is currently being conducted.seven specific projects highlighted deal with subjects asied as insects and mammals of aquatic and marine origi

I. DIRECTIONAL HEARING IN THE PARASITOID FLY

Humans and most large animals generally localsounds by using two mechanisms: time of arrival differenof sounds arriving at the two ears~which can also be considered as a phase difference cue!, and the difference in theintensity of sounds arriving at the two ears caused by diffrtion of sounds around the head and body. However, theof insects are often fractions of a wavelength apart, makthese two cues relatively ineffective. Yet insects and otsmall animals can and do localize sounds using very difent mechanisms such as pressure gradient receivers incrickets~Michelsen, 1992!.

The female Parasitoid fly,Ormia ochraceamust findand deposit her parasitic larvae on a live field cricket.locates the host cricket at night, apparently using acouscues. However, the fly’s ears are not physically separatedare contained within a common air-filled chamber, onabout 450 to 520mm apart~Miles et al., 1995!. The smallseparation distance between tympanum makes the encoof time difference and intensity difference cues impracticA group from Cornell University~Robertet al., 1992, 1994!has been studying the hearing processes of the parasoito

2433 J. Acoust. Soc. Am. 101 (5), Pt. 1, May 1997 0001-4966/97/1

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and has discovered a new mechanism of sound localizainvolving a mechanical coupling between the tympanmembranes of the fly’s hearing organ~Miles et al., 1995!.

Figure 1 is a schematic showing the external anatomlocation of the ears, and an electron micrograph of the typanal membranes. The bulbae acoustica are attached tmembranes through a stiff cuticular rod that connects totympanum at the tympanal pit~TP!. The cuticular structureconnecting the tympanum pits to each other and to the ppoint is known as the intertympanal bridge. Each bulba actica contains about 70–75 auditory reception cells thatinnervated by the auditory nerve.

Miles et al. ~1995! measured the response of the tympnal membranes to sounds arriving at different angles bying a laser vibrometer, as depicted in Fig. 2. The laser licould be focused to an area of approximately 5 mm in diaeter allowing Miles and his colleagues to measure the mbrane vibration at a large number of locations. The incidsound was also measured using a small microphoneshown in Fig. 2. The acoustic stimulus was a burst of balimited white noise lasting 0.01 s and having a frequenrange of 1 to 30 kHz. The signals from the laser vibromeand the microphone were digitized and the transfer functof the different locations on the tympanum were measure

Miles and colleagues found that the tympanal pit~TP!which was closest to the sound source~ipsilateral! respondedwith as much as 20 dB greater amplitude than the tympapit farthest from the sound source~contralateral!. The tym-panal structure rocks about the central pivot point withtwo ears moving in nearly opposite phase. A mechanmodel of the tympanal structure on a pivot is depicted in F3. When a sound first arrives at the ipsilateral tympanalthe structure deflects downward and this motion produforces on the contralateral ear via the intertympanal bridwhich tend to cancel the effects of the external acoustic psure. This mechanical action essentially produces an interal intensity difference that would normally not exist sinthe ears are closer than 1/200th of a wavelength at 5 kH

II. ACOUSTIC DETECTION OF PINK BOLLWORMS

Pink bollworm infestation of cotton bolls is a seriouproblem for cotton growers. The present methods for exa

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ining samples of cotton bolls to determine if a field has beinfested are laborious and prone to error. In order to sothis problem and devise a more expedient method of exining cotton bolls for bollworms, Robert Hickling and coleagues from the National Center for Physical Acousthave turned to acoustic detection of larvae. Larvae constaeat and move, generating sounds that can be detectedsensitive acoustic sensor.

Boll cutting is the traditional method to determine if boinfestation has occurred. The cotton boll are pulled fromplant and are cut open and examined for pink bollworm l

FIG. 1. ~a! External anatomy of theOrmia orchraceashowing the locationof the ears.~b! Frontal view of the ears with the head removed in left paand a frontal scanning electron micrograph view of the ears. The prostetympanal membranes~PTM! show radial corrugations which converge upothe tympanal pit~TP! to which the internal sensory organ is attached~fromMiles et al., 1995!.

FIG. 2. Schematic of the experimental geometry~adapted from Mileset al.,1995!.

2434 J. Acoust. Soc. Am., Vol. 101, No. 5, Pt. 1, May 1997

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vae. However, the larvae are usually very small, and sotechnique is not only error prone but also time intensive. Isecond method, cotton bolls are stored and left in boll boat the appropriate temperature and daylength regime fotime. Larvae within the bolls finish feeding and cut out of tboll. The number of damaged bolls and larvae/pupae pduced are then counted. However, this technique requireto 2 weeks and therefore has limited value.

The prototype multiple acoustic sensor developedHickling et al. ~1994! to detect the sounds of pink bollwormlarvae eating and moving in cotton bolls has four major coponents: the sound isolation box, sensors, amplificationfiltering components, and power supply. The individual sesors are based on low cost fetal monitors consistingadapted electret microphones connected to stethoscope hphones that were developed at the National Center of Phcal Acoustics. A photograph of the prototype multiple acoutic sensor consisting of 48 sensors is shown in Fig. 4. Tsensor signals are amplified and bandpass-filtered soonly a narrow band of low-frequency sound is detected.

Bolls from a Pima cotton field in Coolidge, Arizonwere used in a test of the acoustic detection system. Thhundred bolls were cut using standard procedures, and wput into boll boxes and held in an insectary. Three hundother bolls were used with the multiple acoustic sensorthen were carefully cut to verify sensing results. Binocumicroscopes were used in several cases to find ‘‘suspeclarvae for the bolls tested with the acoustic unit. The bowere warmed to a temperature of 38 °C prior to sensing. Tincreased audible larval activity thereby increasing the prability of sensing the larvae.

Results of the test are shown in Fig. 5. The acoussensor found as many or more larvae than did the omethods. Furthermore, acoustic sensing took less timelabor. As with the standard boll cutting and storing tecniques, the acoustic sensor unit was not error free. Casewhich a larva was heard, but could not be found~false posi-tive! occurred as shown in Fig. 5, along with cases in whthe sensors did not detect larvae but one was found u

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FIG. 3. Mechanical model of the parasitoid fly ears. The intertympabridge is assumed to be two rigid bars connected at the pivot througcoupling springk3 and dashpotc3 ~from Miles et al., 1995!.

2434Whitlow W. L. Au: Animal topics

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cutting ~false negative!. The results indicate that use ofmultiple acoustic sensor may be a good way to examinfestation of bollworms.

III. HEARING IN MANATEES

Manatees were once very plentiful in the warm watersFlorida but were placed on the endangered species lis1973. A variety of causes could account for the decreaspopulation: destruction of habitat, loss of sea grasses,collisions with motorboats. Boating accidents involvinmanatees have been particularly disturbing to Florida ocials as the number of boats using Florida’s waterways ctinues to increase. It seems strange that manatees do

FIG. 4. Photograph of the multiple acoustic sensor system~courtesy of R.Hickling!.

FIG. 5. Results of test one using Pima cotton~from Hickling et al., 1994!.


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detect the approach of on-coming boats and avoid collisby swimming away. Lack of knowledge on the hearing cpabilities of manatees has made it difficult to solve the bcollision problem without placing heavy constraints on tuse of Florida’s waterways by boaters.

A team of scientists led by Edmund Gerstein has beperforming auditory experiments with the West Indian mantee~Trichecus manatus! in order to address the boat collisioproblem. They have performed experiments to measureunderwater hearing sensitivity as a function of frequen~audiogram! for two 7-year-old captive born males~Gersteinet al., 1993!, and their masked hearing threshold and soulocalization capabilities~Gersteinet al., 1995!. The hearingstudies are being conducted at the Lowry Park ZooTampa, Florida.

The hearing sensitivity of the manatees was determiby a controlled psychoacoustic experiment in which thwere trained to place their head into a hoop facing atransducer. The J-9 sound projector, H-56 measuring hyphone, a hoop station, and a manatee approaching theare shown in Fig. 6~a!; and the manatee in the hoop stationdepicted in Fig. 6~b!. A pure-tone signal was then playedthe animal. If it could hear the tone, the animal would rspond by touching the signal-present paddle, and if a sigcould not be heard, the animal would respond by touchthe signal-absent paddle. The signal level was systematicdecreased after correct detection trials and increased afteanimal missed or failed to respond to a sound. Thus,

FIG. 6. ~a! Picture of the experimental apparatus showing the J-9 soprojector, H-56 measuring hydrophone, a hoop station and a manateproaching the hoop.~b! Manatee in the hoop station awaiting an acoussignal ~courtesy of Edmund Gerstein!.


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signal level was varied in an up–down staircase fashionpending on the response of the subject.

The audiogram of one of the subjects~Gersteinet al.,1997! is shown in Fig. 7 along with the audiograms forCalifornia sea lion and for a harbor seal in the top panel;for a killer whale and a bottlenose dolphin in the bottopanel. The manatee audiogram has a typical u shape inhearing range from 500 Hz to 38 kHz. The data also incated that a manatee can hear low-frequency sounds~below16 kHz! as well or better than other marine mammals.

Manatees are agile enough to avoid oncoming bo~Gerstein, 1994, 1995!, if they are aware of impending collision, but yet they do not. However, the low-frequency nture of boat noise may preclude detection at sufficient rabecause of elevated hearing sensitivity at low frequen~below 1 kHz!. It is also possible that a manatee may hearoncoming boat but not be able to localize the direction ofsound. Finally, their hearing in noise may also not be goGerstein and colleagues are continuing to study the heaof manatees by conducting a masked hearing threshold sand a sound localization study.

IV. ACOUSTIC ENVIRONMENT OF DIVING ELEPHANTSEALS

Many marine animals rely on acoustics to capture pravoid predators, reproduce, and navigate. Yet the oceanvery noisy environment, especially at low frequenciBuney Le Boeuf and his colleagues at the University of Cfornia at Santa Cruz have spent many years studyingnatural behavior and physiology of the northern elephanton the Ano Nuevo rookery off the coast of California. The

FIG. 7. ~top panel! Audiogram of a manatee compared with a California slion and a harbor seal.~bottom panel! Manatee audiogram compared witthat of a bottlenose dolphin and a killer whale~from Gersteinet al., 1997!.


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have recently expanded their research to consider the efof low-frequency sounds, including the ATOC signal othese animals~Fletcheret al., 1996!.

Special electroacoustics instrument packages have bdeveloped that can be attached to elephant seals to studacoustic environment of these animals when they are ousea. Adult female elephant seals spend 83% of the yeasea, and 90% of the time they are submerged~Le Boeufet al., 1988!. They are deep divers, averaging 500-m deon each dive with maximum dives to 1500-m depth. Thare one of few marine mammals that can dive down toaxis of the deep sound channel in temperate waters. Thfore, the kinds of sounds that these animals encounter, ecially when they are in the deep sound channel are of minterest.

The objectives of Le Beouf’s project are to determine~a!if diving seals make active sounds,~b! the frequencies andlevels of sounds diving seals encounter in their environmeand ~c! the best package design and attachment processminimizing flow noise. Instrument packages were attachejuvenile seals, on their return to An˜o Nuevo Island fromMonterey Bay. The first model of the attachment packacontained a Sony digital audio tape recorder with a hydphone, along with time depth recorder and swim speedtectors and the appropriate datalogger. Later models uTattletale data logger which also performs the analogdigital conversion of the incoming acoustic signals astores the data on a hard disk. A picture of the latest etroacoustic package is shown in Fig. 8. The hydrophonemounted on the back of the package to minimize the effeof flow noise. The top panel of Fig. 9 shows a seal bereleased from a boat with an electroacoustics packagetached to its back. The bottom panel shows the seal onbeach at Ano Nuevo Island.

An example of the acoustic signal received by a seashown in Fig. 10, with the depth of dive shown above tcolor sonogram. Most of the received signals had frequenin the range of 20–200 Hz. Snapping shrimp, cetacsounds, boat noise, and seal swim strokes and heart beaclearly audible in some of the data. Flow noise correlawith swim speed, suggests that optimal time for acous

FIG. 8. Picture of the electroacoustic package showing the hydrophlocated on the back of the package to minimize flow noise~courtesy of W.Burgess!.


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sampling would be when the seals are swimming slowResults of several deployments have indicated that it isfeasible to obtain long-term, reliable, quantitative, and ninvasive cardiac monitoring of elephant seals and other

FIG. 9. The top panel shows a seal with an instrumentation package breleased at sea and the bottom panel shows a seal on the beach a˜oNuevo Island~courtesy of B. LeBeouf!.


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rine mammals~Burgesset al., 1996!. This capability hasbeen a important bonus to the project. Overall, this projdemonstrates that it is feasible to monitor the acoustic eronment of free swimming seals, and, that such electroactics packages could provide important data on the seal’svironment.

V. AUDITORY-EVOKED POTENTIAL FROM TRAINEDDOLPHINS

Auditory-evoked potentials on dolphins and on smwhales have been measured with the animals housedsmall container where there is a considerable amounacoustic reflection from the container walls~Ridgwayet al.,1981; Supin and Popov, 1985!. The length of these containers is generally just larger than the animal’s length makindifficult to position a transducer directly on the beam axisthe animal’s auditory reception system. Furthermore, brastem-evoked potentials have been measured with eithclick or short tone-burst stimuli. With the click signal, it iimpossible to specify the amplitude that would relate tothreshold obtained in a behavioral study in which the rsound-pressure level at threshold is used. A click signaalso broadband so that it is difficult to determine what partthe spectrum the animal’s auditory system is respondingTone bursts also have some frequency and amplitude aguities associated with them.

Auditory-evoked potentials recorded from the humscalp have been shown to follow the periodicity of pure-tosinusoidal signals for frequencies up to about 1 kHz~Moush-egianet al., 1983! and have been termed the frequency flowing response. Scalp-recorded responses have alsofound to follow the envelope of continuous sinusoidally aplitude modulated stimuli in humans~Richards and Clark,1984! and in gerbils~Dolphin and Mountain, 1993! usingcarrier frequencies considerably higher than the pure-tphase locking capability of the frequency following r

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FIG. 10. Example of the acoustic signal received by the instrumentation package on a diving elephant seal~courtesy of W. Burgess!.


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sponse. Continuous sinusoidal amplitude-modulated~SAM!signals produced by amplitude modulating a high-frequesinusoidal carrier with a low-frequency sine wave canexpressed as

s~ t !5sin~2p f ct !@11m sin~2p fmodt#, ~1!

wheret is time,m is the modulation depth,fmod is the modu-lation frequency, andf c is the carrier frequency. Such a signal will have energy only atf c andf c 6 fmod. However whensuch a signal is passed through a nonlinear detection syssuch as a mammalian auditory system, the signal is demlated and energy atfmodwill appear. The evoked responsesuch a signal contains significant energy at the modulator envelope frequency of the stimulus~Dolphin and Moun-tain, 1993!. Hence this phenomenon has been termedenvelope flowing response~EFR!. With such a continuoussignal, the rms value of the acoustic pressure can be reameasured and associated with the behavioral measurhearing sensitivity measured at the carrier frequency.

Amplitude-modulated signals can also be producedsumming two sinusoidal or tonal signals of different frequecies so that

s~ t !5sin~2p f 1t !1sin~2p f 2t !, ~2!

where f 1 and f 2 are the frequencies of sinusoid 1 andrespectively. A two-tone signal~TT! contains energy only athe frequencies of the individual tones. However, the amtude of the signal will be modulated with a period corrsponding to the absolute difference in the frequency betwthe two tones, orf 2.15u f 2 2 f 1u.


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Researchers from Boston University and the Universof Hawaii have been performing auditory-evoked potenmeasurement, using continuous amplitude-modulastimuli, with three species of trained dolphins~an Atlanticbottlenose dolphin, a Risso’s dolphin, and a false kilwhale! in an open water environment~Dolphin et al., 1995!.The animals are trained to wear suction cup electrodesstation themselves within a hoop which has its center adepth of 1 m. The electrodes were silver discs~2.4-cm diam-eter! imbedded in custom designed latex suction cuEvoked responses were recorded differentially from the sc

FIG. 11. A false killer whale in a hoop station with suction cup electrodon its head, facing a J-9 sound projector.

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FIG. 12. ~a! Example of the two-tone amplitude modulated acoustic stimulus in the time domain.~b! Frequency domain representation of the two-tone sign~c! The evoked potential response in the time domain.~d! The evoked potential response in the frequency domain~from Dolphinet al., 1995!.

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source between the parietal~noninverting! just posterior tothe blowhole and the mastoid~inverting!. A ground electrodewas placed either on the melon or trunk region. A J-9 souprojector is located 1 m from the animal as shown in Fig. 1The white suction cup electrodes on the animal’s headalso be seen in the figure.

An example of the signal is depicted in the time doma@Fig. 12~a!#, showing the amplitude modulation, and in thfrequency domain@Fig. 12~b!#, showing the energies atf 1and f 2 . The evoked potential response averaged overpresentations of the stimulus in 164-ms blocks are showFig. 12~c! and ~d!. The nonlinear property of the auditorsystem demodulates the amplitude modulated signal soenergy at the modulated or envelope frequency is presethe evoked response. The question of what would bemost ideal relationship between the frequencies of the caand modulated signals can be addressed by measuringmodulation rate transfer function. This was determineddolphins by modulating the primary or carrier tone at frquencies between 18–4019 Hz and measuring the respmagnitude and phase atfmod or f 2.1. Examples of the modulation rate transfer function obtained fromPseudorcausingboth TT and SAM signals of different carrier and primafrequencies are shown in Fig. 13. From the figure onesee that there are fairly broad areas in which the magnitof the evoked potential responses does not vary muchthe modulation or difference frequency.

The hearing sensitivity of the three dolphin specieslow-frequency sounds~below 1.6 kHz! has been determinebehaviorally and by measuring evoked potentials. The resof both techniques agree very well, suggesting that determing hearing sensitivity with continuous amplitude modulatsound may be a good method to measure the hearing stivity of large whales that are beached or trapped in nets

VI. HEARING AND ECHOLOCATION BY DOLPHINSAT DEPTH

Hearing and echolocation research on dolphins has bconducted in shallow waters with the animals usually with2 m of the surface. However, dolphins can dive to depgreater than 300 m~Ridgway and Kanwisher, 1969! and theeffects of a high-pressure environment on hearingecholocation is not known. Sam Ridgway and colleaguethe Naval Command Control and Ocean Surveillance Cein San Diego have been studying the effects of depth onhearing sensitivity and the echolocation signals of beluwhales,Delphinapterus leucas~Ridgway and Carder, 1995!.

Two beluga whales were trained to dive in the opocean and station on a test platform at depths of 6~surface!,100, 200, and 300 m. The test platform with a whale aproaching on a bite plate is shown in Fig. 14. On the tplatform are video cameras to monitor the animal’s behava sound projecting transducer for audiometric test, a sotarget and a hydrophone to measure sonar signals. Ridgand his team have been working in the deep waters offClemente Island. The whales are trained to ‘‘boat follow’’inflatable boat to travel to the test site about 1 mile off sho

A trial consisted of a whale diving to the test platforsuspended below the data collection boat and stationing


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the bite plate. Hearing thresholds were determined by psenting a sound stimulus at random intervals. Every timewhale heard a sound, it would produce a whistle, which wdetected by another hydrophone and sent topside for ansis. The amplitude of the test tone was varied in a staircafashion. Echolocation was tested by presenting a targetline with the whale’s longitudinal axis, 1 m away. Each timea door hiding the target on the platform opened, the whawas trained to echolocate. The animal responded by proding a whistle if it perceived the target or remained silewhen the target was absent. The echolocation signals wdetected by a hydrophone and the information sent topsfor recording.

Preliminary results indicate that the whales hearing sesitivity is not affected by the high pressure at depth. Thethresholds are as good or slightly better at depth than atsurface. However, preliminary results indicate that the a

FIG. 13. Examples of the modulation rate transfer function obtained withfalse killer whale~from Dolphinet al., 1995!.


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FIG. 14. The test platform used to assess the hearing sensitivity and eccation signals of beluga whales at depth~courtesy of S. Ridgway!.

mals’ whistles at depth have larger high-frequency harmics, as can be seen in Fig. 15. The lower frequency comnents of the whistles are present at depth and close tosurface, however, the whistles at depth have larger highquency harmonics than at the surface. The increased prescauses various air sacs and chambers in the nasal systechange in shape and volume, affecting their resonancequencies. This increase of whistle frequency may haveportant implications concerning the mechanisms that pduce whistles. Specific sound generation mechanismscetaceans have not yet been pin pointed. However mtheories exist, based mainly on anatomical consideration

VII. DISCUSSION

There has been an increasing interest on how maaffecting the environment in which we live, work, and rereate. Animals are an important part of this environment, awe are slowly becoming aware that our use of naturalsources can have important consequences to many diffelife forms. Therefore, interest in animal bioacoustics hslowly been building over recent years. The U.S. milita~Air Force, Army, and Navy! all have some type of bioacoustics programs to study the effects of military exercispractices and operations on the environment of animals.Air Force has an extensive program to determine the effelo-

FIG. 15. Example of the beluga whale whistle at the surface and at depth~courtesy of S. Ridgway!.


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of aircraft noise on a wide variety of animals, from Musk Oand Arctic foxes in the high northern latitudes to tortoise areptiles in desert locations, and birds such as the Peregfalcons in mountainous regions. The Army also has antensive program studying their effects on the habitat of vous endangered birds and animals. The Navy has a progto determine the effects of low-frequency underwater souon marine mammals. In these programs, various typesnew, small, portable, and state-of-the-art devices are bdeveloped to monitor location, physiological parameters,environmental noise in these programs~Carter, 1996!. In or-der to monitor and study the effects of man-made noisedifferent animal species, it is important to understand thauditory capabilities. Therefore, within the military’s acoutic environmental assessment program, the basic hearingpabilities of different species are being determined. The mtary’s programs are too extensive and broad to be discuhere.

Biologists are increasingly becoming aware of the ufulness of monitoring the vocalizations of animals as a goway to study them in their natural habitat with a minimumdisturbance. New electronic instrumentation that can be cnected to laptop computers has been a boon to field reseers. The continual development of sophisticated signal pcessing and analysis software has made these tools acceto many researchers who would not even have dreameusing these techniques a decade ago. The maturing of slite tracking and monitoring instrumentation has also contruted significantly in studying the life cycle and the useacoustics by animals in the wild. The use of sophisticalaser techniques and other measurement techniques hacreased the capabilities of many researchers to studyacoustic functions of different species in new and innovatways. In short, we have entered into an exciting and inesting period for people interested in animal bioacoustics

ACKNOWLEDGMENTS

The author acknowledges and publicly expresses hispreciation to several people who provided various writmaterials, 35-mm slides, and data used in this paper. Thindividuals are Dr. William Burgess of Stanford ResearInstitute, Dr. Edmund Gerstein of Florida Atlantic Univesity, Dr. Robert Hickling of the National Center for PhysicAcoustics, Dr. Burney LeBeouf of the University of Califonia at Santa Cruz, Dr. Sam Ridgway of the Naval CommaControl and Ocean Surveillance Center, and Dr. Daniel Rert of Cornell University. This is Hawaii Institute of MarinBiology Contribution 1021.

Burgess, W. C., Tyack, P. L., LeBoeuf, B. J., and Costa, D. P.~1996!.‘‘Acoustic measurement of cardiac function on northern elephant seaJ. Acoust. Soc. Am.100, 2709~A!.


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Carter, M.~1996!. ‘‘USAF monitors for recording aircraft noise received banimals,’’ J. Acoust. Soc. Am.99, 2575~A!.

Dolphin, W. F., Au, W. W. L., Nachtigall, P. E., and Pawloski, J.~1995!.‘‘Modulation rate transfer functions to low-frequency carriers in three scies of cetaceans,’’ J. Comp. Physiol. A177,235–245.

Dolphin, W. F., and Mountain, D. C.~1993!. ‘‘The envelope followingresponse~EFR! in the Mongolian gerbil to sinusoidally amplitudemodulated signals in the presence of simultaneously gated pure tonesAcoust. Soc. Am.94, 3215–3226.

Fletcher, S., Le Boeuf, B. J., Costa, D. P., Tyack, P. L., and Blackwell, S~1996!. ‘‘Onboard acoustic recording from diving northern elephaseals,’’ J. Acoust. Soc. Am.100,2531–2539.

Gerstein, E. R., Gerstein, L. A., Forsythe, S. E., and Blue, J. E.~1993!.‘‘Underwater audiogram of a West Indian manatee~Tricheschus mana-tus!,’’ Proceedings, Tenth Bien. Conf. Biol. of Mar. Mamm., GalvestoTexas, p. 53~A!.

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