 
Infrasound From the Giraffe
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Invited to the Sept. 2001
American Zoological Association Conference, presented at the regional
Acoustical.Society of America Conference 2001
von Muggenthaler, E., Baes, C., Hill, D., Fulk,
R., Lee, A., (1999) Infrasound and low frequency vocalizations from the giraffe;
Helmholtz resonance in biology, Published in the proceedings of Riverbanks
Consortium
Abstract
The giraffe, a savanna
ungulate, possessing limited auditory vocalizations, was found to
produce infrasound. Recordings were made of 11 Giraffe (Giraffe
camelonardalis reticulata), at the North Carolina Zoological Park in
Asheboro, North Carolina and the Riverbanks Zoo and Botanical Garden in
Columbia, South Carolina. A portable system (7 Hz – 22 kHz) was used to
record the vocalizations. Analysis was conducted in real-time in the
field using a portable trigger oscilloscope and National Instruments
Polynesia. Real-time analysis consisted of Hamming FFT’s and time domain
displays. The signals were also analyzed in the lab using Polynesia and
Momentum Data System’s DSP Works. Each signal was low-pass/high pass
filtered, and FFT’s and spectrographs were performed. Audible signals
contained frequencies from 11 Hz (75dB+/-3) to 10,500 Hz (80dB +/- 3)
with dominant frequencies between 150-200 Hz. Inaudible vocalizations
detectable by real-time analysis and trigger scope, measured from 14 Hz
(60 dB +/-3) to 250-275 Hz (30dB +/-3) with dominant frequencies between
20-40 Hz. A behavior known as a neck throw appeared to be correlated
with the signals, leading the researchers to theorize that Helmholtz
resonance, (V=c2S/(4p 2Lf2)
was responsible for the production of the vocalizations. Additionally,
the decibel levels of the inaudible signals decreased rapidly over 40
Hz, which suggests that the vocalization may be designed to be a covert
form of communication. The hypothesis that giraffe, like okapi,
elephant, whale, and rhinoceros produce infrasound, was supported.
I. Introduction
Within the last 12 years
several large land mammals have been found to produce infrasound,
including elephants, (Payne et al., 1986), rhinoceros, (von Muggenthaler
et al., 1991), and okapi, (von Muggenthaler, 1992; Lindsey, Green, and
Bennet, 1999). Other animals may also use infrasound, and low frequency
vocalizations may be an important component of large animals’ signals.
Payne et al. (1986) found that
Asian elephants communicate both at sonic and infrasonic frequencies.
The infrasonic signals ranged in frequency from 14 to 24 hertz with
decibel ranges between 70 and 100 dB. African elephants also produce
infrasound (Poole et al., 1988) in the range between 14-35 hertz with
decibel levels up to 90 dB, and can perceive the calls of other
elephants at distances up to 4 kilometers (Langbauer et al., 1991). Male
African elephants have been shown to walk silently for more that 1.5 km
toward a loudspeaker playing the female elephant’s distinctive, low
frequency estrous call (Langbauer et al., 1991).
The long wavelengths of
infrasound mean that these low frequency signals are only reflected by
very large objects. Therefore, there is little attenuation of infrasonic
signals due to scattering by objects in the environment, making
infrasound ideal for the long distant communication (Pye and Langbauer,
1998) of elephants.
Infrasound is useful for
communicating in dense forest as well and for long-distance
communication on the open savanna. Sounds above 1 kHz attenuate much
more quickly in forest than would be expected by the inverse-square law
(Eyring, 1946; Dneprovskaya et al., 1963). Low frequency sounds (below
100 Hz), however, show little excess attenuation in forest environments
(e.g., Marten and Marler, 1977; Marten et al., 1977; Wiley and Richards,
1978). The okapi, a rainforest giraffid, produces infrasonic calls at
around 14Hz that are likely used to maintain mother/infant contact
(Lindsey et al., 1999). Okapi infants spend their days hidden in a nest
while their mother browses in the forest.
Giraffes are found in the
African savannah from the scrub and grasslands of the Sahara almost to
Capetown, not including the Central African rain forest. Their range
seems to be closely linked to the presence of acacias (Pellew, 1984).
Giraffes inhabit large roaming areas, and those of giraffe cows can
extend up to 43 square miles. Immature males tend to range even further
than either the sexually mature bull or cows (Pellew, 1984). Roaming
areas of individual giraffes overlap considerably, and loose
congregations of several animals, particularly females, are common.
These congregations, however, are often not stable and individual
animals may leave the herd, and others join it. Giraffes appear to
recognize each other "personally", and to be in visual contact with one
another over large distances (Pellew, 1984).
The authors noticed two giraffe
behaviors that are very similar to behaviors seen in okapi. Giraffes
produce a behavior where the head and neck starts at about chest level,
is thrown back over the body and curled upwards until the nose is
straight up in the air. The behavior was termed a neck stretch. A
similar behavior was also observed but involved only the head in which
the chin is lowered and quickly raised so that the nose is pointing
straight up into the air. This behavior was called a head throw.
The savanna habitat and
wide-ranging, loose heard structure of giraffes is similar to that of
African elephants. While giraffes have the advantage of height, and can
see conspecifices over relatively long distances, vocal communication
could also be useful for individuals to keep up with the locations and
movements of other giraffes. Infrasound would be well suited to this
type of communication much as it has been demonstrated to be for
elephants. This and the demonstration of the use of infrasound by their
closest relative, the okapi, suggests that giraffes may produce
infrasonic signals.
II. Methods
A. Subjects and site
Eleven reticulated giraffe (Giraffe
camelonardalis reticulata), two adult females, two adult males, a
juvenile male and a yearling male and female, at the North Carolina
Zoological Park in Asheboro North Carolina, and three adult females, one
male, and one yearling were recorded at the Riverbanks Zoo and Botanical
Gardens in Columbia South Carolina. The animals at both zoos were
recorded inside their barns. At the North Carolina Zoological Park, the
giraffe were recorded from the second floor of the enclosure, between 1
and 8 meters from the animals. At the Riverbanks Zoo, the animals were
recorded from both the ground floor, and the second story at a distance
of approximately 5 meters.
B. Recording protocol
Recordings were made for a
total of 20 hours during daylight hours from June to November 1997 at
the North Carolina Zoological Park. The three giraffe at the Riverbanks
Zoo were recorded for 2 hours in January 1998, and then again for 5
hours in October 2000. Recordings were made in the barns at both
facilities without the giraffe present, to determine the specific
acoustical characteristics of each structure. All acoustical
interference in the barns such as running water, or electrical fence
generators, were marked and eliminated before the recording sessions. A
Sony TCD-D8 Digital Audio Tape recorder (DAT), and several Statham
Radio’s LIZ microphones were used to record the signals. The Sony’s
frequency response at 48 kHz sampling rate tested with a Larson-Davis
2800 Precision Analyzer was +/- 3 dB from 7 Hz to 22 kHz and +/- 1 dB
from 11 Hz to 20 kHz. The LIZ microphone (1.5 Hz –22 kHz; 66.1 dB/(Hz)1/2
@ 1P), is powered by an emitter/follower buffer and is housed in the
AT8410A Audio Technica shock mount, which has an attenuation of 8 – 10
dB. An Audio Technica windscreen, with 10 – 20 dB attenuation is also
used. The signals were monitored using a portable Techtronix Tekscope
THS710 storage and trigger oscilloscope attached to the recorder. A
portable computer running Polynesia, a real-time FFT and time domain
analysis program, was also used. This equipment helped to determine when
infrasound occurred, including sounds from manmade sources, such as
airplanes. When manmade interference was noted, recording was halted
until the signal disappeared. Records were also kept of the head throws
and neck stretches that occurred during the recording session. The time
readout on the DAT recorder was noted when each head throw or neck
stretch occurred so that the behavior could be correlated to the
presence or absence of an infrasonic vocalization. FFT’s were performed
when each head throw or neck stretch occurred, to determine if a signal
had been produced.
1. Calibration
Calibration of the microphones
is performed at Statham Radio every 6 months, using a Bruel and Kjaer
2608 measurement amplifier, a Bruel and Kjaer 4231 calibrator, an Audio
Precision System 1, and an Electrovoice 1821 compression driver for
measurements below 40 Hz. All Statham Radio calibration devices are
certified by the manufacturers annually. Calibration of the recording
system in the field to insure accurate levels during recording,
analysis, and playback is performed using a Bruel and Kjaer 4226
multifunction acoustic calibrator. The 4226 generates accurate and
stable signals ranging from 31.5 Hz to 16 kHz in octave steps.
C. Signal Analysis
Signals were analyzed using
Momentum Data System’s DSP Works, digital signal processing software,
and National Instruments’ Polynesia. DAT tapes were scanned and all
signals marked using DSP Work’s real-time spectrographic function, and
Polynesia’s real-time FFT and time domain functions. Signals were then
isolated by Polynesia’s Snapshot function. After being marked, the
signals were transferred to computer hard drive. Full analysis was then
run on signals that corresponded with neck stretches or head throws.
Signals were analyzed by means of DSP’s FFT functions. Additionally the
signals were examined using Polynesia, and were lowpass filtered at 275
hertz. FFT's and spectrographs were again performed after the filtering
process. Each infrasonic signal was made audible using Polynesia, and
was then stored on Compact Disk as a wave file.
III. Results
The recordings contained 255
examples of vocalizations with fundamental frequencies around or below
20 hertz. There appeared to be two types of signals. Spectral analysis
showed that 221 or 87% of the signals were between 14 Hz (at 60dB +/-3)
to 250 Hz (at 30dB +/-3) as shown in Fig. 1 and Fig. 2. The decibel
levels of these signals decreased rapidly over 35-40 hertz and were not
audible to the researchers. The second type of signal occurred 22 times
or 9% of the total number of signals and were audible; 11 Hz (75dB +/-
3) to 11,200 Hz (89dB +/-3) as shown in Fig. 3 and Fig 4. Twelve signals
or 4%, were removed from the results due to background interference. The
lowest frequency, 11 Hz, came from a very large adult bull.Vocalizations
accompanied 35 out of 37 neck stretches or 95% of the time.
Vocalizations occurred 54 times out of 218 head throws, which is 25% of
the time. No signals occurred without the neck stretch or head throw
behavior. Vocalizations occurred more frequently in adult males and
young females when giraffes were separated, although the adult females
at North Carolina were unable to be recorded in the barn separated from
sight of their offspring for safety reasons.
IV. Conclusions
This study has determined that
giraffe emit vocalizations that are infrasonic. The mechanism for the
production of infrasonic vocalizations by giraffe has not been examined.
Further investigation of giraffe anatomy and how air is moved from the
lungs, through the neck and out the sinuses will be necessary. One
possibility is that since giraffe vocalizations have only been seen to
occur during the head throw behavior, infrasound and other components of
giraffe communication might result from large volumes of air being
forced up the neck, and/or possibly channeled through hollow posterior
sinuses. During the study, observers noticed a "shiver" or vibration
extending from the chest up the entire length of the trachea that
occurred during some neck stretches that accompanied vocalizations. It
is possible that this "shiver" is air movement, and could be responsible
for the signal. If air is moving up the giraffe’s neck is producing
infrasound, the mechanism may be Helmholtz resonance, which occurs when
an enclosed volume of air is coupled to the outside free air by means of
an aperture. It is a system of one degree of freedom and is a very
simple type of resonator system, (Kinsler and Frey, 1962.) It is
suggested that future studies involve creating a database of giraffe
vocalizations, so that upon natural death the dimensions of the giraffe
lung and trachea and the corresponding calculations for Helmholtz
resonance can be produced to see whether they correspond. A brief
example is the following:
Calculations of Giraffe Neck Resonance
and Helmholtz resonance:
Neck resonance
Assume a neck length of 1.5 m. Then f = c/2L = 330
(m/s)/ 2 x 1.5 m = 108 Hz. Assume the 14 Hz component is
Helmholtz resonance. Find the lung volume needed to produce the
Helmholtz resonance at this frequency.
Assume Speed of sound c = 330 m/s
Throat diameter = 5 cm = 0.05 m. Then
throat area S = p x (0.05)2/4 = 0.002 m2.
Neck length L = 1.5 m
Then V = c2S/(4Lf2)
= 3302 x 0.002/(4p 2 x 1.52 x
142) = 0.0188 m3 for two lungs=
0.009 m3 for one lung
This is equivalent to a cube
0.2 m (20 cm) on a side, or 18 liter capacity for both lungs. An adult
human is capable of exhaling 4.5 liters, and human athletes 6.5 liters.
It is therefore reasonable to assume that the giraffe, weighing a ton,
could have lungs capable of exhaling 3 times the amount of a human. The
lungs are a flexible membrane; therefore, membrane compliance or
equivalent volume would have to be taken into consideration when
performing calculations.
Another topic for future
research will be to establish whether the vocalizations have
communicative value, as this study was not designed to study the
response of one giraffe’s vocalizations to another. If the giraffe are
communicating, it would be very advantageous for them, being prey, to be
able to communicate "covertly" using signals designed to blend in with
background noise. The most common giraffe vocalizations have frequencies
up to 275 Hz, which should be audible. However, the decibel level of
these signals drops significantly above 40 Hz, and the audible portion
of the signal is lost in ambient noise. The giraffe’s signal may be
designed to be audible at low frequencies, which are less directional,
and inaudible at the higher, directional frequencies. This would
effectively hide the location of the sender.
Future research should focus on
whether the signals are associated with identifiable and stressful
contexts, as this has implications for the care and welfare of captive
giraffes. In the giraffe’s natural environment, the frequency levels
between 10 and 20 hertz are a relatively "quiet" bandwidth, only a few
other animals and natural sounds exist to inhibit their communication.
This is not true in urban areas. Infrasound can travel virtually
unimpeded for great distance; the signals can be interrupted or jumbled
because of other sources of infrasound such as large generators, trains,
freeways and other man made objects. Every effort should be made,
therefore, to see that captive animals are placed as far as possible
from extraneous sources of infrasound, so their vocalizations remain as
intact and perceptible as possible.
This study is dedicated to the memory of
Aaragorn, Maxine, Azog and Dr. Melvin Kreithen. Many thanks to Steve Wing and
Dr. Zuckerwar.
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