Book Read Free

Elephant Sense and Sensibility

Page 10

by Michael Garstang


  orifices extending 10–13 cm (4–5 in.) above the nostril openings and the ability

  to open or constrict the nostril openings is unknown.

  The elephant is a very large mammal with a unique vocal tract in the form

  of an elongated proboscis or trunk. The lung weight of an African elephant is

  about 22 kg (48.4 lbs), the vocal folds measure around 7.5 cm (3 in.), and the

  total length of the vocal tract measures between 3 and 4 m (9 and 13 ft). The

  elephant can extend this vocal tract by as much as one-fifth (20%) and can

  produce sounds as loud as 117 decibels (dB), where 120 dB is the level beyond

  which human hearing might be damaged (Soltis, 2010).

  SOUND DETECTION

  The hearing mechanism of the elephant is also large. Each outer ear or pinna

  measures on the order of 1.8 × 1.1 m (5.76 × 3.52 ft or more than 20 ft2) and is

  highly mobile so that it can be extended outward and raised (together with the

  60 Elephant Sense and Sensibility

  FIGURE 9.4 Raised head with fully spread ears. (Pen and ink drawing by author.)

  head), indicative of listening (Figure 9.4) (Moss, 1988; Poole et al., 1988, see Figure 18). The middle ear structures of the African elephant are also extremely

  large compared with other mammals (Soltis, 2010). The area of the eardrum or

  tympanic membrane is about 9 cm2 (1.4 in.2) compared to that of a human of

  about 0.7 cm2 (0.07 in.2). The mass of the malleus, incus, and stapes (278, 237,

  and 22.6 mg, respectively) are some 10 times that of the human equivalent. The

  large sizes of the elephant’s auditory structures may well enhance their ability

  to detect frequencies down to and below 10 Hz.

  The eardrum vibrates in response to a sound pressure wave and the vibrations

  are transferred by the three small bones known as ossicles. The size, shape, and

  attachments of the ossicle vary greatly among mammals. In general, the larger the

  mammal, such as the elephant, the larger the ossicles. The size of the ossicle is

  inversely related to frequency. The ossicles serve to both amplify the sound and

  transmit it to the oval window of the cochlea or inner ear. The precise relationship

  between the structural features of the external and middle ears and their audi-

  tory functions are not well understood in any mammal, let alone in the elephant.

  Sensitivity to low frequencies is, however, thought to be associated both with large

  tympanic membrane areas and large, compliant middle ear spaces. In Loxodonta,

  the linear dimensions of the malleus in the middle ear are about twice those of the

  incus. The long arm of the malleus (the manubrium) in Loxodonta is nearly verti-

  cal, placing it perpendicular to the horizontal plane and possibly increasing the

  sensitivity to low-frequency sounds (Rosowski, 1994, p. 173).

  The cochlea has the characteristic spiral shape and is filled with fluid. The

  cochlea in Loxodonta has two spiral turns and a total length of the basilar mem-

  brane of 60 mm (von Békésy, 1960, pp. 506–509). von Békésy (1960) measured

  the resonance of the cochlea dissected from an elephant, finding it responsive

  Communication Chapter | 9 61

  down to 30 Hz. von Békésy also concluded that of the animals studied, the

  elephant cochlea exhibited the sharpest resonance, indicating that elephants may

  be well equipped to distinguish between frequencies (Long, 1994, pp. 26–27).

  SOUND LOCATION

  The size of the elephant’s head and ears may govern how well it can locate

  sounds, particularly using low frequencies at large distances. Heffner and

  Heffner (1982, 1984) show that an Asian elephant can localize low-frequency

  sounds (<1 kHz) to within an azimuth angle of 1°, which is about as well as

  humans can. Langbauer et al. (1991), in a playback experiment, was able to

  show that African elephants can locate the source of a low-frequency call at an

  estimated range of 2 km (1.25 mile). Sounds are localized by using interaural

  time differences (ITDs) and interaural level differences (Palmer, 2004). Low

  frequencies work best in generating a time delay between the ears, whereas

  highs and lows (peaks/troughs) in the sound record work best at high frequen-

  cies. The ITD for humans is on the order of 0.7 s but is unknown and probably

  much larger for an elephant based both on the size of the head and pinnae as

  well as the path traveled by the sound wave between the eardrums. The path

  traveled by a wave front between the eardrums is larger than the skull perimeter

  for low frequencies but equal to the skull perimeter for high frequencies (Kuhn,

  1977, 1987). For humans, the effective acoustic circumference for low frequen-

  cies is 150% that of high frequencies (Brown, 1994, pp. 64–69). The size and

  position of an elephant’s ears and the presence of the trunk and forehead struc-

  ture would suggest that this expansion in perimeter distance for low-frequency

  signals might be considerably greater for elephants (Kuhn, 1977).

  Whether and how animals determine range using infrasound is poorly un-

  derstood. Elephant behavior with respect to low-frequency calls by familiar and

  unfamiliar elephants indicates an acoustic perception of range (Moss, 1988;

  Payne, 1998; Poole, 1996). McComb et al. (2003) used low-frequency playback

  calls, which provide evidence of a sense of distance between the listener and

  the sender of 1–2.5 km (0.6–1.5 mile). From personal observations in the field

  (Etosha National Park, in 1999), the time from initial reaction of elephants at a

  waterhole to the arrival (10 min) of a new group of elephants and the pace of the

  incoming group (8.0–9.6 kmph) suggest that the range at which their presence is

  first detected is between 1.5 and 2.5 km (0.9 and 1.5 mile). Distances in excess

  of about 100 m (328 ft) eliminate sight (but not smell) from the potential cues

  being used in determining these ranges.

  However, it is important to distinguish between analog and digital characteris-

  tics of calls. Information contained within the call in analog form depends on the

  structure of the call. This structure, even in long-range, low-frequency calls, is at-

  tenuated more rapidly than the call itself. When information is transmitted in digital

  form, best visualized as Morse code, then the loss of signal (and meaning) is greatly

  reduced compared to the analog form and one animal can hear and interpret the call

  of another at much greater distances (see discussion below on estrous calls).

  62 Elephant Sense and Sensibility

  Only candidate theories exist that might explain how and how well elephants

  determine the distance of the sound source from their location: there is clearly a

  loss of higher frequencies with distance such that ultimately only low frequen-

  cies remain. This change in the frequency spectrum may be used by elephants

  to estimate range.

  For hearing processes to be interpreted, we also need to know the sensitivity

  of the elephant’s hearing at different frequencies. This quantity too is poorly

  known. The only existing study (Heffner and Heffner, 1982, 1984) suggests that

  the threshold of hearing at 17 Hz is 50 dB SPL for an Asian elephant.

  The distance or maximum range over which one elephant can hear the call of

  another is important for a number o
f reasons that influence the survival of the spe-

  cies. In the matrilineal society of an elephant, where mature males are not in the

  herd and may be many kilometers away, a means to reach these males is essential.

  This need is heightened by the fact that the female may only come into heat for a

  short period every 4–5 years. Although at this point the estrous cycle may last for

  4 months, the estrous periods are short (2–6 days) (Leong et al., 2003). Only when

  offspring are not successfully reared do females cycle over shorter intervals. Not

  only must the female be able to let the male know that she is ready to mate, but

  she must pass this message on to a number of males such that genetic selectivity

  can be exercised (increasing the probability that the fittest and strongest male is

  selected for mating). The need then for calls of females in estrous to be heard by

  males over a long distance and thus over a large area may well be an added reason

  why elephants have retained the ability to generate and hear low-frequency calls.

  THE ROLE OF THE ATMOSPHERE

  For this to happen, the atmospheric conditions that were pervasive in the forests

  in which elephants evolved must be replicated over the dry subtropical savannas

  of current-day elephants. Carbon 13 data show that forests (C-3 vegetation) con-

  tracted progressively around 10 million years ago (MYA) being replaced by grass-

  lands (C-4 vegetation). Early elephants changed fairly rapidly from feeding on

  C-3 vegetation to feeding on C-4 vegetation about 8 MYA. This progression from

  forest to grassland, with some woodland still present, resulted in mixed feeding

  by elephants, which changed little over their subsequent history (Lister, 2013).

  As outlined in the Introduction, the dry and often cloudless atmosphere of

  the habitat of Loxodonta africana and Elphas maximus, the African and Indian subtropical savannas, go through a pronounced cycle of daytime heating and

  nighttime cooling at the surface. When translated to heat being transferred from

  the earth’s surface to the air immediately above the surface, the heat is lost from

  the surface to the atmosphere before sunset until more than an hour after sunrise.

  This results in a strong capping nocturnal inversion forming a channel or duct in

  which low- (as well as other) frequency sound can be propagated. This layering

  of the atmosphere (cold air at the surface about 100 m (328 ft) thick and warm

  air above it), decouples the stronger winds higher up in the atmosphere from

  the surface like a layer of oil sliding over water. Calm or low winds occur at the

  Communication Chapter | 9 63

  surface during the early part of the night, gradually increasing later in the night.

  This increase in wind speeds as the night progresses is due to cold air drainage

  over sloping terrain. By dawn the cold air has pooled and the winds at the surface

  once again subside (Garstang and Fitzjarrald, 1999; Garstang et al., 2005).

  Langbauer et al. (1991), in his playback experiment in Namibia, used a

  sound pressure level (SPL) at 1 m (3 ft) of 112 dB and a threshold of hearing

  of 46 dB, yielding a range of 1 km (0.6 mile). He then argued that because the

  sound production of the loudspeaker was limited to half power, the actual range

  of hearing was double the calculated value (i.e., 2 km (1.2 mile)).

  If, however, the atmosphere is stratified and the sound signal ducted under

  an inversion, spherical spreading does not occur and the sound can travel much

  further before dropping below the threshold of hearing. Propagation of sound

  in a stratified atmosphere can be determined by approximating a solution to a

  complicated equation called the Helmholtz acoustic wave equation.

  The Propagation of High- and Low-Frequency Sound

  The pervasively cold floor of the forest compared to the warmer canopy results in

  an inversion of temperature with denser air at and near the floor (lower tempera-

  tures) and less dense air at and near the canopy (higher temperatures). Sound in a

  fluid (air) travels faster at high temperatures and slower at low temperatures. The

  result in the forest is to channel or duct the sound within the canopy.

  High-frequency sounds (typically above 1000 Hz or cycles per second) behave

  like a beam or ray of light. While such a beam of sound may propagate over long

  distances in an unobstructed environment, the short wavelengths (a few centime-

  ters or less) of high-frequency sound in a forest will be intercepted by the vegeta-

  tion and will quickly dissipate.

  Low-frequency sound (below 20 Hz) emanates from its source not as a beam

  but as a uniformly propagating surface of a sphere. This too diminishes as a func-

  tion of the distance from the source. However, it now decreases as the square of

  the distance from the source such that it loses 6 dB for each doubling of the radius

  of the sphere. If, however, the low-frequency sound is contained within a duct

  (the inversion of the forest canopy), it will propagate down the duct, decreasing

  as a function of the linear distance from the source and not as the square of the

  distance from the source, traveling now like the high-frequency sound as a ray of

  light. But with a huge difference: The low-frequency infrasound has a wavelength

  of greater than 17.5 m or 40 ft compared to high-frequency sound with a wave-

  length of a few centimeters or less than an inch. The long wavelengths of infra-

  sound simply go “around” the trees and vegetation whereas short wavelengths of

  high-frequency sound crash into the trees and vegetation and are dissipated.

  As elephants evolved in the forest they took advantage of the “inversion or

  ducting” conditions, developing the capability of generating and detecting low

  frequency or infrasound. They subsequently emerged from the forests onto the

  savannas with this infrasonic capability. The amazing fact is that these hot, dry

  savannas, which during the day destroy almost all sound propagation, flip sides

  before sunset until after dawn to create a pronounced duct or inversion at the

  surface ideally suited to the propagation of infrasound.

  64 Elephant Sense and Sensibility

  This equation has no solution and must be approximated using computers to

  successively arrive at an acceptable estimate of a solution. Because helicopter

  rotors generate infrasound, they can be detected when flying below radar

  (terrain) surveillance height, which simply means flying below the height of

  the surrounding topography. The Department of Defense and the National

  Acoustics Laboratory in Mississippi spent a large amount of time and money

  developing the Fast Field Program (FFP) (Franke and Swanson, 1989; Lee

  et al., 1986; Raspet et al., 1985) that would yield a timely and acceptable result.

  These researchers, in collaboration with scientists at the University of Virginia

  (Garstang et al., 1995; Larom et al., 1997), then applied the FFP to elephant

  communication in the presence of these nocturnal inversions.

  RANGE OF ELEPHANT CALLS

  In the presence of a low-level (surface) inversion, one elephant can hear a loud,

  low-frequency call of another elephant 10 km (6 mile) away. In the middle of the

  day, however, this distance may be reduced to less than 1 km (0.6 mile). These

  calc
ulations were made for calm wind conditions. Calm or low-surface wind

  speeds typically occur under strong inversion conditions. Strong winds occur

  around the middle of the day when heat-driven turbulent mixing is at a maxi-

  mum (Figure 9.5). The effect of wind on the propagation and hearing of sound is complex. Turbulent motions in the atmosphere typically form vortices and rolls

  in the atmosphere that are highly destructive to sound of almost all frequencies.

  Standing on a bridge over a fast-flowing river, the eddies and whirlpools visible

  50

  40

  30

  % min

  20

  10

  12

  18

  0

  06

  12

  Hour of day (LST)

  FIGURE 9.5 Average frequency (%) of wind speeds higher than 4 m s−1 (2.1 mph) (thick black lines) in each hour for the 17 days in eastern Etosha National Park, Namibia. The thin vertical lines delineate net daytime heating (08:00–18:00) from net nighttime cooling (18:00–08:00) at the surface. After Garstang et al. (2005).

  Communication Chapter | 9 65

  at the water’s surface, gives one a visual picture of what occurs in the atmo-

  sphere. Turbulence is certainly the most destructive atmospheric effect on the

  range of low-frequency sounds used by elephants. Turbulence near the surface is

  closely related to wind speed, with higher turbulence occurring in higher winds.

  Since friction constrains wind to be zero, at some point very close the surface,

  wind speeds increase with height producing layers of faster-moving air over lay-

  ers of slower-moving air. This change in wind speed with height produces a

  shearing motion of one layer of air over another, enhancing mixing and turbu-

  lence. Wind-driven refraction of sound shortens upwind and lengthens down-

  wind communication. Finally, wind creates noise and in particular flow noise

  across the elephant’s ears. Hunters and game-watchers know that animals tend to

  seek shelter and lie low on windy days. This may be mainly due to the fact that

  they have difficulty hearing and thus detecting danger (predators) on windy days.

  The net result of the relationship between thermal stratification, wind, and

  the propagation and detection of sound is that one can conclude that optimum

  communication conditions occur when both thermal stratification and low

 

‹ Prev