AMER.ZOOL., 41:1157±1170 (2001)

Exploring the Potential Use of Seismic Waves as a Communication Channel by and Other Large Mammals1

C. E. O'CONNELL-RODWELL,2*L.A.HART,² AND B. T. ARNASON³ *Center for Conservation Biology, Department of Biological Sciences 371 Serra Mall, Stanford University, Stanford, California 94305-5020 ²Department of Population Health and Reproduction, University of California, Davis, California 95616 ³Tezar Inc., P.O. Box 26235, Austin, Texas 78755-0235

SYNOPSIS. Bioseismic studies have previously documented the use of seismic stim- uli as a method of communication in arthropods and small mammals. Seismic

signals are used to communicate intraspeci®cally in many capacities such as mate Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021 ®nding, spacing, warning, resource assessing, and in group cohesion. Seismic sig- nals are also used in interspeci®c mutualism and as a deterrent to predators. Al- though bioseismics is a signi®cant mode of communication that is well documented for relatively small vertebrates, the potential for seismic communication has been all but ignored in large mammals. In this paper, we describe two modes of pro- ducing seismic waves with the potential for long distance transmission: 1) loco- motion by animals causing percussion on the ground and 2) acoustic, seismic- evoking sounds that couple with the ground. We present recordings of several mammals, including , rhinoceroses, and elephants, showing that they generate similar acoustic and seismic vibrations. These large animals that produce high amplitude vocalizations are the most likely to produce seismic vibrations that prop- agate long distances. The seems to be the most likely candidate to engage in long distance seismic communication due to its size and its high amplitude, low frequency, relatively monotonic vocalizations that propagate in the ground and have the potential to travel long distances. We review particular anatomical fea- tures of the elephant that would facilitate the detection of seismic waves. We also assess low frequency sounds in the environment such as thunder and the likelihood of seismic transmission. In addition, we present the potential role of seismic stimuli in communication as well as the impact of modern anthropogenic effects on the seismic environment.

INTRODUCTION brations of the earth substrate (Ewing, Bioseismic cues are known to be impor- 1989). tant for many arthropods (Cocroft et al., Vibration signal energy depends mostly 2000), ®sh, reptiles, amphibians and small on the mass and available muscular power of the signal producer (Markl, 1983). The mammals in intraspeci®c and heterospeci®c source signal intensity and attenuation dur- communication, prey detection and predator ing transmission, together with the sensitiv- avoidance and navigation (see O'Connell- ity and depth of receptors in the receiver, Rodwell et al., 2000 for review). Two pri- and the threshold at which the receptor will mary methods of initiating bioseismic cues be stimulated relative to the frequency and are: 1) percussion that causes an impact strength of the stimulus de®ne the spatial with the earth and produces waves in re- extent of vibration signals. The Weber- sponse to direct contact and 2) vocaliza- Fechner law states that the magnitude of an tions which produce wave movements that observer's psychological response is direct- are then coupled with the earth to cause vi- ly related to the logarithm of the intensity of the stimulus (Landing et al., 1998). Sig- nal detection theory (SDT) further stipu- 1 From the Symposium Vibration as a Communi- lates that detection also depends on the ex- cation Channel presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 3±7 pectation, motivation, in situ conditions, January 2001, at Chicago, Illinois. sensitivity, decision making and ®nally, 2 E-mail: [email protected] noise level (Tanner and Swets, 1954).

1157 1158 C. E. O'CONNELL-RODWELL ET AL.

It appears there is a ``sweet zone'' for may also have used seismic cues at seismic signal transmission ranging from 10 one time as a means of long distance com- Hz to 40 Hz, where there is a maximum munication and prey detection. ef®ciency of transmission of seismic energy (O'Connell-Rodwell et al., 2000). Ambient Seismic vibrations produced by percussion seismic noise on land from ocean waves Foot-drumming of banner-tailed kanga- creates peaks at about 0.14 Hz and about roo rats (Randall, 1989) and the chela 0.07 Hz (White, 1965). With increasing fre- drumming of the male ®ddler crab (Aicher quency, these low frequency and storm mi- and Tautz, 1990) are percussion-induced croseisms sharply decline to negligible lev- seismic signals. Markl (1983) suggests that els by 10 Hz. Although noise due to micro- drumming-induced communication is a seisms decreases to trivial levels above 10 close-ranged communication system. More Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021 Hz, the attenuation of seismic pulses in- recent studies of the Cape mole rat suggest creases with frequency (Frantii et al., that seismic signals produced by drumming 1962). Pre-historically, the range around 20 propagate at least an order of magnitude be- Hz was a quiet seismic region, carrying yond acoustical signals (Narins et al., 1992, only vibrations associated with thunder and 1997), providing evidence that seismic sig- earth tremors making it available to ele- nals might also be used in long distance phants and other large mammals. communication. Both acoustic and seismic waves are sub- Large terrestrial mammals inevitably ject to interference and alteration due to en- cause far greater impact on the seismic en- vironmental factors. Wind shear and tem- vironment than the invertebrates and small perature gradients in¯uence the acoustic mammals. Trunk banging displays of the propagation of sound, whereas the soil type produce a booming sound and heterogeneity are among the factors in- heard for a great distance (Tennent, 1867; ¯uencing the propagation of a seismic sig- Sanderson, 1878). The sound seems to be nal (O'Connell-Rodwell et al., 2000). Air- produced by the sudden percussion of the borne sound waves spread spherically rath- column of air in the trunk as it is expelled er than cylindrically, attenuating more rap- (Krishnan, 1972). A female may drum her idly than ground surface waves such as trunk following the birth of a calf (Vincent, Rayleigh waves, losing 6 dB for every dou- 1946), when a bull joins the herd bling of distance as opposed to 3 dB. (LAH, unpublished observations), as a There is also an outer limit to airborne threat to an intruder (O'Connell-Rodwell, transmission (Uman, 1984) which is not the unpublished observations), or even when case for surface seismic waves. In this pa- testing the soundness of a bridge (Baker, per, we address the production of both 1890/1988). acoustic and seismic waves by large terres- An elephant seal lying on the ground re- trial mammals, especially Asian and Afri- sponds to a seismic wave caused by drop- can elephants, that are known to produce ping an object at a distance of 20 m (Shi- high amplitude acoustic vocalizations. pley et al., 1992). Although the psycho- The primary aim of this paper is to pre- physical parameters have not yet been sent a conceptual framework for examining worked out, perhaps these animals obtain bioseismic cues produced by large verte- seismic information about the size and brates through percussion and the coupling strength of an opponent in an episode of of low frequency vocalizations, especially con¯ict. where such signals might be used for long The locomotion of large mammals pro- distance communication. We review what is duces ground-borne vibrations. An elephant known about biological organs that mock charge ends in a foot stomping be- could potentially be used to detect seismic havior that produces a substantial seismic signals and discuss the data that support the signal, modeled to be capable of traveling possibility of the elephant being capable of up to 32 km (O'Connell-Rodwell et al., detecting the seismic signals produced by 2000). The seismic energy generated by a conspeci®cs. We address the possibility that stampede of bison was apparently detect- SEISMIC WAVES AS A COMMUNICATION CHANNEL 1159 able by Native Americans and used as a phone, with a ¯at response of 20±20,000 form of prey detection. A herd of zebra or Hz. Seismic signals were recorded using a giraffe running may propagate a series of Mandrel 10 Hz MD-79 vertical polarized seismic waves with characteristics unique geophone with a transduction coef®cient of to that species. The characteristic gaits of 0.230 V/cm/sec. Geophones were buried 10 four-legged animals relate to the size and cm into the ground. structure of the body and differ with species The roars were recorded at approx- (Hildebrand, 1995). The natural period for imately 300 m. Rhino vocalizations were a walking elephant is 1.6 to 2.2 sec, for the recorded at approximately 100 m and the horse, 1.2 to 1.8 sec, and for the deer, 0.8 African elephant rumble was recorded at 20 to 1.0 sec (Hildebrand, 1985), each setting m. The Asian elephant rumble was recorded in motion a characteristic seismic wave. In from a female at 5 m. The thunder was re- Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021 theory, a seismic eavesdropper, such as a corded at varying distances approximately lion on the African plain, could assess 1 km away. whether vibrations are from antelope or ze- The seismic components of vocalizations bra, making more selective hunting forays were ®ltered and ampli®ed separately from for preferred food. the acoustic signals to compensate for not having a preampli®er for the geophones. Seismic vibrations produced by coupling Since the software was not designed to deal In some cases, vocalizations couple with with the transduction coef®cient in order to the earth and propagate separately within calculate true ground motion based on the the earth, at a different velocity than the voltage ouputs, the dB scale for the SPL on same vibration travels in the air. In the the spectrograms is arbitrary. In addition, Asian elephant, 20 Hz rumbles propagate since each channel had to be ®ltered sepa- separately in the ground and are modeled rately, the time axes for the acoustic and to travel seismically up to 16 km seismic spectrograms are slightly shifted. (O'Connell-Rodwell et al., 2000). These data were not used to measure time To explore this aspect of seismic activity lags between the acoustic and seismic chan- associated with vocalizations of large mam- nels, but merely to demonstrate the similar- mals and other seismic events, we collected ity in frequency modulation and duration of acoustic and seismic measurements from signals in both the air and ground. vocalizations produced by lions (Panthera For recording the human jumping, three leo), black rhinoceroses (Diceros bicornis) channels of a data acquisition system ac- and both African and Asian elephants (Lox- quired and processed signals from 3 geo- odonta africana and Elephus maximus). We phones as a 175-pound man jumped at also measured the acoustic and seismic pa- known distances from the instruments. In rameters of thunder and the seismic waves addition to two of the previously mentioned created from a man jumping on the ground. geophones, a Mark Products Model L-4 seismometer was used (coil resistance, METHODS 5,500 ohms; frequency, 1.0 Hz; 35.9 K We recorded acoustic and seismic waves dynes/amp; 981.5 g mass). produced by lions, rhinoceroses, elephants, Spectra Plus sound analysis software was thunder, and a man jumping. Simultaneous used to generate spectrograms and time se- recordings of airborne and seismic signals ries plots. Sound Forge software was used were made on a TASCAM 2-channel DAT to ®lter sounds with a low pass and para- recorder during periods of sound produc- metric ®lter to increase the gain of the 20 tion by lions, rhinoceroses, thunder, and Af- Hz signal and reduce other frequencies con- rican elephants in Etosha National Park in taining background noise. Namibia, and by Asian elephants in Nagar- ahole National Park in Karnataka, India. RESULTS Airborne signals were recorded using a Lion roaring was recorded seismically at Neumann KM 131 omni-directional, free- a distance of approximately 300 m (Fig. 1). ®eld equalized pressure transducer micro- The seismic record appears very similar to 1160 C. E. O'CONNELL-RODWELL ET AL. Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021

FIG. 1. Spectrogram of a lion roar at 300 m. The fundamental frequency is in the range of 65 Hz. The top trace represents the microphone recording and the bottom trace the geophone recording. the acoustic record. The lowest component second two of which are of slightly longer of the roar is as low as 30 Hz, with peak duration and higher energy (``bark'' starting amplitude centering at 130 Hz, the begin- at event 5 in Fig. 1). The 65 Hz fundamen- ning of the second harmonic, ranging be- tal and second harmonic are visible on the tween 130 and 160 Hz. The fundamental seismic spectrogram, particularly in the frequency is more pronounced in the ``bark'' portion of the roar, but at much ``bark'' portion of the roar centering at 65 lower energy than the second harmonic that Hz. We de®ne the ``bark'' portion of the starts at 130 Hz and peaks at about 160 Hz. roar to occur after the few initial roars, the The black rhinoceros has a variety of call SEISMIC WAVES AS A COMMUNICATION CHANNEL 1161 Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021

FIG. 2. Acoustic and seismic spectrogram of a rhino vocalization. types, ranging from high-pitched whines to for the geophones, and the distance from low frequency moans and bellows. In Fig- the source as well as the amplitude of the ure 2, the vocalization event includes a low vocalizations. It is clear that the vocaliza- moan and bellow, both having fundamental tion is present in the ground, but the lower frequencies centering around 35 Hz. Both frequencies are not as evident as the higher of these vocalizations are detectable by ones. geophone at 100 m (Fig. 2). The seismic The African elephant low frequency rum- portion of the calls is not as distinctive as ble vocalization has a fundamental frequen- other animals recorded but this is probably cy centering around 20 Hz, the second har- due to the lack of ampli®cation available monic typically having a higher sound pres- 1162 C. E. O'CONNELL-RODWELL ET AL. Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021

FIG. 3. Acoustic and seismic spectrogram of an African elephant vocalization. The fundamental frequency is approximately 20 Hz, with more harmonics and frequency modulation than the vocalization of the Asian ele- phant. sure level than the 20 Hz fundamental. As fundamental frequency appears to be stron- a result, the 40 Hz signal is also stronger in ger in the Asian elephant than the African the ground, but the 20 Hz signal is present elephant, relative to the second harmonic. (Fig. 3). This monotonic fundamental frequency of The structure of the fundamental fre- the Asian elephant would propagate well in quency is most evident in both the acoustic the ground. It is virtually ideal since a and seismic record for the low frequency modulated sound in a dispersive medium rumble of the Asian elephant (Fig. 4). The like the earth is jumbled with distance, SEISMIC WAVES AS A COMMUNICATION CHANNEL 1163 Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021

FIG. 4. Acoustic and seismic spectrogram of an Asian elephant vocalization. The fundamental frequency is at 20 Hz, with few monotonic harmonics, the second and third harmonics carrying the most power. making it undecipherable and energy-inef- mic thunder is below 20 Hz. It is a fairly ®cient. monotonic signal ranging from one to two Thunder is clearly present in both the seconds in duration. acoustic and seismic channels at varying Seismic measurements were made while distances within a range of one km. The a 170-pound man jumped at varying mea- signals look very similar, except that there sured distances from the source. At 350 m is a substantial portion of the signal in the from the source, the SNR ϭ 20 (Fig. 6). air that is not present in the ground (Fig. The ®rst two traces represent the near geo- 5). The fundamental frequency of the seis- phone (NG) and the third trace represents 1164 C. E. O'CONNELL-RODWELL ET AL. Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021

FIG. 5. Acoustic and seismic spectrogram of thunder. The fundamental frequency of the seismic energy is below 20 Hz. the remote geophone (RG). The furthest de- higher velocity body waves, most probably tectable signal was at 1105.2 m, over one p waves. Each high amplitude seismic dis- km away, at a SNR ϭ 1 (data not shown). turbance represents one jump. The lag between the ®rst two geophones and the third geophone is equivalent to DISCUSSION velocity (approximately Production of seismic waves 240±260 m/sec). Arrows depict the high amplitude ground surface waves with ve- The acoustic and seismic recordings re- locities equivalent to Rayleigh waves, and veal that each of the four species contain SEISMIC WAVES AS A COMMUNICATION CHANNEL 1165 Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021

FIG. 6. Time series data from three geophones of a 170 lb. man jumping with a SNR of 20 at 350 m. Each large seismic event labeled as Rayleigh wave represents a jump. some seismic component. The lion roar vo- cues, which would provide a greater range calization remains remarkably intact in the for coordinated movements. ground. The acoustic components of lion Elephants transmit (Payne et al., 1986; roars are known to differ between males Poole et al., 1988), detect (Heffner and and females, and between individual males, Heffner, 1982), and respond to (Langbauer the number of roars increasing with wind et al., 1991) low frequency vocalizations in strength (Stander and Stander, 1987). Fe- the air in the range of 20 Hz. Elephants male lions were found less likely to ap- produce rumbles with high amplitude (120 proach playbacks of three intruding females dB; Langbauer et al., 1991) at 20 Hz, than of a single intruding female (McComb whereby both acoustic and seismic waves et al., 1994) and also avoided roars of un- are generated, for Asian and African ele- familiar males (McComb et al., 1993). If phants (Figs. 4 and 5). Perhaps the more lions were able to detect seismic roars, their monotonic structure of the Asian system would be enhanced. vocalization with a strong fundamental fre- Rhinoceroses produce infrasonic vocali- quency and fewer harmonics indicates a se- zations (E. Von Muggenthaler, described in lection toward long distance communica- Baskin, 1991), though their vocal repertoire tion in a forested environment where the is still largely undocumented. The black higher frequencies and any modulation rhinoceros is thought to be a solitary ani- would be attenuated by the vegetation. In mal, yet it is a common phenomenon that the open savanna, the higher frequencies they arrive at waterholes from separate lo- would not be attenuated as quickly and thus cations at similar times of night more modulation and more harmonics (O'Connell-Rodwell, personal observa- could provide more information for the Af- tion). Although it is possible that rhinos rican elephant. Of the four species that we drink at the same time of night and do not collected acoustic and seismic data from, have many choices for seeking water in the Asian elephant seismic vocalization re- some places, it is also possible that these cording appears most suited to long dis- synchronous water hole visits may be tance transmission. Since the Asian ele- prompted by acoustic or possibly seismic phant has the largest volume of cerebral 1166 C. E. O'CONNELL-RODWELL ET AL. cortex available for cognitive processing of body used in acoustic communication of all extant terrestrial animal species (Hart et dolphins, for ``jaw hearing,'' may be used al., 2001), perhaps they are best equipped for the detection of vibrational signals by to integrate multimodal signals. bone conduction in mole rats (Rado et al., Localizing vocalizations centered around 1987, 1998). The role of ``acoustic fat'' is 20 Hz, a frequency with a wavelength of best known for dolphins, where it is found about 17 m, when the inter-ear distance is only in the mandibular channel and the mel- only about 0.5 m is most likely a challenge. on (Varanasi and Malins, 1971; Varanasi et Using the feet and trunk would add some al., 1975). For reception, the fat of the man- advantage in detecting a phase difference, dible causes a two-fold increase in intensity since the distance from the front to back of sound, serving as an impedance match- feet is about 2 to 2.5 m. A further bene®t ing mechanism. The oil-rich lipid in the Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021 would accrue from working with the seis- melon serves as an acoustic lens that ef®- mic waves that we recorded which had a ciently couples acoustic energy to the water shorter wavelength, 12.4 m, than acoustic (Au, 1993). The cartilaginous, fat-®lled la- sound waves of 17 m. As seismic cues are cunae of the Manatee zygomatics is thought an important localization tool for insects to play a role in coupling sound to the man- (Cocroft et al., 2000), perhaps this is also atee's ear (Ketten et al., 1992). Norris true for the elephant. (1968) also suggested that the structure of During thunder an elastic exchange cou- the manatee skull, incorporating unique fat ples acoustic and seismic vibrations, com- deposits, may function to conduct sounds. plicating the accurate assessment of the im- Anatomical features such as the stiff car- pact of acoustic versus seismic cues. tilage and dense fat found in the head and Ground motion was recorded directly cou- foot of the elephant resemble ``acoustic fat'' pled to that of air, with frequencies of in- (C.O'C.-R., L.A.H., B.T.A., T. Hildebrandt, dividual strikes varying from 6 to 13 Hz, unpublished observations) and could facil- suggesting an acoustic-seismic mechanism itate coupling as impedance matching (Kappus and Vernon, 1991). Our thunder would also be a problem at the air-ground recordings reveal acoustic coupling, where interface. The fatty tissue in the foot does seismic energy is recorded below 20 Hz not change volume seasonally, even though (Fig. 5). As there is an outer limit to the elephants deplete fat reserves around the acoustic propagation of thunder, elephants kidneys, the stomach, and other internal or- would bene®t if they could cue into seismic gans during winter (Haynes, 1991). Oils thunder events as an early signal to migrate from this foot fat are valued by indigenous towards rain. people (Crader, 1983). It is not yet known A man jumping on the ground generates whether this fatty tissue has a similar com- a seismic disturbance measurable at one km position to the dolphin's acoustic lens but it away. These signals travel much further seems likely that this digital cushion with than previously expected. The furthest, pre- cartilaginous nodes may improve sensitivity viously recorded seismic disturbances from of the elephant to substrate-borne vibra- a human were made at 50 m (Department tions. of Defence, 1965). The measurement of a The circumference of the elephant foot man jumping was made to con®rm that just above the toenails increases up to 10% even relatively small seismic disturbances during weight bearing from the non-weight generated by a large mammal are measur- bearing state (n ϭ 4; M. E. Fowler, unpub- able over great distances. lished observations). The fatty digital cush- ion is smaller in the rear foot than the fore- Anatomical adaptations for seismic foot in both African and Asian elephants detection (Haynes, 1991). Elephants at times lean for- Anatomical adaptations for receiving or ward on their front feet, which are directly sending a seismic signal provide evidence in line with the ear due to the unique gra- of the importance of this medium as a chan- viportal structure of their forelimbs. They nel for communication. The mandibular fat appear to exhibit this behavior during times SEISMIC WAVES AS A COMMUNICATION CHANNEL 1167 when seismic stimuli would be highest, pri- the cranium has been proposed as possibly or to the arrival of a new herd to a water being related to low frequency resonance hole (O'Connell-Rodwell, 2000). Early re- facilitating detection of low frequencies ports have described Asian elephants vig- (Gerstein et al., 1999). All of these similar- orously responding to earthquakes (Jack- ities between elephants and manatees, in- son, 1918), or even trumpeting at the ap- cluding the placement of possible acoustic proach of an earthquake (Nicholls, 1955), fats, ear anatomy, and bone structure, may adding further evidence to the elephant's be remnants of an aquatic ancestry for the ability to detect seismic stimuli. elephant, and may also facilitate seismic The elephant's shows the sharp- sensitivity in a terrestrial environment. est resonance among seven species studied

(von Bekesy, 1944/1960). The elephant is and hearing Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021 more capable of hearing low frequency Higher vertebrates have several types of sounds than any mammal previously tested cutaneous sensory organs that are thought (Heffner and Heffner, 1980). The best to act as mechanoreceptors (Saxod, 1996). acoustic sensitivity of an Asian elephant Pacinian corpuscles, or pressure receptors, was reported to be at 1,000 Hz (Heffner and are the largest peripheral mechanoreceptors Heffner, 1982), much higher than the fre- in mammals (Stark et al., 2001). Pacinian quencies being discussed here. These mea- corpuscles are deeply placed whereas the surements may underestimate the acuity at Meissner corpuscles or touch receptors, are the lower ranges, as the longer waveforms super®cial. In humans, the peak sensitivity of lower frequency sounds (16 Hz) pre- of the Pacinian corpuscles is around 250 Hz sented require a longer window of time for with a frequency range of 65±400 Hz, detection. Meissners corpuscles being equally as sen- The elephant may use bone conduction sitive between 10±65 Hz (Makous et al., in hearing low frequency sound, as heavy 1995). ossicles and the elastic coupling between Some animals have more of their sensory the ossicles and the skull would make the world dedicated to seismic stimuli than any elephant more sensitive to low frequencies other, such as the star-nosed mole whose than revealed in audiograms (Reuter et al., snout is surrounded by 22 ¯eshy and mo- 1998). Recent molecular data place ele- bile appendages covered with thousands of phants, sirenians and the golden mole in the mechanoreceptive Eimer's organs, which same clade (Stanhope et al., 1998), the acts like a tactile eye (Catania, 1999). Lam- golden mole having relatively enormous ellated corpuscles, similar to Pacinian cor- mallei, their ossicles adapted towards the puscles, have been found in the legs of kan- detection of ground vibrations (Mason, garoos and are thought to detect ground- 1998). The elephant hypertrophied mallei borne vibrations (Gregory et al., 1986). Vi- may also be an adaptation for seismic sen- brationally sensitive Herbst corpuscles have sitivity (Reuter et al., 1998). been found in the legs of the pigeon and Paleontologic analyses (Barnes et al., may act as a warning device (Shen and Xu, 1985; Ketten, 2000) and the morphology of 1994). Cats have Pacinian corpuscles in the fetal African elephant ear (Fischer, their paws and knees (Madey et al., 1997). 1990) indicate that have common The tip of the Asian elephant trunk contains ancestral traits with elephants. Immunolog- both Pacinian and Meissner corpuscles ical evidence (Gaeth et al., 1999) further (Rasmussen and Munger, 1996). The trunk suggests that elephants and sirenians have tip, when placed on the ground, can appar- a common aquatic ancestor. All head bones ently detect vibrations caused by the feet of of the African elephant's skull are aerated running humans or animals' hooves (Gale, by sinuses (van der Merwe et al., 1995). 1974). If such receptors are present in the Except for the solid mandible, the cranium elephant foot, elephants may be able to de- consists of in¯ated bones compartmental- tect their seismic vocalizations and loco- ized to form diploe (Shoshani, 1996). In motion via their feet (O'Connell et al., manatees, this condition of aerated bones in 1999). 1168 C. E. O'CONNELL-RODWELL ET AL.

Cutaneous receptors in human skin are speci®c communication and possibly as a capable of localizing sound sources ap- cue to migrate. proaching that of the auditory sense (Borg, Animals that produce high amplitude vo- 1997). The human foot is capable of de- calizations at low frequencies would be tecting indentations of 300 microns from a those most likely to produce seismic vibra- tactile probe at velocities of 100±400 mi- tions capable of propagating for long dis- crons/msec (Simonetti et al., 1998). Hu- tances. In this paper, large mammals have mans demonstrate the ability to compensate been found to generate acoustic and seismic for the lack of one sensory modality early vibrations which have characteristics in in development (Levanen et al., 1998). Vi- common. brotactile stimuli activate the auditory cor- Lions and rhinoceroses are likely candi- tices in the congenitally deaf, suggesting dates for seismic communication due to Downloaded from https://academic.oup.com/icb/article/41/5/1157/343557 by guest on 30 September 2021 that the cortical areas that normally sub- their size and vocal structure. Elephants are serve hearing may process vibrotactile in- the ideal candidates for seismic communi- formation in those whose auditory abilities cation as they are very large, have low fre- are lacking (Levanen et al., 1998; Levanen quency high amplitude vocalizations that and Hamdorf, 2001). This demonstration of propagate in the ground and maintain a neural plasticity and the capacity to focus consistent signal over long distances and on seismic cues supports our suggestion they are equipped to detect seismic signals, that humans may once have used seismic at least in the trunk. Whether or not other stimuli as a mode of communication when large mammal vocalizations propagate in the ground was a quieter environment. the ground or whether the signal detection Low frequency drums or the digeredoo, abilities of these animals would make it a low frequency long wooden tube played possible to detect these signals in the on the ground by the Australian Aborigines ground has yet to be determined. could have been and may still be a form of Humans may have used seismic signals seismic stimulation, as well as traditional at one time as a traditional means of com- dance involving stomping. This type of munication, but the seismic channel may no communication may not be necessary today longer be necessary or even possible for hu- with telephones, computers and modern mans to tap into. The increase in seismic transportation, nor may it be possible in noise is pervasive throughout the world, yet most parts of the world do to anthropogenic the effects of this ``bioseismic pollution'' seismic noise. on seismic communication are unknown for Vehicular noise, amusement parks, gen- any species. erators and water pumps are all sources of Seismic communication is a relatively unexplored modality of communication in bioseismic noise. Doppler-shifted, low fre- large mammals. We may ®nd new answers quency noise caused by jet aircrafts also to old questions and generate new questions generate substantial seismic noise (J. Fett, by exploring this modality further. personal communication). Thus, it is almost impossible to assess the use of seismic sig- ACKNOWLEDGMENTS nals by animals in an urban setting without We are indebted to Peter Narins for his dealing with considerable background noise support in ongoing discussions. This re- (Lewis and Narins, 2001). search was supported by a grant from the University of California, Davis. CONCLUSIONS Many more species may use the seismic REFERENCES environment as a modality for communi- Aicher, B. and J. Tautz. 1990. Vibrational communi- cation than has been documented in the lit- cation in the ®ddler crab, Uca pugilator: Signal erature, particularly large mammals. Seis- transmission through the substratum. J. Comp. Physiol. 166:345±354. mic stimuli are used to navigate within the Au, W. W. L. 1993. The sonar of dolphins. Springer- environment, enhance localization, to detect Verlag, New York. predators or prey, in mutualism, for intra- Baker, S. W. 1890/1988. Wild beasts and their ways: SEISMIC WAVES AS A COMMUNICATION CHANNEL 1169

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