Nota Bene: Texas Elephants Stomp to Victory

Abstract

T he old saying “putting an ear to the ground” may turn out to be literally true for elephants, according to a recent report by O'Connell-Rodwell and her colleagues ([1][1]). When elephants generate their low-frequency vocalizations (rumbles), acoustic sound waves traveling through the air are accompanied by seismic waves that travel through the ground. Given that a variety of creatures, from insects to rodents and even the enormous elephant seal, use the generation and detection of terrestrial vibrations to communicate, O'Connell-Rodwell and her team wondered if elephants, too, have this seismic signaling capability. Eschewing the grasslands of Africa for a residential facility in Texas, the scientists analyzed acoustic and seismic signals generated by two captive Asian elephants. Whenever the elephants “rumbled” or stomped their feet during mock charges, seismic data were collected with geophones—sensitive microphones placed 10 m and 30 m from the elephants' pen that transformed terrestrial vibrations into electrical signals. Simultaneously, acoustic data were collected with audio equipment. The seismic and acoustic waves generated by rumbles and foot stomps had similar frequencies (20 to 24 Hz) that fell within the ideal range for the long-distance transmission of low-frequency sounds. Intriguingly, the two sets of waves had different velocities (248 to 264 m/s in the ground, and 309 m/s in the air), so that they were no longer in phase as they traveled further away from their source. This hints that elephants may make their rumbles and foot stomps loud enough to produce separate acoustic and seismic waves. ![Figure][2] CREDIT: BEN AND LYNETTE HART Through mathematical modeling, the investigators estimated that the seismic waves created by their stomping elephants traveled at least 36 km. Conceivably, these long-distance seismic signals may enable elephants to communicate with other herds. The location of a distant herd could be pinpointed by assessing the time delay between the arrival of seismic and acoustic signals. Elephants are known to move toward thunderstorms that are more than 25 km away, too far for them to hear the sound of thunder. It is possible that they can detect the terrestrial vibrations associated with distant storms, presumably a major advantage in their search for new water sources. Generating seismic signals is easy, but what about detecting them? The elephant's trunk has mechanoreceptors that respond to mechanical pressure. There may also be similar receptors in the elephant's well-innervated foot pads. These mechanoreceptors may explain foot-lifting behavior during which elephants lean forward and lift up one foot, possibly to improve their sensitivity to ground vibrations. Establishing that elephants use seismic signaling for long-distance communication is no easy task. Undaunted, O'Connell-Rodwell and her team plan to train their elephants to respond to seismic waves by pulling a lever with their trunk or pressing a button with their feet—with, of course, the provision of a reward for the correct response. 1. [↵][3]1. C. E. O'Connell-Rodwell, 2. B. T. Arnason, 3. L. A. Hart , J. Acoust. Soc. Am. 108, 3066 (2000) [journal]. [OpenUrl][4][CrossRef][5][PubMed][6][Web of Science][7] [1]: #ref-1 [2]: pending:yes [3]: #xref-ref-1-1 View reference 1 in text [4]: {openurl}?query=rft.jtitle%253DJournal%2Bof%2Bthe%2BAcoustical%2BSociety%2Bof%2BAmerica%26rft.stitle%253DJournal%2Bof%2Bthe%2BAcoustical%2BSociety%2Bof%2BAmerica%26rft.aulast%253DO%2527Connell-Rodwell%26rft.auinit1%253DC.%2BE.%26rft.volume%253D108%26rft.issue%253D6%26rft.spage%253D3066%26rft.epage%253D3072%26rft.atitle%253DSeismic%2Bproperties%2Bof%2BAsian%2Belephant%2B%2528Elephas%2Bmaximus%2529%2Bvocalizations%2Band%2Blocomotion.%26rft_id%253Dinfo%253Adoi%252F10.1121%252F1.1323460%26rft_id%253Dinfo%253Apmid%252F11144599%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [5]: /lookup/external-ref?access_num=10.1121/1.1323460&link_type=DOI [6]: /lookup/external-ref?access_num=11144599&link_type=MED&atom=%2Fsci%2F291%2F5505%2F842.atom [7]: /lookup/external-ref?access_num=000165780300039&link_type=ISI