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Long-Distance Communication of Acoustic Cues to Social Identity in African Elephants

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Long-Distance Communication of Acoustic Cues to Social Identity in African Elephants ANIMAL BEHAVIOUR, 2003, 65, 317–329 doi:10.1006/anbe.2003.2047, available online at http://www.sciencedirect.com Long-distance communication of acoustic cues to social identity in African elephants KAREN MCCOMB*, DAVID REBY*, LUCY BAKER*, CYNTHIA MOSS† & SOILA SAYIALEL† *Experimental Psychology, School of Biological Sciences, University of Sussex †Amboseli Elephant Research Project, Kenya (Received 22 October 2001; initial acceptance 11 December 2001; final acceptance 15 July 2002; MS. number: 7106) Research on long-distance vocal communication in mammals has tended to focus on the maximum distances over which a vocal signal might be physically detectable. For example, because elephants and some whales communicate using infrasonic calls, and low frequencies are particularly resilient to attenuation, it has often been assumed that these species can communicate over very long distances. However, a wide range of acoustic characteristics typically carry information on individual identity in mammalian calls, and frequency components crucial for social recognition could be distorted or lost as distance from the source increases. We used long-distance playback experiments to show that female African elephants, Loxodonta africana, can recognize a contact call as belonging to a family or bond group member over distances of 2.5 km, but that recognition is more usually achieved over distances of 1–1.5 km. We analysed female contact calls to distinguish source- and filter-related vocal characteristics that have the potential to code individual identity, and rerecorded contact calls 0.5–3.0 km from the loudspeaker to determine how different frequencies persist with distance. Our analyses suggest that the most important frequency components for long-distance communication of social identity may be well above the infrasonic range. When frequency components around 115 Hz become immersed in back- ground noise, once propagation distances exceed 1 km, abilities for long-distance social recognition become limited. Our results indicate that the possession of an unusually long vocal filter, which appears to incorporate the trunk, may be a more important attribute for long-distance signalling in female African elephants than the ability to produce infrasound. 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved. It has commonly been assumed that the maximum could still be extracted from calls by conspecifics at these distances over which a species can use an acoustic signal distances. In mammalian calls, a wide range of acoustic to communicate are equivalent to the distances over characteristics typically carry information on individual which components of that signal are physically detect- identity, and frequency components that may be crucial able. For example, because African and Asian elephants in social recognition could be distorted or lost as distance (Loxodonta africana and Elephus maximus, respectively) from the source increases. and some whales (e.g. Balaenoptera physalus) have calls Langbauer et al. (1991) obtained responses to playback with infrasonic fundamental frequencies (less than indicating that female African elephants could detect a 30 Hz), it has been suggested that these species can variety of infrasonic calls at 1.2 km from the source; communicate over very long distances (e.g. Payne & males were able to detect the calls at 2 km from the Webb 1971; Payne et al. 1986; Garstang et al. 1995). source. Because playback volumes were lower than the However, although it is theoretically possible that funda- maximum sound pressure levels at which some of these mental frequencies in the infrasonic range may still be calls had been recorded in the wild, the authors extrapo- detectable at large distances from the caller because of the lated from their data to conclude that elephants could unusual resilience of such low frequencies to attenuation, communicate over distances of at least 4 km. Others it is unsafe to conclude that socially relevant information (Garstang et al. 1995; Larom et al. 1997a, b) have since used computer modelling based on this estimate to Correspondence: K. McComb, Experimental Psychology, School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, predict that, under optimum atmospheric conditions, U.K. (email: [email protected]). C. Moss and S. Sayialel are at elephants could communicate over distances in excess of the Amboseli Elephant Research Project, P.O. Box 15135, Nairobi, 10 km. None of the above studies considered whether Kenya. socially relevant information can be extracted from 317 0003–3472/02/$30.00/0 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved. 318 ANIMAL BEHAVIOUR, 65, 2 calls by other elephants at the proposed transmission at distances from a loudspeaker of 3.0–0.5 km and distances. quantified levels of degradation. Female African elephants are more vocal than males and communicate using a variety of low-frequency calls (Poole 1995). Although these calls often have infrasonic METHODS fundamental frequencies and are referred to as ‘infra- Study Population sonic’, harmonics of the fundamental usually extend well into the audible range. In the case of the contact call, Fieldwork was conducted in Amboseli National Park, which is the low-frequency call most regularly used by Kenya, where data on life histories and association females for long-distance communication with social patterns have been obtained for more than 1700 individ- companions, sound energy is often present up to fre- ual elephants over 28 years by the Amboseli Elephant quencies of at least 1 kHz (K. McComb & D. Reby, Research Project (Moss & Poole 1983; Moss 1996). The personal observation). Previous studies have not con- park encompasses 390 km2 and covers a varied ecosystem sidered the potential importance of noninfrasonic including open grassland sparsely scattered with Acacia frequencies in coding individual identity and social trees, dense patches of palms, permanent and semi- meaning, nor have they given due consideration to permanent areas of swamp and the seasonally flooded whether elephant hearing could, over long distances, bed of lake Amboseli (see e.g. Moss & Poole 1983). All readily extract information contained in an infrasonic adult individuals in the population are recognized from a fundamental frequency contour alone. combination of natural features, particularly patterns of In earlier playback experiments, we showed that adult tears, holes and veins in the ears, and ear and tusk size female African elephants in Amboseli National Park, and shape (Moss 1996). Kenya, are familiar with the contact calls of about 100 other females in the population (McComb et al. 2000). Sound Recording and Playback Equipment The primary social unit in these elephants is the female family unit, composed of adult females that are usually Contact calls were recorded on digital audiotape using matrilineal relatives and their immature offspring. Close equipment specialized for low-frequency recording: a social ties also exist within bond groups, which are made Sennheiser MKH 110 microphone linked to either a Sony up of family units that associate often and greet one TCD D10 DAT recorder (with DC modification) or HHb another when they meet (Moss & Poole 1983). Females PortaDAT PDR 1000 DAT recorder, through an Audio can distinguish the calls of female family and bond group Engineering Ltd power supply (which incorporated a members from females outside these categories, and 5-Hz high-pass filter). With this equipment, the fre- beyond this, they discriminate between the calls of other quency response for recording was flat (1 dB) down to families based on how frequently the females encounter at least 10 Hz. The system for playback was composed of them (McComb et al. 2000, 2001). In response to play- a custom-built sixth-order bass box loudspeaker with two back of the calls of family or bond group members, sound ports (Aylestone Ltd, Cambridge, U.K.) linked to listening females typically give a distinctive reaction either a Kenwood KAC PS 400M, Kenwood KAC923 or characterized by contact calling and/or approaches to the Kicker Impulse 1252 xi power amplifier and a HHb area from which the call came (McComb et al. 2000). In PortaDAT PDR 1000 DAT or Sony TCD D10 recorder (with contrast, on hearing playbacks of calls from families DC modification), to give a lower frequency limit of outside these categories, females either listen and remain 10 Hz and a playback response that was flat 4 dB from relaxed if they have associated frequently with the caller, ca. 15 Hz on one sound port and ca. 20 Hz on the other. or bunch into defensive formation if they have encoun- tered the caller only rarely, but they do not call back or Social Recognition Distances approach the loudspeaker (McComb et al. 2000, 2001). To investigate the distances over which social recogni- Playbacks were conducted between June 1993 and tion is possible, it is necessary to identify a diagnostic January 2000. All contact calls used as playback stimuli response that shows that subjects have not only detected had been recorded from adult female elephants (at least the call but also categorized the social identity of the 11 years old), at distances of less than 30 m from the caller. To do this, we used the distinctive calling/ caller and in conditions of low air turbulence (see also approach response described above, which is given when McComb et al. 2000, 2001). In each playback, a single subjects have identified a call as belonging to a family or contact call was played at peak sound pressure levels of bond group member (McComb et al. 2000). In the present 1073 dB at 1 m (measured with a CEL-414/3 sound study, calls of family and bond group members were level meter, C weighting), to represent a call given at high played to subjects from successively closer distances until volume (McComb et al. 2000, 2001).
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