Elephants Show Risk-Avoidance Behaviour In

Noise matters: elephants show royalsocietypublishing.org/journal/rspb risk-avoidance behaviour in response to human-generated seismic cues Beth Mortimer1, James A. Walker1, David S. Lolchuragi2, Michael Reinwald1 Research and David Daballen2 Cite this article: Mortimer B, Walker JA, 1Department of Zoology, University of Oxford, Oxford OX1 3PS, UK Lolchuragi DS, Reinwald M, Daballen D. 2021 2Save the Elephants, Marula Manor, Karen, Nairobi 00200, Kenya Noise matters: elephants show risk-avoidance BM, 0000-0002-7230-3647; MR, 0000-0001-8481-8392 behaviour in response to human-generated seismic cues. Proc. R. Soc. B 288: 20210774. African elephants (Loxodonta africana) use many sensory modes to gather https://doi.org/10.1098/rspb.2021.0774 information about their environment, including the detection of seismic, or ground-based, vibrations. Seismic information is known to include elephant-generated signals, but also potentially encompasses biotic cues that are commonly referred to as ‘noise’. To investigate seismic information Received: 1 April 2021 transfer in elephants beyond communication, here we tested the hypothesis that wild elephants detect and discriminate between seismic vibrations that Accepted: 8 June 2021 differ in their noise types, whether elephant- or human-generated. We played three types of seismic vibrations to elephants: seismic recordings of elephants (elephant-generated), white noise (human-generated) and a combined track (elephant- and human-generated). We found evidence of both Subject Category: detection of seismic noise and discrimination between the two treatments Behaviour containing human-generated noise. In particular, we found evidence of retreat behaviour, where seismic tracks with human-generated noise caused elephants to move further away from the trial location. We conclude Subject Areas: that seismic noise are cues that contain biologically relevant information for behaviour, ecology elephants that they can associate with risk. This expands our understanding of how elephants use seismic information, with implications for elephant Keywords: sensory ecology and conservation management. Loxodonta africana, noise, risk-avoidance, vibrational communication, seismic vibration 1. Introduction Author for correspondence: African elephants use a variety of sensory modes, whether for communication through signals [1,2] or for information gathering by detecting cues generated Beth Mortimer by other animals (e.g. predator odour [3]), humans (e.g. voices [4], roads/ e-mail: [email protected] railways [5]) or natural earth processes (e.g. rainfall [6]). Information transfer through ground-based, or seismic, vibrations is the least well understood [7]. Elephants likely detect seismic information using the Pacinian corpuscles on the feet and/or the inner ear, picking up ground vibrations via bone conduction Downloaded from https://royalsocietypublishing.org/ on 30 June 2021 [8]. Elephants are thought to use seismic signals to communicate with each other, as seismic vibrations are generated by elephants during certain infrasonic vocalizations, known as rumbles [9–11]. The rumbles contain both acoustic and seismic components in the frequency range 20–40 Hz and under [12–14], which are modelled to propagate to a maximum of 6 km under differing favourable conditions [15–17]. Sensory modes can be investigated using playback experiments, including seismic information transfer in elephants. Behavioural changes during these experiments indicate what signals and cues animals can detect and what they can discriminate between, which involves classifying the potential information according to the source identity and status [18]. A series of acoustic (no seismic component) playback experiments with elephants has shown that elephants can Electronic supplementary material is available detect, for example, elephant rumbles, the sound of bees, human voices and big online at https://doi.org/10.6084/m9.figshare. c.5470431. © 2021 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited. (a) (c) 2 100 royalsocietypublishing.org/journal/rspb each experimental trial: 80 60 2 min 2 min 2 min 40 silence treatment silence (a1) (b) (a2) (Hz) frequency 20 3 types 0 0 30 60 90 120 (b) (d) 100 100 80 80 Proc. R. Soc. B 60 60 40 40 frequency (Hz) frequency 20 20 288 0 0 : 20210774 0 30 60 90 120 030 60 90 120 time (s) time (s) Figure 1. Design of playback tracks. (a) Two-minute treatments (light grey) were between 2 min of silence (dark grey) for each playback track. Treatments were three types: (b) elephant-generated seismic vibrations (Ele); (c) white/human-generated noise (WN); (d) combined track of WN superimposed on elephant track (Ele&WN). Colour gives spectral amplitude on a log scale from dark blue (1E−5 V) to yellow (1 V). Note that propagation alters the tracks, as seen when a geophone records the seismic vibrations 10 m from the source (electronic supplementary material, figures S1 and S2). (Online version in colour.) cat growls [4,12,19–21]. Elephants can also discriminate (when the signal magnitude is constant) [15]. Therefore, between subtle differences in acoustic sources, changing their detecting and responding to seismic noise can allow elephants behavioural response, for example human voices versus bees to mitigate these negative effects (e.g. by using repeated or [11], or even gender, age and ethnicity of human voices [4]. louder calls [18]) or choose environments where they are Two studies to date have used seismic playback experiments more likely to be able to communicate effectively through seis- to explicitly test elephant responses to ground-based mic vibrations (the seismic domain). However, seismic noise is vibrations [22,23]. These showed that seismic playback of inevitable as it is superimposed during wave propagation [15]. (the acoustic component of) an elephant alarm rumble is suffi- SNR naturally decreases as propagation distance increases, cient to elicit a behavioural response [23] and the elephants so elephants must have methods to cope with increasing responded significantly to seismic playback of alarm rumbles noise levels since they can respond to acoustic rumbles over from familiar elephants, but not from unfamiliar elephants or a variety of distances, up to the kilometre range [12,27]. artificial warble tones [22]. Despite the potential sources of seismic information, it Yet potential seismic information for elephants is wider remains untested whether and how elephants respond to than elephant-generated signals, encompassing what may seismic vibrations beyond elephant-generated signals (their traditionally be considered as noise, but can be used to infrasonic rumbles) [22,23]. Here, we focus on whether seis- inform biological decision making, making them cues [24]. mic ‘noise’ acts as a cue for elephants, testing how wild What is generally referred to as seismic noise is generated elephants respond to seismic vibrations of different noise by a mixture of biotic, abiotic and human sources, so can types (elephant- versus human-generated noise). Since there Downloaded from https://royalsocietypublishing.org/ on 30 June 2021 be natural or artificial (human-generated). These seismic is increasing scope for anthropogenic seismic noise within vibrations could potentially provide direct reliable infor- the elephants’ natural and captive landscapes [28,29], deter- mation about the seismic source for decision making in mining whether human-generated seismic noise can act as elephants, so are potential seismic cues. Relevant seismic cues for elephants has important implications for their con- sources include elephants or other animals that will generate servation management. This includes understanding the seismic vibrations as they move around [13,15,25], the pro- potential impacts on elephants of land-use change that is cesses of the natural environment (e.g. thunder [14]), and expected to increase noise, for example due to infrastructure humans that purposefully or incidentally generate seismic development (roads, railways, wind farms, etc.) [30], which vibrations (e.g. wind turbines [26]). Elephants could use includes seismic noise [26,31]. these seismic vibrations as cues to determine the presence of potential threats, for example from the movement patterns of other animals or from human activity [4,11]. 2. Methods Aside from potentially acting as cues, the detection of seismic noise is also useful for decision making as higher noise (a) Seismic playback tracks and calibration decreases communication efficacy [18]. A lower signal-to- Three 6 min seismic playback tracks of different noise types noise ratio (SNR; i.e. higher noise level) reduces the ability to were generated as stimuli to play to wild elephants in the field detect and discriminate between seismic signals and cues (figures 1 and 2a; see also electronic supplementary material, (a) seismic playback set-up seismic playback treatments: 3 royalsocietypublishing.org/journal/rspb 2. Ele WN Ele&WN 1. (b) (c) 20 distance moved 20 distance moved a1b a2 a1b a2 15 15 10 10 = 0 s (m) = 0 s (m) t t Proc. R. Soc. B 5 5 since since mean distamce moved 0 0 0 60 120 180 240 300 360 mean of the distance moved a1b a2 time (s) experimental period 288 Figure 2. Elephant behavioural responses suggest detection and discrimination of seismic treatments that differ in noise

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