Assessing Raspberry Shake and Boom Sensors for Recording African Elephant Acoustic Vocalizations Oliver D
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1 Assessing Raspberry Shake and Boom sensors for recording African Elephant acoustic vocalizations Oliver D. Lamb1;∗, Michael J. Shore1, Jonathan M. Lees1, Stephen J. Lee2, and Sean Hensman3 1Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA; 2US Army Research Laboratory/Army Research Office, Research Triangle Park, NC, USA; 3Adventures with Elephants, Bela Bela, South Africa *Correspondence to: O. D. Lamb, [email protected] 2 ABSTRACT 3 In this work we assess the performance of the Raspberry Shake and Boom (RS&B) sensor 4 package for detecting and monitoring African elephants (Loxodonta africana). This is the first 5 documented test of this particular unit for recording animal behaviour; the unit was originally 6 designed for detecting tectonic earthquakes and low frequency (<50 Hz) atmospheric acoustics. 7 During a four day deployment in South Africa we tested five RS&B units for recording acoustic 8 and seismic vocalizations generated by a group of African elephants. Our results highlight a 9 varied degree of success in detecting the signals of interest. The acoustic microphone recorded 10 fundamental frequencies of low-frequency (<50 Hz) harmonic vocalizations that were not clearly 11 recorded by more sensitive instruments, but was not able to record higher frequency harmonics 12 due to the low sampling rate (100 Hz). The geophone was not able to consistently record clear 13 seismic waves generated by vocalizations but was able to record higher harmonics. In addition, 14 seismic signals were detected from footsteps of elephants at <50 m distance. We conclude that 15 the RS&B unit shows limited potential as a monitoring tool for African elephants and discuss 16 future directions and deployment strategies to improve the sensitivity of the sensor package. 17 Keywords: Seismic; Acoustic, Wildlife monitoring; Loxodonta africana; Raspberry Pi 1 INTRODUCTION 18 Acoustics are an important component of many habitats and in-situ sound recordings offer potentially 19 rich information about the abundance, distribution, and behaviour of vocalizing animals in the target area. 20 Cost-effective and scalable acoustic sensors are therefore being increasingly used in ecological research 21 and conservation efforts for monitoring animals (Blumstein et al., 2011; Browning et al., 2017). Results 22 from these studies are providing new insights into animal acoustic signal features (e.g. Stoeger and Baotic, 23 2016), communication processes within social groups (e.g. Poole et al., 1988), seasonal variability in 24 acoustic behaviors, and spatio-temporal variability of acoustic habitats in which the animals reside (e.g. 25 Thompson et al., 2010a). Acoustic techniques allow researchers to survey wild populations at ecologically 26 meaningful scales without intruding on animal activity and causing unintended stress (Blumstein et al., 27 2011). 28 African elephants (Loxodonta africana and Loxodonta cyclotis) have been observed to make broad use 29 of strong low-frequency acoustic vocalizations, or ‘rumbles’ (Poole et al., 1988; Langbauer et al., 1991; 1 Lamb et al. RS&B sensors for tracking elephants 30 Langbauer Jr., 2000; Soltis, 2009; Thompson et al., 2010a; Zeppelzauer et al., 2015; Keen et al., 2017). 31 ‘Rumbles’ are highly harmonic and modulated sounds with fundamental frequencies for adult male African 32 elephants ranging from ∼19 Hz (Stoeger et al., 2014) to as low as ∼10 Hz (Narins et al., 2016; Stoeger 33 and Baotic, 2016). Studies have suggested that ‘rumbles’ are used to coordinate movement and spacing 34 between social groups, help individuals locate each other, and prompt exploratory or defensive behaviour 35 (Poole et al., 1988; McComb et al., 2000, 2003; Narins et al., 2016). The frequency characteristics and 36 structure of ‘rumbles’ may also vary with age (e.g. formant frequencies, min/max frequencies; Stoeger et al., 37 2014), and changes in reproductive (Poole et al., 1988) and emotional states (Soltis, 2009). Elephants have 38 also demonstrated an ability to respond to seismic waves generated by the acoustic waves from ‘rumbles’ 39 coupling with the ground as well as those generated by high-force locomotion behaviors such as stomping or 40 rapid running (O’Connell-Rodwell, 2007; Mortimer et al., 2018). Wild elephant herds have been observed 41 to respond to artificially transmitted seismic recordings of elephant alarm vocalizations (O’Connell-Rodwell 42 et al., 2006). Responses include defensive ‘bunching’ behaviour, greater vigilance by individual animals, 43 orientation towards the source of the signal, and expeditiously leaving the immediate area (O’Connell- 44 Rodwell et al., 2006). This ability may provide elephants a means for long-range communication at 45 distances up to and over 3 km (Garstang, 2004; Mortimer et al., 2018) while discriminating between calls 46 from familiar and unfamiliar individuals (O’Connell-Rodwell et al., 2007). It has been demonstrated that 47 the calling rate of ‘rumbles’ is a useful index of L. africana abundance (Payne et al., 2003). Acoustic 48 sensors can also be used for detecting other acoustically active species (e.g. L. cyclotis; Thompson et al., 49 2010b; Keen et al., 2017) as well as monitoring for anthropogenic noises from illegal activities such as 50 poaching (Thompson et al., 2010a). Presently, elephants remain at risk due to poaching, habitat loss, and 51 human-elephant conflict arising from damaged crops and buildings, along with infrequent human fatalities 52 (Douglas-Hamilton, 2008; Zeppelzauer et al., 2015). 53 Acoustic and seismic recording devices offer a means for installing non-intrusive monitoring systems 54 that can autonomously detect elephants and monitor their location, including real-time alarm systems 55 for elephants approaching human settlements or sensitive food supplies (e.g. Zeppelzauer et al., 2015). 56 The choice of which instrumentation to purchase and deploy is fraught with trade-offs. One must take 57 into consideration all of the following: size and weight, power demands, data format and storage, ease 58 of deployment, sensor performance and costs (for purchase and deployment). Of these, performance of 59 the sensor in recording acoustic and/or seismic waves from the source of interest may the most difficult 60 to assess. In recent years, seismologists have begun exploring the use of low-cost (<$1000), rapidly 61 deployable stations (i.e. sensor, data recorder, and power source) in temporary seismic deployments (e.g. 62 Anthony et al., 2019). This includes recently developed multi-sensor and data recorder packages such as 63 the Raspberry Shake and Boom (RS&B) that are designed as a plug-and-play solution. The device, and 64 similar variations of the device, are becoming increasingly popular, mainly for home use, educational 65 purposes, and outreach. However, the potential for using the RS&B device for ecological purposes is still 66 unexplored. Here we present the first results of a pilot test performed in South Africa to record African 67 elephant vocalizations and locomotions using multiple RS&B devices. In the following sections we provide 68 a more detailed technical description of the device, describe our data acquisition set-up, highlight key 69 results in seismic and acoustic data, and discuss the overall performance of the RS&B. This is a preprint and has not been peer reviewed 2 Lamb et al. RS&B sensors for tracking elephants 2 MATERIALS AND METHODS 70 2.1 The Raspberry Shake and Boom sensor 71 The RS&B is an all-in-one plug-and-go sensor package designed for seismological and atmospheric 72 acoustic applications developed by Raspberry Shake (Fig. 1a). The unit integrates vertical geophone and 73 omni-directional pressure sensors together with a 24-bit digitizer, period-extension circuits, and Raspberry 74 Pi 3 Model B computer into a single enclosure with dimensions of 135x110x70 mm. The power supply 75 is 5 Volts DC (2.5 A) and consumption is estimated as 3.1 W at start-up and 1.9 W during running time. 76 Data is recorded at sampling rates of up to a maximum of 100 Hz and are saved on a local SD card (8 Gb, 77 but larger cards can be installed if needed) with an estimated data amount per channel of ∼15 Mb/day. 78 By default, time synchronization is based on Network Time Protocol, but a USB GPS module can be 79 connected for situations where an internet connection is not available. Further technical details on the RS&B 80 sensor are detailed in Appendix A and at https://manual.raspberryshake.org/specifications.html#techspecs 81 (last accessed November 2020). Other variations of the RS&B (but not assessed here) include that 82 with a single vertical geophone (Raspberry Shake 1D), three orthogonal geophones (Raspberry Shake 83 3D), 1 vertical geophone with 3 orthogonal MEMs accelerometers (Raspberry Shake 4D), and a single 84 pressure sensor (Raspberry Boom). So far, two studies have evaluated the performances of the above 85 sensors for environmental seismology monitoring. Raspberry Shake 1Ds were successful in recording and 86 discriminating rockfall activity above a glacier in the Swiss Alps, demonstrating their potential for use 87 in scientific investigations (Manconi et al., 2018). Raspberry Shake sensors were also used to estimate 88 local magnitudes for earthquakes recorded in Oklahoma, USA, and were found to be suitable for the 89 characterization of local and regional seismicity (Anthony et al., 2019). Despite limitations in design (e.g. 90 sampling rate, weather-proofing), both studies concluded that the relatively low cost of the units make 91 them a realistic candidate for complementing existing seismic networks or for deployment in locations 92 unfeasible for other sensors. 93 2.2 Experimental set-up 94 Seismic and acoustic recordings were collected from a group of seven African elephants (three female 95 adults, two male adults, and two calfs; aged between 3 and 23 years) located at Adventures with Elephants, 96 Bela Bela, South Africa.