Although more famous for their trademark trumpet sounds, elephants also make loud low frequency vocalizations known as “rumbles”, which extend below the range of human hearing. In a new study, a team of geoscientists has tracked the acoustic and seismic signals of rumbles among a family of elephants at a reserve in South Africa. They did this using a relatively low-cost sensor kit – showing promise as an affordable tool for wildlife studies and conservation projects.
In ecology, there is a growing understanding that monitoring animal sounds can reveal a lot about animal behaviour. At the same time, the increasing availability of cost-effective and scalable acoustic sensors is leading to a growing number of animal studies that make use of this technology. Tracking animals in this way also removes the need to physically tag them, which can be logistically difficult and stressful for the animals.
Elephants are big. Given the vast size of their vocal tracts, elephants can produce sounds that are loud and low – with frequencies below the 20 Hz limit of human hearing. As well as travelling through the air, this infrasound can also couple with the ground and propagate through the Earth as seismic waves. Earlier research, reported in Physics World, observed that the seismic component of rumbles could play an important role in long-range communication among elephant herds.
“There is evidence from previous studies that rumbles are used to coordinate movement and spacing of social groups, help individuals find each other, as well as triggering defensive or exploratory behaviour,” said Oliver Lamb of University of North Carolina at Chapel Hill in the US, who led the latest study and presented it last week at EGU2020: Sharing Geoscience Online.
Equipment costs – the elephant in the room
Lamb’s group set out to see if they could get meaningful results using an off-the-shelf device known as a Raspberry Shake and Boom (RS&B). Resulting from a Kickstarter campaign in 2016, the RS&B – based on the Raspberry Pi computer – is popular among educators and amateur seismologists due to its accessibility and cost. While conventional geophysical sensor systems tend to cost thousands of dollars, the RS&B seismo-acoustic is currently sold to researchers for approximately $865.
Lamb and his team took five RS&B units to study a family of seven African elephants at Adventures with Elephants – a 300 hectare area reserve in the north-east of South Africa. During a four-day period in October 2019, the group focussed on reunion events, which tend to involve a significant amount of vocalization. To calibrate readings, they also deployed a collection of more sensitive – but more costly – monitors.
To the researchers’ surprise, the acoustic component of rumbles below 50 Hz was more clearly recorded using the low-cost device than with the more sensitive microphone. This extra information would be useful in ecology studies for distinguishing individual animals by age and size. On the down side, the acoustic range was limited to roughly 400 m, which would only be of practical use in locations where elephants are known to congregate, such as waterholes and food sources.
Unfortunately, the seismic range of the device was even smaller. Individual rumbles were detected to within 100 m, while elephant footsteps were limited to just 50 m. Describing their results in a paper submitted to Bioacoustics, the researchers suggest they could extend the range up to 1 km by anchoring sensors to rocks rather than burying them in the ground.
Stomping towards a monitoring system
In fact, the clarity and range of seismic signals is likely to be affected by a number of factors, according to Tarje Nissen-Meyer, a geophysicist at the University of Oxford in the UK, who was involved in a previous seismic study of elephants in Kenya. “We showed that the distances up to which seismic cues may be detectable were strongly dependent on many factors, including local soil and geomorphology, geological structures, noise environment and the type of elephant motion or vocalization,” he said.
Elephant rumbles travel more than 6 km through the ground
Nissen-Meyer describes the new study as promising and believes the range could be extended with just a few alterations to the experiment set up. “I personally maintain that seismic monitoring of wildlife in the savanna can indeed be a viable path forward since seismic signals are, as seen in the Lamb study, detectable with little effort compared to approaches such as tagging after tranquilizing.”
But even with range limitations, the study could still represent an important step towards developing a practical system for elephant monitoring. One of the goals of tracking wild animal vibrations in Africa is to develop early warning systems that could alert park wardens of distressed animals or even the presence of poachers.
“Aside from the noises generated by the elephants, the Raspberry Shake and Boom can also pick up noises generated by humans either from the vehicles they use and, if the conditions are right, the footsteps of the humans,” said Lamb. “We are exploring the potential of integrating the Raspberry Shake and Boom sensor into some kind of package that can be deployed in game farms, game reserves and national parks, especially those that cannot afford the expensive technologies found at Kruger National Park and some private reserves.”