All vibrations great and small

elephant

Animals are able to detect changes in their environment in a number of ways, for example, receiving vibrations. Photo credit: Ally Darnton, The Oxford Scientist


Auditory and visual waves are commonly used by animals across many taxa to receive information over long distances. Waves can be described as fluid (travelling through gases or liquids) and substrate-borne (travelling through solid objects).

The conditions of a fluid medium (for example, high pressure air and warm water) influence how these waves travel. This affects the efficacy of signal communication and has various biological implications. For example, humpback whales can reportedly communicate with others over hundreds of kilometres. This is thanks to the density of water, allowing sound to travel faster. If the fluid medium were to be less dense, the information transmitted in whale song may not be received as intended.

Waves can also travel through solid material and animals similarly exploit this natural phenomenon to extract information. A surface-borne wave can be thought of as a substrate-borne wave that travels along said substrate’s surface. 

‘Biotremology’ is the study of how waves are produced, dispersed, and received by organisms and the impact this has on an organism’s behaviour. Waves can be described as fluid. Often the same wave travels through different media or a signal sent by one animal induces multiple waves that travel through different media, and are thus processed in different ways by different—intended or unintended—receivers.

‘Biotremology’ is the study of how waves are produced, dispersed, and received by organisms and the impact this has on an organism’s behaviour.

Substrate-borne waves, although often not initially considered when considering animal communication, are a common occurrence in the natural world. For example, for the majority of the world’s approximately 200,000 insect species, substrate-borne waves are the exclusive mode of communication. 

All tremors small

Life for a web-dwelling spider is an intense sensory experience—it secures its food by paying attention to vibrations in its web. But not everything that induces a vibration makes for a good lunch: wind, foliage, and large indigestible animals are all common examples of false positives for the spider. Not to mention, the brain power that it would need to process all these stimuli, oftentimes simultaneously. 

Life for a web-dwelling spider is an intense sensory experience—it secures its food by paying attention to vibrations in its web.

To overcome this, many spider species have evolved a passive way to filter ‘false positive’ waves. Studies reveal that these innovations form a ‘bridge’ between the external substrate (the spider’s web) and the mechanosensory site, where the wave is actively processed by the spider. This concept of a ‘bridge’ encapsulates how morphology and behaviour are imperative to how substrate-borne waves are processed via morphological computation, in other words, not directly using the spider’s brainpower.

The morphological aspects of the ‘bridge’ include the size of the spider’s legs, the weight distribution of its cephalothorax (the front part of its main body) and abdomen (the often-larger back part), and the distance between the legs on its cephalothorax. Tuning these parameters has been shown, through experimental modelling, to result in different output vibrations given the same input vibration. 

The resulting output vibrations are further influenced by the spider’s behaviour: the spatial position of the legs, where the spider positions its abdomen, whether it is carrying a silk-sac full of eggs, and where it chooses to camp on its web. These factors passively influence the transfer of the wave from the external substrate to its mechanosensors. 

One way in which the incoming wave is morphologically computed through the ‘bridge’ is by frequency filtering. Here, certain frequencies are either amplified or diminished, influencing the amplitude of the wave that reaches the mechanosensor. This happens in all cases of wave transfer through physical substrates, but the spider can “choose” the course of frequency filtering conducted by the substrate itself. 

Evolution has selected for important frequencies to be amplified and costly amplitudes—those that would decrease the fitness of the receiver—to be diminished. The amplification of a substrate can be achieved by matching its resonant frequency, because all things vibrate and the frequency of waves is additive. 

For spiders, the substrate is its web. For other animals, it may be the tree it lives on, the hive it inhabits, or perhaps the rock it basks on. This is exemplified well in honeybees as the resonant frequency of the honeycomb it dwells on amplifies the vibrations the bee makes when it communicates via the waggle dance. Okay now, back to spiders.

Another aspect of a spider’s morphological computation is its amplitude damping. A vibration must meet some threshold amplitude to be computed when getting rid of background frequencies. This is largely influenced by the web the spider spins but also where it chooses to position itself on its web. When a vibration is spreading across the web it is being dampened and the spreading also occurs in the spider’s body. The magnitude of the source also determines if the threshold is met and if amplitude damping has an effect. For example, a large animal, likely producing a vibration that breaks the threshold, conveys important information to the spider concerning danger. 

All tremors great

Apologies to all the arachnophobes reading this. Perhaps a larger, and to some a friendlier and cuter example of the scope of biotremology will earn your pardon. Let’s consider elephants. 

Elephants are large creatures and hence the vibrations they generate when vocalising are also rather great. Moreover, their herding behaviour increases the magnitude of the waves generated by the group. Aside from their sheer size, their substrate also makes for interesting and unique biotremology. Indeed, the savannahs and grasslands inhabited by African elephants are flat and regular, and so encounter less dampening than in other biomes. This allows for the low frequency seismic waves generated by elephant vocalisation to travel great distances. 

Furthermore, elephants are interesting biotremological models as they have tactile sensors in their feet and trunk, and the morphology of their legs allows for the amplification of some frequencies when they reach their inner-ear. This opens the possibility for elephants to communicate with other herds without having to risk conflict from meeting. A question that arises is if communication is intentional or not, but a lot of the time it does not have to be. Frequencies generated when elephants are fleeing danger and vocalising just need to be interpreted as ‘danger!’ by the other herd. If the response to the signals means the receiving herd avoids danger or death, it is plausible for this behaviour to be selected for. 

…elephants are interesting biotremological models as they have tactile sensors in their feet and trunk, and the morphology of their legs allows for the amplification of some frequencies when they reach their inner-ear.

Long-distance communication in elephants is still actively being researched. An interesting consideration is if herds can use this form of communication to trick other herds into responding unnecessarily and thus starting some sort of intraspecies arms race. 

All creatures

Biotremology can be found across the natural world, ranging from the smallest to the largest of creatures. This can prove useful to them in life-or-death situations—for example, some species (including elephants) have been known to react to seismic waves ahead of tsunamis—a useful skill achieved by humans through elaborate technological developments. Thus, greater understanding of biotremological phenomena may lead to such next-generation technology (like bioinspired wave dampening search and rescue robots designed for earthquake scenarios), or even provide the basis of yet-to-be-discovered technologies of the future.