With the right kind of sensory organs, a tremor in the earth can become an entire conversation.
From the terrifying throes of an earthquake, to the steady pulse of a loved-one’s heartbeat, vibrations carry vital information about our surroundings. For many animal species, vibrational signals serve as a practical, if not vital, means of communication. But in our human world, dominated by visual and acoustic signals, the study of communication by vibration has long been neglected. That is, until recently. A new field of study, known as biotremology, is quickly gaining traction.
Understanding vibrational communication requires that we first understand sound. Imagine, for instance, a hand beating against a drum, causing the drumskin to vibrate. These vibrations spill into the surrounding air and create changes in air pressure that emanate in waves from the source. It is these waves that eventually reach our ears, and are interpreted by our brain as sound. With vibrational communication, the difference is in the medium: the waves travel through a solid substrate, such as earth or plant material, instead of the open air.
Travelling through a stable material has its advantages, as the waves maintain their integrity over larger distances. Take, for example, jumping on the spot. Though it doesn’t produce much noise, the seismic disturbance it creates can be detected a kilometre away using a geophone, a device for measuring seismic vibrations. For animals like the wolf spider, which seldom encounter other members of their own species, vibrations allow signalling without the need for close contact. In the subterranean world of mole rats, visual and acoustic signals simply don’t do the job; vibrational communication is the most efficient means of signalling.
How an animal creates these tremors depends largely on its mass. For the miniscule, vibrational communication is typically a funky business — it’s all about percussion. The male brush-legged wolf spider bounces his body against a substrate, producing the sweet beats that will hopefully attract a special lady. Not to be outdone, small mammals like the blind mole rat adopt a similar strategy. They head-bang against tunnel walls, creating vibrational cues capable of reaching distant neighbours.
Many small invertebrates, such as the wolf spider, use vibrations to find each other. Katja Schulz/Flickr (CC BY 2.0)
Vibrational communication in larger animals is an entirely different, and less laborious, process. Caitlin O’Connell-Rodwell, a biotremologist at the Stanford University School of Medicine, understands this better than anyone. Her work explores the signalling strategies in our planet’s largest terrestrial animal, the elephant. While smaller species like mole rats use percussion to create vibrational cues, an elephant’s seismic signals are a by-product of its vocalisations. Their immense size means that they can produce loud, and hence high-energy, acoustic signals called rumbles, which cost them very little.
"Elephant vocalisations are produced at 120 decibels," an amplitude equivalent to the explosive force of a thunderclap, says O’Connell-Rodwell. But the energy in these sound waves doesn’t just dissipate into the surrounding air. It passes into the ground and creates a seismic disturbance.
In all forms of animal communication, the signaller transmits information to a receiver in the hope of altering their behaviour. So how can we be sure that these vibrations, whether produced by percussion or as a by-product of vocalisations, are being received and interpreted? The answer lies in each species’ behaviour and sensory organs: Has evolution gifted them with the heightened sensitivity needed to detect vibrations?
The task of detecting vibrations typically falls to the Pacinian corpuscles, pressure receptors that are found in a variety of species, including humans. During her research, O’Connell-Rodwell observed that elephants adopted certain postures when they were exposed to the vibrational component of another elephant’s rumble. Specifically, they orientated themselves so that either the front or back regions of their feet were in better contact with the ground.
In a 2007 study, Donna Bouley and colleagues examined cross-sections of elephants’ feet and discovered clusters of Pacinian corpuscles in these very spots. Like us, an elephant’s Pacinian corpuscles allow it to detect vibrations. But as O’Connell-Rodwell explains, “the larger size and clustered distribution may facilitate superior sensitivity”, indicating that these pressure receptors have far greater importance in an elephant’s foot than in our own hands.
Elephants are no strangers to noise, but also communicate via vibrations transmitted through the earth. katja/Pixabay (CC0)
But increased sensitivity to tremors is only half of the equation. Animals using vibrational communication must also “hear” tremors to interpret them. Bone conduction allows interpretation of vibrations using similar mechanisms to those used for hearing sound. In mammals, hearing depends on a chain of events involving the outer, middle and inner ear. Airborne acoustic signals first cause the tympanic membrane, or ear drum, to vibrate. Three tiny bones in the middle ear amplify these vibrations, before passing them to the inner ear for interpretation by the brain.
But with substrate-borne cues, there is nothing to cause the ear drum to vibrate and kick off this chain of events. Animals that communicate using vibration possess larger middle ear bones relative to their body size. These bones bear enough weight to swing like a pendulum within the middle ear cavity. This means that vibrations conducted along the skeleton cause the skull to move around the middle ear bones. This creates vibrations in the middle ear, which are passed on to the inner ear for interpretation. Animals like the mole rat and elephant adopt behaviours that allow them to fully embrace this form of hearing. Mole rats place their jaw against tunnel walls to better receive the head-banging signals of their neighbours, while the elephant’s change in posture is hypothesised to allow vibrations to carry straight to the middle ear.
But we don’t have all the answers yet. Much of biotremology in vertebrates remains a mystery, particularly for larger mammals like the elephant.
Gaining a clear understanding of all forms of animal communication is of growing importance. Studies in light and sound pollution show that human activities have dire consequences for many species, but very few have considered vibrational pollution. Even aside from the obvious sources of vibrational interference, like vehicles or construction, where there is noise pollution there is also likely to be vibrational interference. The issue lies in teasing apart the sources and effects of each.
Peggy Hill, a biotremologist at the University of Tulsa and author of the book Vibrational Communication in Animals, admits that her scientific field still faces some difficulties. Studying vibrational communication “requires cross-disciplinary tools and terminology that most people don’t have”, she says. And with its relatively recent origins, aspiring biotremologists face formidable obstacles. But this is perhaps what makes it so appealing; there is still so much to learn and discover.
It is no wonder Hill remains optimistic about the field’s future. “It is fascinating and exciting and awe-inspiring to work in biotremology at this time in history,” she says. And as the field grows, there’s no telling what groundbreaking discoveries will soon be made.
Edited by Andrew Katsis
