Finding the way with waves


Waves have been used as a navigational tool for centuries, and is ubiquitous throughout the animal kingdom. Photo credit: Jamie Street via Unsplash

Waves arise from motion. It is perhaps not surprising, therefore, that waves often guide motion by providing dynamic cues from which navigational information can be gathered and processed. We find astonishing examples of navigational mastery throughout the Animal Kingdom, such as echolocation in bats, infrasound-sensing in pigeons, and the ancient-practice of wayfinding by sailors, who use ocean waves as directional tools. 

Ultrasonic Echolocation

The phrase ‘as blind as a bat’ is perhaps one of the most misleading idioms in the English language. An individual being ‘as blind as a bat’ is synonymous with them lacking observance and generally being disconnected to their environment. The phrase comes from a quote by Aristotle, based upon the belief that bats relied solely on hearing for flight, thus being ‘blind’ to their visual environment. And yet, whilst their navigational mechanism is, at its core, an auditory behaviour, its output is visual. The processing of these auditory cues facilitates the creation of a visual map so rich with information that the absence of advanced eyesight is no loss. 

Echolocation relies on the detection of reflected sound waves. “Sound,” as we perceive it, arises from sound waves bouncing off objects, with the subsequent changes in pressure or particle velocity being detected by the ear. Bats produce ultrasonic calls which are at frequencies higher than the human ear can detect, above 200Hz. These calls produce sound waves, which propagate through the air until they hit an object, causing them to bounce back to the caller. The time taken for the wave to come back can be used as a measure of distance. 

Calls enable bats to detect, classify, and localise objects to high levels of accuracy, allowing them to prey on species as small as flies. Incredibly, bat calls are specialised for each species’ preferred social and ecological environment, so that bats flying in a cluster produce calls distinctive from those inhabiting more open environments, such as mountainous regions.

Echolocation is relatively widespread in the Animal Kingdom, from sperm whales to swiftlets, and even humans. Some visually impaired people have learned to map their environment by recognising how their own ‘clicks’ resonate. Sonar systems, a technology that revolutionised our understanding of the oceans, work by the same principles. Sonar is not only used as an immediate localising tool, but also as a cartography tool, used to create nautical charts and image the features of the sea floor.

Echolocation is relatively widespread in the Animal Kingdom, from sperm whales to swiftlets, and even humans.

Infrasound Navigation

Whilst echolocation is a vital detection tool over short ranges, sound waves may also be used as a larger-scale navigational tool. On the other end of the spectrum to ultrasonic bat calls is infrasonic, below the lower limit of human hearing. We are constantly surrounded by a cacophony of infrasound, unbeknownst to our naïve ears. These sound waves are generated by natural systems such as oceanic waves and storms, and increasingly, human activity, such as operating machinery and aeroplanes. 

We are constantly surrounded by a cacophony of infrasound, unbeknownst to our naïve ears.

Most significantly, infrasound is produced by the interaction of marine storms with ocean waves (known as “microbaroms”), and the resulting miniature-scale earth vibrations (called “microseisms”). These produce specific infrasonic signatures for everywhere on Earth, which are dependent on the topography, weather systems, atmospheric temperature, and wind patterns of the area. 

Due to the low frequency of these sound waves, they are able to propagate over long distances and, in theory, be used as a ‘homing beacon’ for the likes of pigeons. This forms the basis of the ‘sound map’ hypothesis of pigeon navigation, the brainchild of geophysicist Jon Hagstrum. 

Fascinatingly, the confluence of environmental factors can alter signatures, resulting in examples of extreme disorientation in even the most experienced of navigators. For example, on 29th June 1997, 60,000 pigeons were released from Northern France, bound for their home lofts in England. Pigeon racing like this was routine and, under normal circumstances, 95% of the birds would be expected to successfully return home. But on this occasion, return numbers were staggeringly low, with only a few thousand ever showing up

The reason remained a mystery until Hagstrum discovered that the pigeons’ release coincided with a Concorde flight taking off from Paris. The sonic boom produced by Concorde caused significant infrasound disturbance, leaving the birds completely disorientated.

Bizarrely, homing pigeons in Germany are particularly prone to becoming lost during the winter. This ‘Wintereffekt’ is thought to result from natural infrasonic interference from winter Atlantic storms, which are directed over Europe by stratospheric winds. 

Oceanic Waves and Wayfinding

Humans also have a history of wave-guided travel. Instead of detecting and homing in on the subtle infrasound signatures of oceanic waves, skilled navigators are able to directly observe oceanic swell patterns and use them as a guide. 

These methods are the basis of the practice of Polynesian wayfinding. Swell patterns are altered by the presence of islands, resulting in reflection and refraction.  These patterns are comparably different to patterns in the open ocean. Recognising these signature changes enables wayfinders to detect and localise islands long before they are in sight. This, in a way, could be considered a form of ‘echo-location’, as it relies on the same principle of detecting wave reflection from objects. Wayfinders often also rely on directional cues from animals themselves, due to their more adept navigational capabilities. 

The ability to find one’s way with waves underpins the navigational systems of animals and humans alike. Different waveforms are utilised, whether that be sound waves or visible ocean waves, over a range of scales, from nearby prey localisation to continent-scale homing. These techniques are perhaps more analogous than it would appear on the surface, being underpinned by the basic tendency of waves to propagate, reflect, and refract, and the ability of the navigator to detect and interpret these patterns. 

And yet, humans are becoming increasingly oblivious to the world of waves in the modern era of GPS and electronic navigation. We are all capable of detecting patterns in our environment; of using waves to guide us. This is what allows us to exist in a multi-sensory, multi-dimensional landscape, and respond rapidly to the stimuli of everyday life. 

…humans are becoming increasingly oblivious to the world of waves in the modern era of GPS and electronic navigation.

Understanding the mechanisms underlying these navigational methods is important not only for human mimicry and understanding, but also is vital to the conservation of these species, particularly in the face of anthropogenic change. Mapping the large-scale movements of animals and the cues they utilise for this enables us to select suitable habitats to conserve, and to attempt to minimise human interference with their maps and compasses. We would be wise to look upon these advanced navigational feats with wonder, and to reflect upon which direction we as humans are going in.