Tuning in to nature

Sounds are everywhere. Animals, especially birds, create tailored calls to find mates, to alert others of predators, and to establish territories. Insects, such as crickets, cicadas and grasshoppers, are present in nearly all ecosystems, and typically keep the “rhythm” of a place through their pulsating sound production. Amphibians also contribute to the rhythm of nature; in some places, they do so in such large numbers that it becomes deafening.

Even fish, and other animal life in water, use sound as a means for individuals to locate one another or to navigate. For instance, sounds enable the young of many fish and crustacean species to orient themselves toward coral reefs and the resources they contain. On land, many bird species in tropical rainforests will use the sounds of a river to locate their nesting places along stream banks. 

Scientists are now discovering that night-time biological sounds are more common and more complex than previously acknowledged. As many terrestrial and marine animals are active during the dark hours of the day, sound becomes a major way to sense how the environment is changing, how to communicate with one another, and to find food. Nocturnal foragers must prioritize sound and smell. 

Humans can only hear some sounds in their midst. Sounds above the human threshold of hearing (or “ultrasonic sounds”) occupy a sonic space that many animals use. Scientists continue to discover species that communicate with one another using this sonic space, including many insect and tropical frog species.

Perhaps most famously, bats rely on ultrasound through their use of echolocation. They emit acoustic signals that are used to locate objects, like mosquitoes flying in the air. Those acoustic signals bounce off the object, and the timing of the echo is used to determine its proximity. 

Sounds below the threshold of human hearing are called “infrasonic”. Many large animals, such as elephants, hippopotamuses, rhinoceroses and whales, but also octopuses and squid, communicate in this sonic space. A handful of smaller species, including pigeons, fowl and fish, also use infrasound.

Among all existing species, it is likely that more than half use some form of acoustics to either produce sounds or to use sound to sense how their environment is changing.

Why is all this information so important?

As scientists, we grapple with how to monitor the current biodiversity crisis and assess the extent of species loss. It’s a tricky mission because monitoring animals is so difficult – we need to seek clues in hard-to-reach places like dense tropical rainforests and deserts, during the day and night, and for long periods of time. 

With recent advances in technology, however, we can position acoustic sensors in large networks. The sensors are equipped to operate continuously and over long periods and across large areas, in deserts and rainforests, and especially in biodiversity hotspots like coral reefs. We can also record sounds in the ultrasonic and infrasonic ranges. This technology allows scientists to track animal activity and biodiversity to establish an acoustic record of biological sounds. Artificial intelligence (AI) tools are used to extract and identify sounds in these complex, digital audio recordings; scientists can “teach” computers about the sources of specific sounds, allowing us to develop a catalogue of species for any location.

My research has focused on a new field of science called soundscape ecology, which studies how sound from animals can be used to measure changes in animal biodiversity and to create an archive of all of Earth’s major biomes – sets of ecosystems characteristic of a given biogeographical area – in the most remote places in the world. As part of this "Mission to Record the Earth", 29 of the planet's 32 major land and aquatic biomes have been completed so far.

What the community of soundscape ecologists has found is revolutionizing our understanding of current trends in biodiversity. For example, the sounds of an old-growth forest are often the most diverse, as it supports a huge diversity of animals: birds, insects, mammals, and amphibians. In the Midwestern United States, several ongoing soundscape studies are finding that the greatest animal acoustic diversity occurs late in the summer, after many insects emerge and “mix” with the sounds of birds and frogs, which have been singing since spring. Young-growth forests have much less acoustic diversity than old-growth forests, and the sounds of landscapes dominated by human food production lack biological sounds, especially at night.

I am often interested in capturing what a scientific researcher calls “baseline” or “reference condition” information. That implies going to locations that are least disturbed by humans to deploy an array of sensors, to study how the most “pristine” paleotropical rainforests sound. In general, it takes one year to identify such a location and find a colleague with whom to collaborate. Travelling there can also be long and complicated. 

To reach the eastern province of Brunei on the island of Borneo, we travelled by plane, truck, boat, and foot, for days. The acoustic diversity of this place is staggering! Nearly 100 frog species, over 390 bird species, and dozens of species of cicadas create a biological diversity so complex and crowded that some species, such as the six o’clock cicada, have to select a specific time of day to sing. These limited “acoustic niches” mean that many species have to find unique ways to communicate with members of their own species.

As a result, the soundscapes vary widely by place and time of day. The sounds of Borneo are truly ancient; the subcontinent land masses have barely shifted over the past 300 million years, imparting a “prehistoric” quality. Such soundscapes allow our research community to ask: “What acoustic gaps exist and what kind of animal, based on body size, might be missing from this biophony?” Blending ecological theory with technology helps us find answers.

Visiting and listening to these remote places on Earth has filled me with deep emotions about what I call “the awe of nature”.

Take, for instance, the research station where my formative project in Borneo was completed. A nearby tourist park boasted a 90-metre observation tower, and I had the urge to hear what the forest sounded like from this perch. 

I was astounded! At sundown, gibbons were barking across the valley below, followed by a multi-species concert  with tropical frogs at the forefront, before a long chorus of crickets. Occasionally, high-frequency bat sounds would also pass into my awareness. Oddly, these soundscapes seemed familiar to me. In fact, the sounds were very similar to those of wetlands back home in Michigan. The top of a rainforest supported the same kinds of animals as a wetland in the Midwestern United States: insects and frogs with occasional birds that are active at night.

Indigenous peoples have long used sound to understand changes in their environment but also to relate themselves to nature and the afterlife.  Nature’s sounds and spiritual world are often inextricably linked. In Mongolia, I am collaborating with social scientists and scholars in the humanities to understand how nomadic herders use, in their songs and sonic practices, sounds of the cuckoo, of ice breaking and rustling of rivers, etc. – to sing praise to nature. To understand more deeply what these sounds mean to them, I once asked a Mongolian herder what he thought would be the consequences of the loss of the sounds of the natural world around him.  Without hesitation, he answered "We’d no longer be human”.