Nature’s Hidden Sounds

By Karen Bakker

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How digital bioacoustics is revealing the unsuspected world of animal communication, and creating new possibilities for environmental conservation.
 

The Sound of LIfe, by Karen Bakker

Karen Bakker is the author of The Sounds of Life: How Digital Technology Is Bringing Us Closer to the World of Animals and Plants (Princeton University Press, 2022), an amazing journey into the hidden realm of nature’s sounds—just published by Princeton University Press.

Male peacock tails are one of the most arresting sights in the animal kingdom. But scientists have recently discovered that there is more than meets the eye. When a peacock vibrates its tail, it produces a low-pitched yet loud sound, well below human hearing range. The peacock’s famous mating dance is not primarily a visual display but, rather, a sonic summons.

The scientific mystery began unfolding when biologist Jessica Yorzinski used eye-tracking technology to monitor where peahens directed their gaze during courtship. Rather than looking at the tips of the peacock feathers, peahens focused their attention lower on the tail. In a later experiment, Yorzinski observed that peahens were more likely to notice males that shook and vibrated their tail feathers more.[i]

Angela Freeman, a neuroscientist and behavioral ecologist at Cornell, was intrigued by these findings. Her supervisor, James Hare, had discovered that ground squirrels communicate using ultrasonic alarms (distinct from ultrasonic echolocation, which is well documented in bats and dolphins).[ii] Hare’s squirrels lived in a zoo compound that they shared with peacocks. During one visit, he noticed a peacock shaking its tail at a concrete wall. Initially bemused, he suddenly realized that the huge tail looked a bit like a satellite dish. Could the bird be listening to itself, he wondered?[iii] Freeman set out to answer this question. By using a digital bioacoustics recorder that could detect low frequency sound, she confirmed Hare’s suspicion: the male peacocks were indeed making noise at infrasonic frequencies below human hearing range.

But could other peacocks hear the noise? To answer this question, Freeman recorded the infrasonic sounds made by 46 peacocks, and played them back to both peahens and peacocks. Both sexes responded by becoming more alert, and spending more time walking and running. The males called out in response to the sounds; they even often faced off against the playback speaker as if it were a competitor. To double-check her results, Freeman played back the sounds again, but at a different frequency, within human hearing range. This time, the birds did not respond. With these experiments, Freeman provided the first empirical evidence of a bird producing and perceiving infrasound as a signal.[iv] Later research demonstrated how the biomechanics of the peacock’s tail enable it to act as a resonator in order to produce loud infrasound.[v]

Why would peacocks make infrasound? Infrasound has the advantage of being inaudible to some potential predators; peacocks can use infrasound to warn off potential rivals and protect their territory, without making themselves a target. Infrasound could also be used to attract females, as it could reinforce the size of the peacock tail as a signal of strength and genetic fitness. In the landscapes where peacocks normally live in the wild (scrub brush and woodland), infrasound is a more effective way of communicating; high-pitched calls are muffled even over short distances, but low-frequency infrasound travels long distances, through stones, trees, and even walls.[vi]

How many other species make noise beyond human hearing range? Many. Using digital bioacoustics, scientists have recently documented a vast array of sounds that species use to communicate. Above human hearing, shrews, gerbils, tree mice, and lemmings emit ultrasound as a form of communication or echolocation.[vii] The first evidence of ultrasonic echolocation in tree-climbing mammals was found in the Vietnamese pygmy dormouse: a small rodent with tiny, reduced eyes that can only distinguish light from dark, whose surefooted ability to leap from tree to tree has long intrigued scientists (who speculate the dormouse provides evidence that echolocation evolved before flight in bats).[viii] Many other species—fish, insects, even coral—have been shown to emit and respond to ultrasound. Below human hearing range, low-frequency ultrasound can be made and heard by whales and elephants, tigers and rhinos, hippos and crocodiles. The world is alive with a continuous chorus of sounds, of which humans are simply unaware.

Second, digital bioacoustics has revealed that some species use vocalizations to identify themselves individually to one another. These vocalizations correspond to animals’ “names,” which individuals and others in their communities recognize. For decades, we have known that dolphins possess this capacity. Researchers identified the unique whistle vocalizations that dolphins use to self-label and identify social groups (in non-technical terms, their names). [ix] Dolphins in both captivity and the wild copy one another’s signature whistles, suggesting that these whistles function as a “name” (which scientists refer to as a “learned vocal label”).[x]

But more recent research suggests that many more species than previously suspected—cetaceans, hyraxes, wolves, dolphins, and bats—also use these vocal labels.[xi]

Third, combining bioacoustics with other digital technologies, like sensors and artificial intelligence, researchers have also discovered “functionally referential communication” (which a layperson might call “words,” although scientists are careful to avoid that term).[xii] One example is prairie dog vocalizations. Prairie dogs communicate using unique whistles that are extremely precise descriptors. They can distinguish between shapes (triangles versus circles), and between thin and overweight humans, and even describe the colors of clothing humans are wearing. In a controlled experiment, a human researcher walked through a prairie dog colony wearing a blue T-shirt, and then later wearing a green T-shirt. The prairie dogs’ whistles are specifically coded: sounds for “thin, human, blue” and “thin, human, green” are distinct.[xiii] Vervet monkeys employ a system of warning calls for predators such as eagles (which signal other monkeys to descend from trees), leopards (signaling others to climb trees), and snakes (stand and scan the ground).[xiv] Elephants have specific sounds for honeybee and human; and even distinguish between different types of humans depending on the level of threat. In other words, animals can describe their world to one another in much more specific ways than we previously understood. [xv] The advent of digital bioacoustics means that we can decode such vocal signals in more species, more easily.

Fourth, bioacoustics provides insights into complex social organization in non-humans. If you were to walk into a bat cave, you could not decode the cacophony of sounds, many of which you would be unable to hear. But digital recorders, combined with artificial intelligence, can easily decode these signals. Through using digital bioacoustics, researchers have learned that bats distinguish between genders, help one another, remember favors, hold grudges, trade food for sex, and socially distance when ill; mothers teach their babies to speak much like humans do, cooing at their infants who babble back until they learn to speak adult bat language. Researchers are now using artificial intelligence to study the sounds made by other species. They have found that chimpanzee calls communicate identity-related information, age, social status, and activity (traveling versus feeding). Previous research data had suggested that primates were capable of only much simpler calls; the combination of bioacoustics and automated machine learning reveals a much more complex set of patterns than previously suspected.[xvi]

If non-humans possess symbolic communication, will we ever be able to bridge the interspecies communication divide?

What does all of this mean about our understanding of non-human communication? In layperson’s terms, many more species than we previously understood have much more complex communication than we realized. Some animals have individual names. Some learn languages much like humans do. And many use forms of complex communication to socialize and serenade their mates, to warn of dangers and ward off threats, to teach their children and sing their family songs.

But is this really language? This remains a controversial question. Some scientists distinguish between communication (exchanging information) and language (which includes symbolic communication). Experiments have demonstrated that several species (apes, gray parrots, dolphins) can be trained to use symbols to express their preferences with other members of their own species or with humans.[xvii]  Researchers have used a variety of technologies (touchscreens, interactive keyboards, TV monitors, gestures, acoustic signals and speech),[xviii] combined with social feedback, to demonstrate that some animals can engage in symbolic communication, and also possess numerical competence, form concepts, learn associatively, and self-organize learning.[xix] As digital bioacoustics research continues to develop, researchers are conducting experiments on symbolic communication in a larger number of species. As a result, our understanding of non-human language capacity is evolving: more of a continuum, rather than something that humans uniquely possess.

If non-humans possess symbolic communication, will we ever be able to bridge the interspecies communication divide? Researchers from MIT and University of California, Berkeley are working on decoding sperm whale language. Others are building dictionaries in East African Elephant. Using artificial intelligence, scientists have decoded honeybee language; some have gone further, and encoded bee-like robots with AI-derived signals, enabling the robots to speak honeybee language directly to bees in the hive. Some believe that we will soon be able to use AI to communicate intelligibly with other species, like an interspecies Google Translate.

Beyond these intriguing speculations, digital bioacoustics also creates new opportunities for environmental conservation. Digital recorders provide an easy, low-cost method of monitoring hard-to-reach places, allowing scientists to monitor at night and in inclement weather. And these digital sensors also tend to be less alarming to nonhumans; digital bioacoustics is thus a non-invasive way to monitor species and places at risk. In a few cases, scientists have even discovered entirely new species, like the blue whale species discovered in the Indian Ocean a few years ago, revealed by the unique songs made by the cetaceans who normally dive too deep for humans to see.

Beyond monitoring, digital acoustics creates exciting new ways to protect nature. Conservationists have used acoustic deterrents to save dolphins from fishing nets. Digital bioacoustics devices can detect poachers and alert guards; in hundreds of national parks around the world, automated listening devices discreetly nestled in trees provide a new layer of protection for the world’s most endangered species. Scientists are also using sound as a tool for ecosystem regeneration, like music therapy for non-humans; playing healthy coral reef sounds in degraded reef landscapes can attract fish and coral to repopulate once-empty marine landscapes.

Much like the microscope opened up an entirely new world of microbiology, digital bioacoustics is fundamentally changing the scientific view about the ubiquity of communication and importance of sound across the Tree of Life. Digital bioacoustics is like a planetary-scale hearing aid, revealing the resonant mysteries of nonhuman sound. Although we can’t buzz like an insect or sing like a whale, our computers can do just that. Are we on the brink of inventing a zoological version of Google Translate? What are the ethics of eavesdropping on non-human species without their consent? Can scientists gather digital acoustic data without limit, or does this data belong to the Indigenous communities from whose lands it is harvested, or perhaps even to non-humans themselves?

Conservationists are increasingly using bioacoustics to preserve wild spaces and places. But some tech innovators are attempting to use bioacoustics to domesticate previously wild species. Which will we choose, dominion or communion? Digital bioacoustics can serve as both a tool and a weapon. It is time for a healthy debate in the environmental conservation community about how to use our newfound listening powers.

 

Endnotes

[i] Yorzinski, Jessica L., Gail L. Patricelli, Jason S. Babcock, John M. Pearson, and Michael L. Platt. “Through their eyes: selective attention in peahens during courtship.” Journal of Experimental Biology 216, no. 16 (2013): 3035-3046.

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Yorzinski, Jessica L., Gail L. Patricelli, Michael L. Platt, and Michael F. Land. “Eye and head movements shape gaze shifts in Indian peafowl.” Journal of Experimental Biology 218, no. 23 (2015): 3771-3776.

[ii] Wilson, David R., and James F. Hare. “Ground squirrel uses ultrasonic alarms.” Nature 430, no. 6999 (2004): 523-523.

[iii] Milius, Susan. “Life: Peacock pomp makes a rumble: Male birds emit sounds too low to be heard by humans.” Science News 182, no. 2 (2012): 8-8.

[iv] Freeman, Angela R., and James F. Hare. “Infrasound in mating displays: a peacock’s tale.” Animal Behaviour 102 (2015): 241-250.

[v] Dakin, Roslyn, Owen McCrossan, James F. Hare, Robert Montgomerie, and Suzanne Amador Kane. “Biomechanics of the peacock’s display: How feather structure and resonance influence multimodal signaling.” PloS one 11, no. 4 (2016): e0152759.

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Kohles, Jenna E., Gerald G. Carter, Rachel A. Page, and Dina KN Dechmann. “Socially foraging bats discriminate between group members based on search-phase echolocation calls.” Behavioral Ecology 31, no. 5 (2020): 1103-1112.

Koren, Lee, and Eli Geffen. “Individual identity is communicated through multiple pathways in male rock hyrax (Procavia capensis) songs.” Behavioral Ecology and Sociobiology 65, no. 4 (2011): 675-684.

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[xv] Demartsev, Vlad, Andrew S. Gersick, Frants H. Jensen, Mara Thomas, Marie A. Roch, and Ariana Strandburg‐Peshkin. “Signalling in groups: New tools for the integration of animal communication and collective movement.” Methods in Ecology and Evolution (2022).

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[xvi] Fedurek, Pawel, Klaus Zuberbühler, and Christoph D. Dahl. “Sequential information in a great ape utterance.” Scientific reports 6, no. 1 (2016): 1-11.

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[xvii] Herman, Louis M., David S. Matus, Elia YK Herman, Marina Ivancic, and Adam A. Pack. “The bottlenosed dolphin’s (Tursiops truncatus) understanding of gestures as symbolic representations of its body parts.” Animal Learning & Behavior 29, no. 3 (2001): 250-264.

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[xviii] Egelkamp, Crystal L., and Stephen R. Ross. “A review of zoo‐based cognitive research using touchscreen interfaces.” Zoo biology 38, no. 2 (2019): 220-235.

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[xix] Agrillo, Christian, and Angelo Bisazza. “Spontaneous versus trained numerical abilities. A comparison between the two main tools to study numerical competence in non-human animals.” Journal of neuroscience methods 234 (2014): 82-91.

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Karen BakkerKaren Bakker is a professor at the University of British Columbia, a Guggenheim Fellow, a Fellow of Harvard University’s Radcliffe Institute for Advanced Study (2022/23), and the author of The Sounds of Life: How Digital Technology Is Bringing Us Closer to the World of Animals and Plants (Princeton University Press, 2022). Her research explores the relationship between digital transformation, governance, and sustainability. The author of more than 100 academic publications, she has conducted fieldwork on four continents. She is a member of the Editorial Board of Global Environmental Change, the Internet Governance Forum’s Policy Network on the Environment, and the United Nations Coalition on Digital Environmental Sustainability (CODES).

Header photo by Allan Lau, courtesy Pixabay.

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