Tiger's secret weapon
Tiger’s secret weapon
How science explains the stun effect of tiger’s roar on preys and rivals.
By Sofia Solovieva, Ph.D. Biol. Sci. IPEE RAS
Photo: A roaring tiger (Panthera tigris)
Introduction
Credit for all audio files : Walsh, Edward & Armstrong, Douglas & Napier, Julie & Simmons, Lee & Korte, Megan & McGee, Joann. (2008). Acoustic communication in Panthera tigris: A study of tiger vocalization and auditory receptivity revisited.
This the roar of a tiger. What effect has on you? Listening to it from your safe place in the “civilized” world of humans is not the best situation to “feel” its “magical” power but try to identify with a sika deer (Cervus nippon), a muntjak (Muntiacus) or a Malayan tapir (Acrocodia indica) that hears that frightening sound at a close range while drinking water from a pond or eating grass in a meadow and then listen to it again. Does that sound paralyze your mind and your body for a second? Does it scare you at the point that all your reactions froze? If your answers are a yes, you just experimented the incredible stun effect generated by the tiger’s roar!
Is it magic? No, it is science and it has a name: infrasound.
Infrasound, a misleading term
The common definition of infrasound is: sound waves/oscillations whose frequency is below the frequencies of audible sound of about 16 Hertz (Hz). Several studies (1) seem to prove that a sound remains audible at frequencies below 16 Hz because measurements of hearing threshold has been lower down to 4 Hz for exposure in an acoustic chamber and to 1,5 Hz for an earphone listening. The limit of 16 Hz (or 20 Hz) derives from the “lower frequency limit for which the standardized equal loudness hearing contours have been measured” (Leventhall 2007) and, for this reason, we could define infrasound as a continuation of audible sound into a region that is characterized by a frequency reduction and an hearing threshold increment. The “real” and inaudible infrasonic sounds are, for the most part, natural (volcanic eruptions, ocean waves, meteors, wind), and their frequency can be lower than 1 Hz. Some scholars note that the term infrasound should be used for the aforementioned very low-frequency atmospheric sounds, natural or man-made (large combustion processes and slow speed fans for example), while it would be more correct to choose the definition “low-frequency noise” for other sound waves whose frequency is higher than 1/1,5 Hz. For this brief and non-academic text, we will often use the word infrasound (and infrasonic) for convenience even if, as explained, it isn’t the correct choice in some contexts.
From the whale’s song to the tiger’s roar
Several animals produce and perceive sounds in the infrasonic/low-frequency noise range. Blue and fin whales (Balaenoptera musculus and Balaenoptera physalus) “sing” to call each other and attract mates at very low frequency. Their long and intense sequences of infrasonic sounds have a long-range propagation especially in deep water and, during feeding season in high latitudes, females use infrasonic calls to guide other specimens to high food productivity areas. Humpback whales (Megaptera novaeangliae) communicate through “songs” whose frequency ranges from 20 to 100 Hz, these “marvelous to hear sounds” are simple but they travel hundreds of miles. Each species of whales use different array of sounds and the low-frequency vocalizations have also a geographic and seasonal variability. The most common explanations for whale’s songs are too simplistic and new studies show an astonishing complexity and differentiation (2).
It is now widely recognized (3) that elephants are able to communicate using extremely low frequency sounds and, a study conducted on a death individual in a zoo (Herbst et al.), examined the larynx and the collerated biomechanics of elephant vocal production, discovered that the infrasonic sounds are produced without a neural control but through a self-sustained vocal-fold vibration (that’s the same system that comes into operation when humans sing). Elephants can emit sounds below 20 Hz that can be detectable at a range of 10 km. The behavior of these enormous terrestrial mammals seems to suggest that they respond better and quicker to waves that go through the ground than those that travel in the air because low frequency noise oscillations would move faster in the solid material.
A research (4) conducted in 2003 on three Sumatran rhinos (Dicermoceros sumatrensis) at the Cincinnati Zoo, OH, recorded sounds produced by these animals that in the low-frequency area ranged from 4 to 17 Hz. Sumatran rhinos emit vocalizations described as eeps (70 – 4 Hz) and whistles/blows (17 Hz) that are similar to humpback whales signals and contains high level infrasound that are useful in their natural habitat, travelling fast and far in the dense tropical and mountain forests.
On 17 July 2014, biologists observed (5) a yacare caiman (Caiman yacare) that was producing infrasound during a “water dance” (male specimens vibrate their lungs at a low frequency and moves the water above their backs upward) while swimming in a flooded savanna in central Pantanal, Brazil. Among all species of crocodilians, only the gharial (Gavialis gangeticus) doesn’t seem to emit infrasound and it is probably the exception that proves the rule and researchers think that crocodiles and alligators perform the “water dance” associated with infrasonic sounds to long-distant communications and to impress females and rivals.
Whales, elephants, rhinos, crocodiles and alligators are not the only species that use and perceive low-frequency sounds, we could add a lot of examples but we would steer away from the purpose of this brief text: explaining the “magical effect” of the tiger’s roar.
The secret of the tiger
Elizabeth von Muggenthaler is a bioacoustician, president of Fauna Communications Research Institute (North Carolina, N.C.), and she has always studied the natural sounds and how they propagate in the air and underwater. She was a pioneer in investigating the infrasonic and ultrasonic communications of several animal species. On December 6, 2000, she presented her research (6) on tigers vocalization at the Acoustical Society of America Conference (ASA/NOISE-CON 2000) held at Newport Beach, CA. Von Muggenthaler and her team recorded twenty-two tigers at the Carnivore Preservation Trust (Pittsboro, N.C.) and the Riverbanks Zoological Park (Columbia, South Carolina, S.C.) and they discovered that most of sounds produced by tigers (growls, grunts and chuffles) had a frequency between 40 and 60 Hz but when they roared they created frequencies lower than 18 Hz (17.50 Hz). The researchers tried to stimulate the tigers playing back "low-frequency noise" sounds and they reacted, leaping and roaring towards the speakers.
Infrasound can travel for long distances and through solid objects such as buildings, caves, mountains and forests. It is clear the benefits of low-frequency sounds for tigers because, using infrasonic, they can communicate with each other even if separated by a dense wood.
“The lower the frequency, the farther the distance the sound can travel.” Von Muggenthaler said. “Scientists believe that infrasound is the missing link in studying tiger communication.”
Tigers are solitary hunters and their hunting territories can be extensive such as, for example, those of Amur tigers (Panthera tigris altaica) that rule the Ussuri taiga and occupy the immense deciduous forests of the Russian Far East. Infrasonic acoustic signals and sounds are perfect for long distance communication and biologists think that tigers have developed a complete (low-frequency) vocal repertoire that allows them to find mates, establish and mark hunting areas and, regarding females, to identify dangerous males and escape safely with the cubs.
In 2003, at the 145th Acoustical Society of America Meeting, held in Nashville, TN, Edward J. Walsh and colleagues presented their findings (7) regarding the acoustic communication in Panthera tigris (tiger). They analyzed the auditory brainstem response (ABR) from the scalp of Amur, Sumatran and Bengal tigers, a series of voltage peaks that show the electrical activity of the neurons that are distributed from the VIIIth nerve to the auditory cortex.
Walsh et al. noted that the tigers’ acoustic sensitivity decreased if the signals had a frequency around 32 kHz (kilohertz) and that the animals reached the maximum sensitivity to 500 Hz. This finding showed that Panthera tigris’ most sensitive “range of hearing” was (and is) located in that part of their audible spectrum which has the task to detect the low-frequency sound waves and, taking into account the size of the inner ears of these carnivores, it suggested that tigers could be able to hear acoustic signals in the infrasonic range.
Now listen…
These recorded series of vocal utterances illustrate that tigers produce sounds spectrally “full”, whose spectrum ranges continuously from infrasonic to ultrasonic frequency bands. Listen again and note the first three breathy sounds whose spectral band is nearly flat and then it extends to 800-900 Hz. The four roars that follow are characterized by a sequential increment, the first one possesses a frequency of about 700 Hz (similar to the breathy utterances) while the remaining three roars produce a peak around 300 Hz (closely related with the most sensitive portion of tiger’s “range of hearing”).
Three breathy growls followed by four brief roars characterize the next vocal sequence by an Amur tiger. The roars’ highest peak is at 300 Hz. You can listen to it down here:
And now we come back to the first roar we listened at the beginning of this text.
Listen to it again:
The quick roar is followed by a low growl and this kind of vocal production is used by tigers to face and intimidate rivals. The roar contains a considerable amount of energy in the low-frequency band but, at the same time, it is characterized by a notable acoustic energy in higher frequency bands as well. Analyzing these vocalization, Walsh and his team discovered that a tiger’s roar is “spectrally full” and it incorporates low-frequency sound waves.
The ultimate weapon of the tiger…and it’s science, not magic…
“When a tiger roars, the sound will rattle and paralyze you”, declared von Muggenthaler, “we suspect that it is caused by the low frequencies and loudness of the sound.”
Panthera tigris is present in habitats with limited visibility (dense jungles or forests) and the capacity to hear and emit low-frequency sounds could help this big cat to sense preys and then stun them before the deadly attack (G. T. Huang, J. J. Rosowski, and W. T. Peake, J. Comp. Physiol. A (2000). The infrasound generated by a tiger can paralyze the prey and sometimes it dies before the pantherid catches it. Even the diurnal primates known as Homo sapiens experiment the “magical effect” of the tiger’s roar, a frightening sensation produced (probably) by "low-frequency noise" that can cause a momentary paralysis.
Michael A. Persinger (Behavioural Neuroscience and Biomolecular Sciences Programs, Laurentian University, Sudbury, ON, Canada), described human beings as “both mechanical and energetic organisms.” According to him (8), infrasound can resonate with the human body and generate positive and negative effects as well. We have already seen that low-frequency sound waves can be produced by geomagnetic activity, winds, microseisms and by human made activities such as large machinery and duct systems but human beings are not always aware of being exposed to infrasound because they are often not audible. Without entering into details, we would limit to say that our muscles and body vibrations are within a range of 5 to 40 Hz and that there is a correlation between infrasound and the onset of nausea, fatigue, pain and sleep disturbances.
There is part of the tiger’s roar that we can’t hear, but we can feel it.
That part is correlated with the infrasound and their effect on human body and mind and on those of tiger’s usual preys. The low-frequency sound oscillations produce by a Panthera tigris’ roar is the ultimate weapon in this big cat’s arsenal and, as we have just discovered together, it has a scientific explanation.
In 1998, the overall tiger population was between 5000 and 7000 (Seidensticker et al.) but, due to indiscriminate hunting and habitat destruction and fragmentation, their number decreased to 3200 in 2009 (IUCN Red List). Today, the tigers left in the world are fewer than 4000, the last count, made in 2014, estimated an amount of 3159 individuals. Even if some subspecies, for example the Amur tiger, are recovering and their populations are growing, the total number of tiger declined of about 50% in 16 years and the species is still considered Endangered (EN).
Large carnivores are essential to the ecosystem but they are disappearing, the consequences of their demise will be devastating and will affect an incredible number of animal and plant species. Hearing a wild lion or tiger’s roar is like entering in a time machine and being transported to the past, when we weren't the dominant species and we feared the big cats, the “real” guardians of the natural order, the pillars of biodiversity.
The tiger's roar filled the cave with thunder. Mother Wolf shook herself clear of the cubs and sprang forward, her eyes, like two green moons in the darkness, facing the blazing eyes of Shere Khan. (9)
Rudyard Kipling, The Jungle Book.
Bibliography and citations:
- (1) Geoff Leventhall, What is infrasound?, Progress in Biophysics and Molecular Biology, Volume 93, Issues 1–3, 2007, Pages 130-137, ISSN 0079-6107, https://doi.org/10.1016/j.pbiomolbio.2006.07.006. (https://www.sciencedirect.com/science/article/pii/S0079610706000848)
- (2) Christopher W. Clark, Baleen whale infrasonic sounds: Natural variability and function, The Journal of the Acoustical Society of America, May 2004 115(5):2554, https://asa.scitation.org/doi/full/10.1121/1.4783845
- (3) Herbst, Christian T., Stoeger, Angela S., Frey, Roland, Lohscheller, Jörg, Titze, Ingo R., Gumpenberger, Michaela, Fitch, W. Tecumseh, How Low Can You Go? Physical Production Mechanism of Elephant Infrasonic Vocalizations, Science , 03 Aug 2012: Vol. 337, Issue 6094, pp. 595-599 DOI: 10.1126/science.1219712 http://science.sciencemag.org/content/337/6094/595.abstract
- (4) Muggenthaler, Elizabeth & Reinhart, Paul & Lympany, Brad & Craft, R.. (2003). Songlike vocalizations from the Sumatran Rhinoceros (Dicerorhinus sumatrensis). Acoustics Research Letters Online-arlo - ACOUST RES LETT ONLINE-ARLO. 4. 10.1121/1.1588271. https://asa.scitation.org/doi/...
- (5) Christopher P. Kofron, Patricia A. Farris, Infrasound production by a yacare caiman Caiman yacare in the Pantanal, Brazil, Herpetology Notes, volume 8: 385-387 (2015), https://www.biotaxa.org/hn/article/view/10993
- (6) Elizabeth von Muggenthaler, Infrasonic and low‐frequency vocalizations from Siberian and Bengal tigers, The Journal of the Acoustical Society of America 108:5, 2541-2541, DOI: 10.1121/1.4743417, https://asa.scitation.org/doi/abs/10.1121/1.4743417
- (7) Walsh, Edward & Armstrong, Douglas & Napier, Julie & Simmons, Lee & Korte, Megan & McGee, Joann. (2008). Acoustic communication in Panthera tigris: A study of tiger vocalization and auditory receptivity revisited. The Journal of the Acoustical Society of America. 123. 3507. 10.1121/1.2934400. https://digitalcommons.unl.edu...
- (8) Persinger, M.A. Infrasound, human health, and adaptation: an integrative overview of recondite hazards in a complex environment. Nat Hazards 70, 501–525 (2014). https://doi.org/10.1007/s11069-013-0827-3
- (9) Rudyard K., The Jungle Book, Penguin Books LTD, Penguin Classics, London 2013
