Photo by Hans Hillewaert: A bull Southern savannah giraffe
close to Namutoni, Etosha, Namibia
Do you ever wonder what is happening in frequency ranges outside of your hearing range?
In September, 2015, various articles announced apparently giraffes communicate by low-frequency humming.
"After Dark, Giraffes Carry A Tune" by Vicki Croke, September 22, 2015 can be viewed at:
Scientists have usually described giraffes as tall silent types. But a new study from the University of Vienna shows that that description was only half right. It turns out that under cover of darkness, zoo giraffes do something odd…they hum.
We’ve known that giraffes make some noise. Zoo keepers have always reported hearing the quiet, shy animals snorting or grunting on occasion during the day, but scientists reviewing nearly a thousand hours of audio material collected from three European zoos, from both day and night, discovered something significant—“harmonic, sustained and frequency-modulated ‘humming’ vocalizations during night recordings.”
The study cited is apparently:
"Nocturnal 'humming' vocalizations: adding a piece to the puzzle of giraffe vocal communication" by Anton Baotic, Florian Sicks, and Angela S. Stoeger
In the study, the authors are cautious about inferring giraffe communication via humming:
...It has been suggested (again rather anecdotally) that giraffes do communicate using infrasonic vocalizations (the signals are verbally described to be similar—in structure and function—to the low-frequency, infrasonic “rumbles” of elephants) [27, 30]. It was further speculated that the extensive frontal sinus of giraffes  acts as a resonance chamber for infrasound production. Moreover, particular neck movements (e.g. the neck stretch) are suggested to be associated with the production of infrasonic vocalizations.
Despite these reports of giraffe sounds, there is no clear evidence that giraffes indeed use acoustic signals to communicate with each other . Acoustic communication describes the interchange of information between at least two individuals, where an acoustic signal (typically a vocalization) is being directly transmitted from a sender and perceived by a receiver, that alters the behaviour of the communicating animals [33, 34]. Although grunts and snorts are produced in agonistic interactions [28, for personal observation for a grunting adult female giraffe see Additional file 1], it is unclear what role the acoustic signals play compared to the visual, tactile and olfactory cues....
...In this study we analysed hundreds of hours of acoustic recordings from captive giraffes in three institutions and documented a sound that has never been structurally described in the scientific literature before—the “hum”. Although we could not identify the calling individuals, the giraffes definitely produced the recorded sounds because we documented similar vocalizations in three different institutions without any additional co-housing species.
The “hum” is a low-frequency vocalization with a rich harmonic structure and of varying duration. Since it was not possible to determine the calling individual, we are currently unable to prove that this sound is indeed used for communication or to give information about the behavioural context and prospective information content. Although we cannot provide behavioural data, we would like to note that at all 3 zoos all giraffes where kept under similar housing conditions during night times. At Copenhagen Zoo the pregnant cow was separated from her herd, while at Vienna Zoo the giraffe bull was kept separate from the rest. Berlin Tierpark kept each giraffe in an individual stall, however calves where kept together with their mothers. At Copenhagen Zoo hums occurred approximately within 2 h before sunrise, while at the other two zoos, hums occurred mainly in the middle of the night. These patterns might provide suggestive hints that in giraffe communication the “hum” might function as a contact call, for example, to re-establish contact with herd mates.
Nonetheless, the rich harmonic structure and the frequency modulation indicate that this type of vocalization has the potential to convey relevant information to receivers.
Interestingly, these vocalizations have so far been recorded only at night. Even giraffe keepers and zoo managers stated that they have never heard these vocalizations before. Anatomical investigations indicate that giraffes have excellent vision with potentially long-range visual acuity, which would provide a means of communication between widely separated conspecifics . Recent social behaviour research has shown that giraffes spend a significant portion of their vigilance towards social partners , suggesting that perception and utilization of visual communication cues are highly developed in the giraffe communication system. Giraffes might use vocalizations more often once vision is limited (e.g. at night time). Future studies should test in a well established experimental setting whether giraffes are more vocal when visual communication cues are absent.
We found no evidence for giraffe infrasonic communication in our data set even though it is widely assumed that giraffes communicate in this manner. The lack of systematic assessment, detailed spectrographic descriptions and presentations or sound examples of giraffe infrasonic signals have not prevented researchers from suggesting adaptive explanations (e.g. keeping vocal contact) or from accepting as fact the idea that giraffes produce infrasound (via Helmholtz resonance, not vocal fold production) to communicate [23, 30–32]. We concede that giraffes in captivity, housed within the same enclosure, might not need to use infrasonic signals to communicate (such signals may be used mainly for long-distance communication when vision is eliminated)....
If you have watched a beautiful peacock display, perhaps, like me, you will be surprised that there is an infrasound effect from the display.
Photo by Loke Seng Hon: Hybrid of mostly Pavo cristatus
and some Pavo muticus blood
“Peacocks’ tails make noises too low for humans to hear: The tails of peacocks aren’t just pretty to look at. They also make infrasound that humans can’t hear” by Michael Marshall, 3 March 2015:
Freeman and Hare recorded the sounds made by 46 displaying peacocks. "They shake the feathers," says Freeman. "It just kinda looks like they're vibrating."
To human ears, it sounds like "rustling grass in the wind". But the recordings revealed that the shaking tail feathers also made noises too low-pitched for humans to hear. This infrasound was loud, between 70 and 108 decibels.
When Freeman and Hare played the recordings back to other peafowl, both males and females became more alert and spent more time walking or running. Also, "the males would call in response to these signals," says Freeman....
...A peacock's large tail is a signal of strength and good genes, essentially telling females "I'm in such good condition, I can afford to carry this enormous heavy tail behind me".
The infrasound might reinforce that message, says Freeman. "If you don't have a tail you can't produce the sound," she says.
Producing low-frequency sound might seem pointless, but in peafowls' natural habitat it makes sense. They live in scrubby woodland, so high-pitched calls get muffled even over quite short distances.
"Oftentimes they can't see each other from where they're displaying," says Freeman. "We think that possibly these signals could signal to females and males who are out of sight behind a bush or over the hill."...
The study referred to is apparently:
"Infrasound in mating displays: a peacock's tale" by Angela R. Freeman, , James F. Hare, Department of Biological Sciences, University of Manitoba, Winnipeg, MB, Canada
The following video includes pied peacocks' mating dance displays:
To be continued...