Elephants trumpet, horses winey, owls screech, and dogs bark. Any young grade-schooler could proudly share this knowledge (while another 500 million folks can tell you what the fox says). However does anyone know what the giraffe does? Now experts have found out that while the tallest residents of your favorite zoo might not say anything, they sure do a lot of humming.

That's at least according to a study recently published in the journal BMC Research Notes, which details how  giraffes are surprisingly quiet animals. After collecting nearly 1000 hours of audio material in three European zoos, a trio of researchers from Austria and Germany determined that captive Giraffes do not communicate with any obvious contextual calls or other vocal behaviors.

"Although giraffes do have a well-developed larynx and laryngeal nerves, it was long suggested that due to the long neck, giraffes might have problems to produce an air-flow of sufficient velocity to induce self-sustained vocal fold vibrations," study co-author Angela Stoeger, a researcher with the University of Vienna, explained.

In fact, some of the only recorded instances of newborn giraffes and their mothers vocalizing to one another is when the pairs are separated, and what is heard are instinctual bleats of alarm, not consistent communication. That's apparently unusual even among hoofed mammals, where early vocalizations among goats and deer are known to be important for bonding and recognition. (Scroll to read on...)

One theory was that giraffes are simply communicating on a frequency so low that it is inaudible to humans (infrasound). Interestingly, Stoeger and her colleagues' observations show that this isn't the case.

However, they did find that as an evening goes on and light ebbs, these remarkable animals do start to hum.

"Interestingly, these vocalizations have so far been recorded only at night," wrote Stoeger. "Even giraffe keepers and zoo managers stated that they have never heard these vocalizations before."

And while that while humming isn't infrasound, it occurs at an exceptionally low frequency (~92 Hz) making it difficult to hear. This could explain why it has gone unnoticed by the animals' caretakers.

So what then, could this humming be for? Stoeger and her colleagues are hesitant to call this a means of communication. In fact, they note that because giraffes have a naturally high vantage and exceptional vision, body language can be sufficient even at long distances.

The humming then might be a means for reassuring one another that they are nearby, even as the night's darkness renders even superior vision useless. And that wouldn't be too much of surprise. Just last Oct, researchers determined that mostly-silent fish larvae even 'baby talk' in the dark, while cranes change how they call to one another when flying through thick fog. (Scroll to read on...)

But while this humming might sound reassuring to giraffes, New Scientist's Karl Gruber notes that not everyone will welcome it come nightfall.

Residents living just outside of Paignton Zoo in southwest England have reported a "low frequency noise" escaping the zoo's giraffe house at night.

"It is a humming or droning noise that at times it can be quite loud," one resident, Peter Thorne, complained to the local Herald Express.

"I am very tired," he added. "I am being disturbed in the night and am being kept awake by this."

Still, according to Gruber, the zoo is hesitant to tie the complaint to their giraffes.

"The image of our giraffes humming happily to themselves all night is a delightful one," a representative told New Scientist, with Stoeger adding that it would be very unlikely that even a sensitive listener would hear the humming of giraffe nightlife several houses away.

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