Giraffes have been thought to be relatively quiet animals because of how physically difficult it is for sound to travel up their long necks. While some have suggested that giraffes may make noise in infrasonic sounds that are too low for humans ears, the humming that Dr. Stöger picked up were just high enough for humans to hear.
“I was fascinated, because these signals have a very interesting sound and have a complex acoustic structure," she told New Scientist.
You can listen for yourself, thanks to a recording courtesy of New Scientist:
The giraffes hum at a frequency of about 92 Hz, which is low in comparison to other animals.
However, Dr. Stöger and her colleagues weren't able to observe the giraffes as they hummed, so it's not certain what they were doing as they hummed — possibilities range from snoring to mating to trying to communicate with each other.
“It could be passively produced -– like snoring –- or produced during a dream-like state -– like humans talking or dogs barking in their sleep,” Meredith Bashaw, an animal behavior researched told New Scientist. It could also be a way for the giraffes to signal to each other, "Hey, I'm here," she added.
John Doherty at Queen’s University Belfast thinks he might have an answer: “I have once come across audible vocalization reminiscent of [the] recordings, again in a captive giraffe."
“But, in this case, [the giraffe] was clearly disturbed by a husbandry procedure being carried out on its calf in a separate but visible enclosure.”
His reasoning, however, doesn't explain why the same kind of humming was heard at three different zoos.
We might not know what giraffes are doing or saying while they're nocturnally humming, but it's nice to imagine that when all the zookeepers have gone home for the night, the animals are talking.
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