Late at night, when humans are rarely around, giraffes do something that until now has never been documented.
Researchers attempting to document the sounds made by giraffes recorded almost 1,000 hours of audio at three different European zoos, even leaving their recording equipment in the enclosures at night.
And there, near the edge of human hearing, the researchers picked up the unusual low sounds giraffes make at night -- the mysterious hum.
New Scientist posted audio of one of the 65 "hum" recordings:
The scientists aren't sure why the animals are humming, just that they are and that it only happens at night.
Even zookeepers, they note, have never heard the animals hum.
Giraffes have not been known for their wide range of vocalizations. Describing the animals as "taciturn," the researchers wrote in the journal BMC Research Notes that most sounds identified so far have been described as a "bleat," "brrr," "burst," "cough," "growl," "grunt," "low," "moan," "moo," "sneeze," "snore" or "snort."
The hum is something new, and it may be a form of late-night communication as each zoo had at least one animal separated from the herd at night.
At one location, the creatures were placed in separate stalls. In another, a pregnant female was separated from the herd while the third zoo kept a bull from its group at 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," the researchers wrote. "Nonetheless, the rich harmonic structure and the frequency modulation indicate that this type of vocalization has the potential to convey relevant information to receivers."
But there is another possibility.
"It could be passively produced -- like snoring -- or produced during a dream-like state,” Meredith Bashaw at the Franklin & Marshall College in Lancaster, Pennsylvania, who was not part of the study, told New Scientist. "Like humans talking or dogs barking in their sleep."
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