Giraffes are high-profile animals, yet they're typically seen more than heard. Not only are they literally hard to overlook, but they're famously quiet. Aside from assorted snorts and grunts, these stately mammals mostly seem like the strong, silent type.

But according to a new study, we might just need to listen more closely. A team of biologists has recorded giraffes at three zoos humming at night, a vocalization they describe as "rich in harmonic structure, having a deep and sustained sound."

Before this, it had been suggested giraffes don't vocalize because they can't generate enough airflow in their 6-foot necks. Scientists had also begun to suspect the animals do produce infrasonic sounds inaudible to humans, like elephants do, despite inconclusive evidence. To test that idea, biologists from the University of Vienna and Tierpark Berlin recorded more than 900 hours of audio from giraffes at three European zoos, then scoured the data for signs of infrasonic noise.

While they didn't find any infrasound, they did stumble upon something potentially even more interesting: a low-frequency vocalization that's quiet, yet still within the range of human hearing. Here's what a giraffe humming sounds like:

The hums only occurred at night, with an average frequency of about 92 hertz. No one was there at the time to confirm the source, but the researchers say they're confident these sounds came from giraffes. "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," they write in the journal BMC Research Notes.

There's no video to go with the audio, either, so it remains unclear what the giraffes were doing as they hummed. But due to the harmonic structure and changes in frequency, the researchers point out these sounds at least have the potential to convey information — and thus could be a form of communication.

Wild giraffes have complex social structures, as recent studies have shown, and they seem to live in fission-fusion societies — a trait also seen in elephants, dolphins, chimpanzees and other social mammals that vocalize to communicate. Because most of the captive giraffes in this study were separated from the rest of their herds at night, the authors say humming may be an attempt to stay in touch.

wild giraffesWild giraffe society is complex, but so far humming has only been recorded in zoos. (Photo: Soaring Flamingo/Flickr)

"These patterns provide suggestive hints that in giraffe communication the 'hum' might function as a contact call, for example, to re-establish contact with herd mates," they write. But it's also possible the giraffes were asleep when they made the sounds, as one psychologist who wasn't involved in the study tells New Scientist.

"It could be passively produced — like snoring — or produced during a dream-like state — like humans talking or dogs barking in their sleep," says Meredith Bashaw, a psychology professor at Franklin & Marshall College in Pennsylvania who has also studied social behavior among giraffes in captivity.

For now, no one knows why these giraffes hum at night. More research is needed, both to see what captive giraffes are doing while they hum and to learn if their wild relatives make similar noises. This new audio doesn't rule out the possibility that giraffes also communicate via infrasound, the study's authors note, since other animals often use infrasonic signals for long-distance communication. While that's likely useful on the savanna, it may be unnecessary even in the largest zoo.

Still, this research seems to finally prove giraffes aren't as tight-lipped as we thought. And since their wild populations have plummeted 40 percent in the past 15 years — a trend some conservationists call a "silent extinction" due to its relative lack of publicity — it's now more important than ever that we don't tune them out.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.