Cows moo, lions roar, and pigs oink. But for many years it's been assumed that, except for the occasional snort, giraffes spent most of their lives in a tight-lipped silence. New research from a group studying animal sounds at the University of Vienna suggests giraffes might not be so quiet after all: They spend their evenings humming.

For decades zookeepers reported occasional snorts as the only sounds their charges made. The conventional explanation was that the long necks of giraffes caused their taciturn nature. Giraffes do have a larynx (voice box), but perhaps they couldn't produce sufficient airflow through their 13-foot long (4 meter) trachea to vibrate their vocal folds and make noises.

The researchers suspected the reason no one heard giraffe communication was because the sound frequency was too low for humans to hear. Elephants and other large animals use an ultra-low frequency "rumble" for long-distance communication; why not giraffes?

So they recorded giraffes at three zoos, compiling more than 938 hours of giraffe sounds over an eight year period. That's a lot, and plowing through all that data seems to have worn on the researchers a bit. In their paper they note "exploring giraffe vocal communication turned out to be time consuming, tedious and very challenging ... we strongly suggest developing an automatic system that helps analyzing great amount[sic] of acoustic data."

What made it so very tiresome was the researchers didn't listen to the recordings, they visually examined them. They were looking for specific patterns of low-frequency sounds with harmonic structure. They found them, and the pattern is distinctive:

Baotic et al. 2015

The humming sounds only occurred at night, and the average hum was about 92Hz in frequency, which isn't "infrasound," but still on the low end for human hearing. When the researchers played the sounds back to giraffe keepers and zoo managers, they'd never heard them before.

What does this rare giraffe communication sound like? It sounds remarkably like my ex-husband snoring:

Giraffes have excellent vision, so their primary means of communication is thought to be visual signals during daylight hours. As prey animals, it also makes sense that they might not want to make loud noises that can attract the attention of predators. But when vision is impaired at night, low frequency humming might be a great way to make sure the herd stays together.

I wasn't able to reach the researchers themselves, but I did talk to Christopher Basu of the Royal Veterinary College, University of London about humming giraffes and anatomy. He told me the giraffe larynx (which includes the vocal folds; the things that vibrate to make noise) is similar to the horse. Giraffes have the correct hardware to make vocal noises, but they just don't. Or, we thought they didn't, until this new research.

The scientists calculated the relationship between vocal fold length and sound production, and giraffe humming fell roughly where it should have. Giraffes actually hummed a bit more vigorously than predicted, which suggested some very interesting questions about their 13-foot trachea to Basu:

"To my knowledge no one has ever measured air speed when giraffes exhale! It would be a fantastic experiment, you’d have to persuade a giraffe to blow into a straw."

What are giraffes saying with their nocturnal humming? We don't know. The new research identified the sounds, but provides no context for matching them to behavior, since giraffes only hum in the dark. Matching the sound production to what's happening in a giraffe society is the next step, and will require some fairly high level night-vision instrumentation. And some really dedicated researchers to plow through reams of giraffe data.

Maybe in this case, they are humming because we don't know the words.

Anton Baotic, Florian Sicks and Angela S. Stoeger. 2015. Nocturnal “humming” vocalizations: adding a piece to the puzzle of giraffe vocal communication. BMC Research Notes 8:425. doi:10.1186/s13104-015-1394-3.

Fred B. Bercovitch & Philip S. M. Berry. 2013. Herd composition, kinship and fission–fusion social dynamics among wild giraffe.

African Journal of Ecology. Volume 51(2):206–216 DOI: 10.1111/aje.12024

If you have a strong stomach, you can see an entire dissection of a giraffe's neck here.