What does the giraffe say? Until now, they were only known to make a few grunts, snorts, and bleats. Most of their sounds seem to come from sudden bursts of air out their nostrils. But after listening to nearly 1,000 hours of audio, researchers discovered that giraffes make low-frequency humming noises at night. The findings were published in BMC Research Notes this week. 

Giraffe herds are organized into the same sort of social system as talkative animals like spotted hyenas and chimpanzees who use vocal communication to facilitate group dynamics. However, there has been no evidence that giraffes use vocalizations to communicate. While they do have a well-developed larynx (or voice box) and laryngeal nerves, their long necks were thought to impede air-flow with sufficient velocity to produce self-sustained, vocal fold vibrations. 

To investigate, the University of Vienna’s Angela Stoeger and colleagues collected 947 hours of audio during the day and night from captive giraffes living in three European zoos: Berlin Tierpark, Copenhagen Zoo, and Vienna Zoo. When they analyzed various components of the acoustic signals contained in the recordings, the team detected harmonic, sustained, and frequency-modulated “humming” vocalizations. A total of 65 humming vocalizations were recorded during night sessions, with an average frequency of 92.01 Hz. (You can listen to giraffes humming here.) 

“I was fascinated, because these signal have a very interesting sound and have a complex acoustic structure,” Stoeger told New Scientist. However, because they weren’t able to identify the caller or verify the behavioral context of the calls, the researchers don’t know what sorts of information would be contained in those hums. Vocalizations in other animals with a similar social structure are known to convey information about age, gender, sexual arousal, dominance, or reproductive status.

So why only at night? It’s possible that giraffes communicate visually during the day; after all, they do have excellent vision with potentially long-range acuity, the authors write. 

Furthermore, none of the nocturnal vocalizations they recorded were within the infrasonic range, below the normal hearing range of humans. Anecdotal evidence have previously suggested that giraffes communicate using the same sort of low-frequency, infrasonic “rumbles” of elephants.