Until now, giraffe caretakers had reason to believe that their long-necked vegetarian friends were strictly silent beings. A 13-foot-long trachea isn’t exactly conducive to easy vocalization.
Scientists assumed that, if anything, giraffes—like elephants—might produce infrasonic (ultra-low) sounds below the range of human hearing. But without the data to back it up, researchers couldn’t guarantee that giraffes weren’t simply producing audible noises out of human earshot. So a team at the University of Vienna painstakingly gathered 947 hours of giraffe noises over an eight-year period at three European zoos and measured their spectral characteristics, with the goal of finding out once-and-for-all whether giraffes’ socially-structured society lends itself to vocal communication.
What they found was that at night, the giraffes produced a harmonic and sustained “humming” that varied in frequency over time. Not only were they humming, but these vocalizations were within the frequency range that humans can hear (albeit on the low side, at an average of 92 Hertz).
“Based on their acoustic structure,” the scientists report in the journal BMC Research Notes, these vocalizations might function as “communicative signals to convey information about the physical and motivational attributes of the caller.”
Here’s Gwen Pearson, writing for Wired:
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.
Though the scientists have not yet correlated these sounds to behavior (that will require more extensive research involving night-vision technology), they do know that vocalizations in other species with similar social structure is known to convey information about age, gender, sexual arousal, dominance, and more. The specific qualities of giraffe sounds could lead to further insights on how these animals interact with each other and with other species.