Giraffes do have a larynx (voice box), but perhaps they can't produce sufficient airflow through their 13-foot long (4 meters) trachea to vibrate their vocal folds and make noises.
What does the giraffe say? For decades it's been a simple answer: nothing, except for a snort or grunt every now and then. Though giraffes have a voice box, one line of thought was that due to their long necks it was too difficult for the creatures to generate the airflow needed to "vibrate their vocal folds."
But University of Vienna researchers now say they've determined that giraffes do indeed "produce vocalizations" that may serve as a means of communication.
The researchers recorded animals at three European zoos, amassing 947 hours of recordings over eight years. They then analyzed the recordings visually, in a process, they describe in BMC Research Notesas "time consuming, tedious, and very challenging." They believed the animals might produce "infrasonic vocalizations" — that is, below the level of human perception—and were looking for such low-frequency sounds.
And they did indeed find "structurally interesting humming vocalizations" that occurred mainly at night and hovered around 92 hertz in frequency; as one New Scientist commenter notes, they sound a little like the Kraken (listen for yourself here). The hum isn't infrasound, but it's not exactly easy to hear either; Wired notes the researchers shared the vocalizations with zookeepers, and the sound was unfamiliar to them.
Though the researchers couldn't prove the sound is used for communication, they found "suggestive hints" that the hum might serve as a "contact call, for example, to re-establish contact with herd mates." They speculate the hums may be occurring at night because giraffes' typically keen vision is less effective then. "Future studies should test in a well established experimental setting whether giraffes are more vocal when visual communication cues are absent," they conclude.