Giraffes are not as silent as previously thought. Though it had been assumed that they communicated through infrasonic calls too low for human ears (as elephants do), researchers did not have any concrete evidence that these long-necked animals used vocalizations to communicate with each other at all. 

Yet giraffes are actually pretty chatty, new research in the journal BMC Research Notes suggests. Researchers from the University of Vienna and Teirpark Berlin, a German zoo, recorded almost 950 hours of audio from three different zoos to determine whether giraffes talk to each other. They found that in addition to the snorts and grunts that had previously been observed (but not known to be communication, as such), giraffes make humming noises at night. 

Over the course of their research, they observed 65 humming vocalizations, exclusively heard at night. They report that the hums were “rich in harmonic structure,” with a “deep and sustained sound.” Hear an example below: 

The study is not definitive about what these hums might mean for a giraffe. But because they were harmonic and varied in frequency (as opposed to the atonal grunts previously observed), they could possibly convey information about the caller to other giraffes. The researchers suggest that “in giraffe communication the ‘hum’ might function as a contact call, for example, to re-establish contact with herd mates" when they are out of visual range, like when it's dark. 

[h/t: Washington Post]