Birds chirp and lions roar but did you know that giraffes hum?

Audio recordings taken at three European zoos reveal that giraffes produce a “low-frequency vocalization with a rich harmonic structure of varying duration.” The catch? Giraffes only hum a night.

Researchers know very little about giraffe auditory communication and it isn’t clear why the animals hum, much less why they only hum at night. Giraffes have excellent eyesight, and researchers think that the nighttime humming may be a way for the animals to communicate once darkness has fallen and their eyesight is impaired.

Regardless of the reason, the researchers reported that once the sun sinks below the horizon, some giraffes produce an “eerie tune that sounds like a creaking ship.”