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The researchers didn't take videos of the humming giraffes, so it's unclear exactly what the animals were doing at the time. But Stoeger has a hunch the eerie sound is a method of communication — an idea that needs to be verified in future studies, she said.
"Giraffes have excellent vision and use visual signals for communication a lot," she said. "However, it might be possible that once vision is limited — for example, at nighttime — and they are separated, that they might use vocalizations to keep contact."