Researchers at the University of Vienna are studying vocalizations from giraffes. They captured recordings of the animals "humming" during the night at local zoos. Zookeepers were surprised to learn the animals made such noises as they are not heard during the day. Researcher Dr. Angela Stöger remarked to the New Scientist, "I was fascinated, because these signals have a very interesting sound and have a complex acoustic structure." However, it remains to be determined whether the animals are snoring, humming in their sleep, or actually communicating with one another. You can listen to a recording of their nocturnal sounds here.