Giraffes hum to each other at night

Researchers have discovered that giraffes may communicate with each other by humming at low frequencies in the night.
It has been speculated that giraffes are unable to produce any substantial sounds because it is physically difficult for them to generate sufficient airflow through their long necks to produce vocalisations.
Some biologists have suggested giraffes use low frequency 'infrasonic' sounds that are below the level of human perception.
After reviewing almost 1,000 hours of sound recordings in three European zoos, Angela Stoger at the University of Vienna, Austria, found no evidence of infrasonic communication, 'New Scientist' reported.
She, however, picked up a weird humming coming from the giraffe enclosures in all three zoos at night.

The 'hum' turned out to be a low frequency sound, of about 92 hertz. It is not infrasound and humans can hear it unaided. Stoger and her colleagues said the hum varies in duration and contains a rich combination of notes.

Giraffes have a socially structured system, and for a long time scientists have been trying to figure out how they communicate, said Meredith Bashaw at the Franklin & Marshall College in Lancaster, Pennsylvania.
"This new vocalisation could add a piece to that puzzle," she said.
"It could be passively produced - like snoring - or produced during a dream-like state - like humans talking or dogs barking in their sleep," she said.

Alternatively, it could be a way for giraffes to communicate with each other in the dark, when vision is limited.