Giraffes are known to be quiet animals, and for a long time, it was believed that their necks were too long for them to produce sounds. However, recent research has shown that giraffes do make sounds, including low-frequency humming noises. These humming sounds are quiet and within the range of human hearing, and they are believed to be a form of "contact call" between individuals who have been separated from their herd, helping them to find each other in the dark. It has also been suggested that the humming is giraffes snoring or sleep-talking. In addition to humming, giraffes are known to snort or hiss when threatened, and female giraffes bellow to their young.