The high-profile animals, Giraffes, are generally seen more than heard. Besides assorted snorts and grunts, most of the times, these stately mammals seem like the strong, silent type. But, a study has suggested that human beings might just need to listen more closely to hear humming Giraffes.

A group of biologists has recorded giraffes at three zoos humming at night, a vocalization, which they described as 'rich in harmonic structure, with a deep and sustained sound'.

Previously, it was suggested that giraffes don't vocalize as they can't generate sufficient airflow in their 6-foot necks. Scientists even started suspecting that the animals do produce infrasonic sounds inaudible to humans, just like elephants, in spite of uncertain proof.

For testing that idea, biologists from the University of Vienna and Tierpark Berlin recorded over 900 hours of audio from giraffes at three European zoos, scouring data for signs of infrasonic noise.

Although they didn't find any infrasound, they did notice something potentially even more interesting. They observed a low-frequency vocalization that's quiet, yet belonged to the range of human hearing.

Giraffes hummed only at night, with an average frequency of around 92 hertz. At the time of recording, there was nobody to confirm the source, but the researchers said that they are fully confident that the sounds came from giraffes.

They wrote in the journal BMC Research Notes, "Although we could not identify the calling individuals, the giraffes definitely produced the recorded sounds because we documented similar vocalizations in three different institutions without any additional co-housing species".