Lions roar, wolves howl and elephants trumpet. But what do giraffes do to communicate? They "hum", new research suggested on Wednesday - and only at night.

Unlike other similar species, giraffes were not thought to be strong vocal communicators, producing nothing more profound that an occasional snort or grunt, possibly because of their long necks.

But scientists led by Angela Stoeger at Vienna University made 947 hours of recordings at three European zoos and found instances of a kind of a humming sound with a "rich harmonic structure".

Almost all of the humming occurred at night, with even zookeepers saying they have never heard such noises before, the findings published in scientific journal BioMedCentral showed.

Further research is needed, but the findings suggested that the "hum" might help members of a giraffe herd stay in touch when they can no longer see each other.