London, July 8 : Spotted hyenas are well known for their laughs, but researchers at the University of California in Berkeley, US, have just determined that in intimate social situations, the carnivores often groan softly to one another.
The findings highlight what a complicated vocal repertoire these very social animals possess.
Besides this, each hyena has a rich range of vocal calls, including loud whoops and giggles.
For the study, Frederic Theunissen and colleagues focused on hyena groans.
To elicit the sounds, the researchers presented hyenas at the Berkeley Field Station for Behavioral Research with three things: meaty bones, unfamiliar spotted hyena cubs and an empty transport cage used to contain bones or cubs during other experiments.
The adults groaned more at the cubs than the objects, with cub groans sounding much more melodic and gentle.
When the animals groaned at the bones and the cage, the vocalizations were less tonal and had a lower identified frequency.
"They shift their vocalisations to be even higher in pitch. The data fits with the general idea that low frequency sounds are more threatening and higher frequency sounds are more submissive," New Scientist quoted Theunissen, as saying.
According to Theunissen, the high-pitched, musical groans are designed to reassure the cubs.
The preliminary results of the study suggests that there are at least two classes of hyena groan, but further study may show the groans to be at least as expressive as the hyena whoops and laughs, he said.
The study has been presented at the Acoustics'08 Paris conference in France on 3 July.