Hyenas are famous for their loud laughs, but hyena groans may be an equally important facet of their vocal repertoire.
The has a rich range of vocal calls, including loud whoops and giggles. But in intimate social situations, hyenas often groan softly to one another.
Listen to hyenas groaning to their cubs.
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team at the University of California in Berkeley, US, is using a captive hyena colony in Berkeley Hills to study those groans, which are too subtle to record in the wild.
鈥淸The noises] must communicate something,鈥� says Theunissen. 鈥淗yenas groan every time they approach something of interest.鈥�
Theunissen鈥檚 team recorded hyena groans as the adults approached either an empty cage, a cage containing meaty bones, or a cage containing unfamiliar hyena cubs.
Listen to hyenas groaning to an empty cage and to meaty bones.
Cub care
As both males and females approached the cage, they groaned more frequently if it contained cubs. The calls to the cubs were also higher in pitch and more musical.
Theunissen also recorded the groans of mothers as they approached their own cubs, although there were only two mothers with cubs in the pack at the time of the study.
鈥淭hey shift their vocalisations to be even higher in pitch,鈥� says Theunissen. 鈥淭he data fits with the general idea that low frequency sounds are more threatening and higher frequency sounds are more submissive.鈥�
He thinks that the high-pitched, musical groans are designed to reassure the cubs.
Theunissen鈥檚 preliminary results suggests 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 says.
Community living
Those laughs and whoops are already known to be individualistic 鈥� each hyena can be identified by its laugh 鈥� and Theunissen is studying whether the groans are as idiosyncratic.
鈥淲hen a hyena groans towards another, there鈥檚 usually visual contact at the time,鈥� he says, which suggests groans need not communicate identity.
鈥淏ut interestingly, all of the cubs live in a communal den and a mother groans to summon just her own cubs,鈥� Theunissen says. 鈥淪o there must be something recognisable in the groans.鈥�
at Michigan State University in East Lansing, US, isn鈥檛 surprised that Theunissen鈥檚 work has revealed variation in hyena groans. 鈥淪potted hyenas live in very complex societies, and they have a long, slow development during which cubs must form many different social relationships,鈥� she says. 鈥淕roans may very well play a role in that socialisation process.鈥�
Theunissen presented his results at the in France on 3 July.
