By Jason Palmer
Science and technology reporter, BBC News
The captive animals' vocalisations could be studied more closely than in the wild
Researchers have begun to unravel the information and social content present in the hyena's famed laugh, which they say is only used in times of conflict.
The pitch and variability of the giggles may be used to indicate age or social status, they say.
Younger hyenas tend to have high-pitched giggles, and dominant females of the strongly hierarchical clans tend to have a narrower range of sounds.
The work will be reported at a meeting of the Acoustical Society of America.
The rich social structure of hyena clans gives rise to many vocalisations, ranging from "whoops" that travel great distances to quiet grunts among close individuals.
But it is the laugh of spotted hyenas (Crocuta crocuta) that has given them their more common moniker of "laughing hyenas".
"This is a very complex society of nocturnal animals, so acoustics is a very important communication channel for them," says Nicolas Mathevon, who led the research.
It has been hypothesised by researchers studying hyenas in the wild that the laughing is not, in fact, a sign of good humour.
Yet it remains unclear what social information the short fits of laughter - or giggles, as the researchers call them - convey.
Pitch and role
We guess it's a sign of frustration
To investigate that, Nicolas Mathevon and colleagues at the Field Station for the Study of Behavior, Ecology and Reproduction (FSSBER) at the University of California, Berkeley studied the calls of individual hyenas.
The group of 17 hyenas was recorded when individuals were in competition for a limited amount of food - a time when they are known to emit the famous giggles.
The team were able to record the vocalisations of individual hyenas, analyse their frequency components and compare them.
They found that the giggles' fundamental frequencies were fairly specific to each individual; given a giggle of unknown origin, the team could guess about half the time the individual from which it came.
The giggles probably advertise an individual's age, because the fundamental frequencies tended to decline among older individuals. Before they reach maturity at three years of age, hyenas have noticeably higher-pitched giggles.
And the giggles are likely to be a sign of a given individual's social status; the team found that subordinate females tended to be more variable.
That is, there were more "vowel"-type sounds within a given bout of giggles.
Dominant females - the leaders of the clan - seemed to advertise their powerful role by not giggling as much.
"During the competition between the animals, subordinate animals emit far more giggles than dominant ones," Dr Mathevon told BBC News.
"We guess that it's a call of frustration," he said.
Dr Mathevon notes that songbirds can be identified with 100% accuracy from their calls, so the frequency analysis still has more to discover.
The team also plans to investigate how the hyenas use the information encoded in the giggles, by playing them back to individual hyenas and observing the effect on their behaviour.
One secret remains, though: the giggles, seemingly relevant to close-range interactions, are incredibly loud. That can attract other diners to a feast.
"There's a lot of competition between lions and hyenas, and hyenas always lose," Dr Mathevon said.
"So there's still a question as to why they make a lot of noise that could attract lions."
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