It is seen as extremely rude in most human societies, but it appears apes also use whispers to pass on information they do not want others to hear. Gibbons - better known for their deafening songs that reverberate around the forests they inhabit - have been found to regularly use the secretive form of communication. The apes, which are found in the forest of south east Asia, produce a variety of soft call types, known as 'hoos', that are almost inaudible to the human ear. Scroll down for video . Lars gibbons like this one in Khao Yai National Park Thailand use soft 'hoo' calls that are similar to whispers . Yet scientists have found the animals use these whispers to communicate specific information to each other about predators, foraging and when meeting neighbours. The researchers found that predator 'hoos' in particular were produced at frequencies well below the threshold of hearing for eagles and other birds of prey. A dictionary that decodes the calls produced by gibbons is being developed by scientists. Researchers at Wisconsin University in Madison have been using computer algorithms to analyse the whoops and songs produced by the apes. Gibbons singing usually consists of a crescendo of notes, particularly in response to predators. This usually starts with a series of soft 'hoo' noises that are only audible at close range but this rapidly grows to become much louder. In most studied cases these songs are produced in response to predators but they also duet together in social song. This suggests the quiet calls were designed to alert other gibbons nearby without alerting the raptors. Similarly biologists studying lar gibbons found they used a different whisper call to when a tiger or leopard was in the area. Dr Esther Clarke, who led the research at the University of Durham, said the findings could also help to provide valuable clues about human speech evolved and perhaps even why humans began using whispers. She told Mail Online: '(Hoo calls) have been overlooked in favour of the more conspicuous songs. 'We just didn't know they were using context-specific calls at close range. 'It opens up the possibility that their vocal repertoire is much larger than we previously imagined.' She added: 'These animals are extraordinarily vocal creatures and give us the rare opportunity to study the evolution of complex vocal communication in a non-human primate. 'In the future, gibbon vocalisations may reveal much about the processes that shape vocal communication, and because they are an ape species, they may be one of our best hopes at tracing the evolution of human communication.' Gibbons have one of the loudest calls in the natural world, creating whoops that can travel for miles through the dense forests where they live. Most research until now has concentrated on their loud songs which the animals use to communicate with each other over long distances. However, Dr Clarke and her team, whose work is published in the journal BMC Evolutionary Biology, decided to focus on the softer hoo call types. While these calls have been known about since the 1940s, they have not been studied in detail due to their low volume and because they are almost indistinguishable to the human ear. Lar gibbons used hoo calls to warn each other of predators but also during more intimate moments like this . The researchers used models like of this snake to test whether certain hoo calls were specific to predators . Different hoo calls appeared to be related to the activity the gibbons were taking part in like foraging (above) Using modern recording technology and computer analysis, Dr Clarke and her colleagues have been able to detect subtle differences in frequency, pitch, length and volume of these calls. They recorded 450 hoo sounds and found there were six different types of calls being produced by the gibbons. The researchers have been able to identify which of these calls are associated with particular situations. The researchers spent almost four months following lar gibbons around the forests of Mo Singto in Khao Yai National Park, north east Thailand recording their calls and behaviour. They found that the gibbons produced individual hoo calls in different situations, including when they were foraging, when they encountered neighbours, during duet songs between paired mates and when they detected predators. This model of a bird of prey produced short, low frequency and extremely quiet calls from the gibbons . This model of a leopard also produced low frequency and quiet calls from the gibbons when they spotted it . The researchers used a series of models of predators to test whether the calls were associated with specific predators. They found raptor hoos were less intense, shorter, lower pitch and had a small frequency span. They were the least audible of the calls. Raptors hear best in the 1-4kHz range while the gibbon hoos were consistently below the 1kHz threshold. Tiger and leopard hoos were similar, suggesting that callers perceived these two predators as belonging to the same ‘big cat’ class. While both gibbon sexes displayed similar hoo calls, female calls were lower in frequency than male ones. The researchers say this is surprising, as among mammals, males tend to have lower frequency voices than females. Females also typically did not produce hoo vocalisations when encountering neighbours and often remained passive. This video shows a white handed gibbon singing the more distinctive and audible calls while in a zoo enclosure . Males, however, often interacted with neighbouring individuals. The gibbons also seemed to use whispering during more intimate moments - much like humans do too. Writing in the journal BMC Evolutionary Biology, Dr Clarke and her colleagues said: 'Duet hoos tended to be higher in frequency than the other contexts, significantly so when compared with feeding,leopard, tiger and raptor hoos and raptor and tiger hoos.' The researchers say the study is of direct relevance for the on-going debate about the evolution of human speech. Researchers say the findings could shed light on the evolution of human speech and why we began whispering . The ability to produce calls that are context-specific is necessary for communication to direct the attention of others. This behaviour appears to be widespread and was likely present in the ancestor of modern primates and humans. The acoustic variation seen in gibbon hoos in particular may be similar to human speech, in which subtle acoustic parameters, like pitch, can be important carriers of meaning. Chimpanzees bash out distinctive drum solos on the roots of trees as a form of 'signature' and to tell others in their group where they are. Biologists studying the primates have found over the years that male chimps grip the buttresses of tree roots with their hands and beat them with their feet. The noises produced - which is normally accompanied with loud hooting -  are known as pant calls, and can be heard more than half a mile (1km) away, even in the dense jungle. But now one group of scientists studying the animal in the Sonso Community in the Budongo Forest in Uganda, have found that this drumming contains distinctive rhythmic patterns that can be used to identify a specific individual. Each male used a different number of beats and used a style of doublets and pauses that was distinct to them. The researchers also discovered that the chimps tended to use drumming far more often when their group was travelling. They believe this may help keeps tabs on an individual's whereabouts.
Biologists at Durham University spent four months recording gibbon calls .
They recorded gibbons making 450  'hoo' calls from 25 animals that were impossible to distinguish with the human ear and were often inaudible .
Each call was found to relate to different contexts like foraging, meeting neighbours, singing to mates or warning others about a predator nearby .
Hoos about birds of prey were the quietest so not to alert the predators .