article_snippet.sent: 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.
article_snippet.sent: Lars gibbons like this one in Khao Yai National Park Thailand use soft 'hoo' calls that are similar to whispers .
article_snippet.sent: Gibbons singing usually consists of a crescendo of notes, particularly in response to predators.
article_snippet.sent: 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.
article_snippet.sent: This suggests the quiet calls were designed to alert other gibbons nearby without alerting the raptors.
article_snippet.sent: Similarly biologists studying lar gibbons found Similarly biologists studying lar gibbons used a different whisper call to when a tiger or leopard was in the area.
article_snippet.sent: 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.
article_snippet.sent: Lar gibbons used hoo calls to warn each other of predators but also during more intimate moments like this .
article_snippet.sent: Different hoo calls appeared to be related to the activity Lar 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 Different hoo calls.
article_snippet.sent: Dr Clarke and her colleagues recorded 450 hoo sounds and found there were six different types of calls being produced by Lar gibbons.
article_snippet.sent: The researchers spent almost four months following lar gibbons around the forests of Mo Singto in Khao Yai National Park, north east Thailand recording
article_snippet.sent: The researchers found that Lar gibbons produced individual hoo calls in different situations, including when The researchers were foraging, when The researchers encountered neighbours, during duet songs between paired mates and when The researchers detected predators.
article_snippet.sent: This model of a bird of prey produced short, low frequency and extremely quiet calls from Lar gibbons .
article_snippet.sent: This model of a leopard also produced low frequency and quiet calls from Lar gibbons when they spotted This model of a leopard .
article_snippet.sent: 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.
highlights_snippet.sent: Biologists at Durham University recorded gibbons making 450  'hoo' calls from 25 animals that were impossible to distinguish with the human ear and were often inaudible .
  
highlights_snippet.sent: Hoos about birds of prey were the quietest so not to alert the predators .