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 .