The secret language of gibbons has been translated for the first time, with scientists announcing they have discovered the meaning behind their 'hoo' calls.
Gibbon calls are extremely soft hoo sounds that have been studied since the 1940s – but because of their low volume, they are almost impossible for humans to hear and difficult to record and analyse.
The study, published in the journal BMC Evolutionary Biology, looked at the likely meanings of a number of distinct gibbon calls that respond to specific events or alarms.
Researchers spent four months following large gibbon groups around forests in north-eastern Thailand. The gibbons were followed from first encounter in the morning until night when they went to sleep.
Using modern recording technology and computer analysis, they identified the specific calls – be it foraging, bumping into neighbours, and distinguishing between predators.
They noted their hoos and what event was taking place at the time. Eventually they had recorded over 450 hoo sounds to analyse and they found links between audio patterns and the context of the recording.
In terms of predator distinction, researchers found a range of calls for leopards, tigers, pythons, eagle owls and serpent eagles. Raptor hoos were less intense, shorter and with a smaller frequency span than other hoos – raptors hear best in the range of 1-4kHz, while the hoos were consistently below the 1kHz threshold.
Researchers believe this allows the gibbons to alert one another to the danger without attracting the attention of the predator.
Tiger and leopard hoos were similar, suggesting they are perceived as belonging to the same type of predator.
As well warning calls, they also recorded duet songs from mated pairs and social calls. However, findings showed females did not normally produce hoos when encountering neighbours, while males engaged and interacted with one another.
The team believes their findings provide an insight into the evolution of human speech, in that the ability to produce context-specific calls is necessary for communication.
Some scientists believe this behaviour was probably present in the ancestors of modern primates and humans – and that the acoustic variation in gibbons may be similar to human speech, where acoustic parameters carry meaning.
Lead author Esther Clarke said: "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."