Researchers say that gibbon whispers ‘hoo’ calls could have many meanings depending on specific predators or events. Their utterances may also provide clues as to how humans developed their speech. The loud, conspicuous vocalizations of the Lar gibbons (Hylobates lar), commonly known as white-handed gibbons are a hallmark of Hylobates lar. They can make soft hoo-type sounds. In the scientific journal BMC Evolutionary Biology, a team of scientists from Switzerland, the UK, and the US explained that while gibbons have made subtle sounds in research dating back over seventy-five decades, it is difficult for humans to distinguish them. Researchers identified 450 distinct Lar Gibbon “hoo” sounds. The lead author of the study, Dr. Esther Clarke (a Research Fellow at Durham University’s Department of Anthropology in England), said that the researchers identified four different Lar Gibbon ‘hoos. From the time they met the primates in the morning, to the moment when they found their night sleeping tree at evening, the researchers recorded and logged their sounds and noted any events. Gibbons’ verbal communication may offer important clues about how humans developed their speech. Gibbons possess a large vocabulary. After analysing the recordings, they discovered 450 various hoo sounds. Computer analysis was used to identify specific sounds and link them to their recorded context. The researchers discovered that gibbons have specific hoo sounds for different contexts. These include predator detection, foraging and encountering neighbors. They also use these calls in their mating songs. The researchers also found subtle variations in the hoo calls of gibbons within certain contexts. This allowed them to differentiate between various predators such as tigers, crested serpents eagles and pythons. The ‘Hoo” sounds that warned about birds of prey were made at frequencies that raptors are unlikely to hear. They also presented fake predators with realistic poses to less common animals. The hoos that warned of birds-of-prey (raptors), were more difficult to hear because they had a distinct sound. They were shorter and less intense than the other hoos. Although raptors can hear well in the range of 1kHz-4 kHz, gibbon raptor hoos are consistently lower than the threshold at 1kHz. Researchers believe that gibbons communicated at the lowest frequency possible to avoid predators hearing their raptor hoos. The gibbons interpreted leopard and tiger hoos to be the same, which suggests that they saw them as being part of the same type of predator, the ‘bit cats.’ Gibbon females are more vocal than their male counterparts. The authors stated that both sexes produced similar hoo sounds, but the gibbon females produce lower frequency hoos than those of the males. It was quite surprising as most mammals (including humans) have a deeper voice. Female gibbons were more reserved when they encountered their neighbours. On the other hand, male gibbons interacted with others vocally and engaged in conversation. Their study was directly applicable to ongoing discussions about the evolution of human speech, according to the research team. They wrote that “the ability to make calls that are context specific is essential for communication where an actor refers to an external event to a receiver’s attention,” It is a common behavior that was likely present in humans and their ancestors. According to the authors, gibbon hoos’ acoustic variations may have similarities with human verbal communication. In which subtle parameters like pitch can be used to convey meaning, acoustic characteristics such as pitch and frequency are able to carry information. Citation: Esther Clarke and Ulrich H Reichard, “Context-specific close range “hoo”, calls in wild gibbons. (Hylobates Lar), Klaus Zuberbuhle BMC Evolutionary Biology. 2015., 8 April. DOI: 10. 1186/s12862-015-0332-2.
