Researchers led by Dr Esther Clarke of the Durham University and the University of St Andrews, UK, have revealed that lar gibbons (Hylobates lar) produce distinct ‘hoo’ calls in response to specific events, such as foraging and encountering neighbors, and that subtle differences even distinguish between different predators.
The lar gibbon (Hylobates lar) in the Kaeng Krachan National Park, Phetchaburi, Thailand. Image credit: JJ Harrison / CC BY 3.0.
The lar gibbon, also known as the white-handed gibbon, is a species of primate in the family Hylobatidae.
These primates are found in northern Sumatra, Indonesia, throughout Peninsular Malaysia, north through southern and eastern Myanmar, most of Thailand, and marginally into southern China. They also occur in a small area of northwestern Lao PDR (west of the Mekong River).
Lar gibbons are dichromatic in pelage. They wear either a dark coat, which may range from gray to black to brown, or a light coat of light cream color to light brown. The hairless face is surrounded by a ring of very short white or lightly colored fur; and the hands and feet are both white. Their elongated forelimbs, hands, and feet are adaptations for brachiation, which is their primary mode of travel through forest canopies.
These primates do not have tails. Adult males are 43 to 58 cm long and weigh 5.0 to 7.6 kg, while females weigh 4.4 to 6.8 kg and are 42 to 58 cm long.
They live in evergreen, semi-evergreen, and mixed evergreen-deciduous forest, and are known to utilize regenerating secondary forest and selectively logged forest. In northwestern Thailand, they utilize patches of dry evergreen, mixed deciduous, and bamboo forest near Karen settlements if they are not hunted.
While lar gibbons are mainly known for their loud and conspicuous songs, they can also produce a number of soft call types known as hoos.
These calls have been alluded to in studies dating back to 1940, but due to their volume, they are virtually indistinguishable to the human ear and have been difficult to record and analyze.
A new study published in the journal BMC Evolutionary Biology reveals the likely meaning of these calls.
“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,” Dr Clarke said.
“In the future, gibbon vocalizations 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.”
Dr Clarke and her colleagues spent almost 4 months following groups of lar gibbons around the forests of north-eastern Thailand.
The gibbons were usually followed from the first encounter in the morning until they had located their evening sleeping tree, while the scientists recorded their hoo calls and noted the event that elicited the response.
From the recordings they extracted over 450 hoo sounds and used computer analysis to find links between audio patterns and the context in which they were recorded.
The gibbons reliably produced individual hoo calls for different contexts, including foraging, predator detection, encountering neighbors, and as part of duet songs by mated pairs.
In addition to differences between contexts, the researchers also discovered subtle hoo variations within contexts, for example to distinguish between different types of predator.
They investigated the responses to a range of predators including clouded leopards, tigers, pythons, and raptors including eagle owls and crested serpent eagles.
In addition to real predator observations, they presented fake model predators in realistic poses for the rarer animals.
Raptor hoos were acoustically distinct – less intense, shorter and with a smaller frequency span than the other hoos, making them the least audible. Raptors hear best in the range of 1-4kHz, while gibbon hoos are consistently below the 1kHz threshold. The raptor hoos were the lowest frequency of all and could help gibbons avoid attracting the attention of the predator.
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.
“This is surprising, as among mammals, males tend to have lower frequency voices than females,” the scientists said.
“Females also typically did not produce hoo vocalizations when encountering neighbors and often remained passive and removed, while males engaged and interacted with neighboring individuals.”
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Esther Clarke et al. 2015. Context-specific close-range “hoo” calls in wild gibbons (Hylobates lar). BMC Evolutionary Biology 15: 56; doi: 10.1186/s12862-015-0332-2
