Mature male orangutans produce what scientists call ‘long calls’, which can be heard for one kilometer in all directions even in dense forests. New research in Ethology has uncovered that these calls are employed for a number of reasons and provide information about who is calling and why.
“Orangutans have a rich repertoire of calls, however only sexually mature, flanged males emit long-distance calls with a series of long booming pulses and grumbles,” explains co-author Dr Brigitte Spillmann. “Individual recognition is important in long distance communication when individuals are separated beyond visual contact, we examined whether individual identity and context were also encoded into a long call.”
 Orangutan in Borneo. Photo by: Rhett A. Butler. |
Researchers followed three males in a nature reserve in Tuanan, Boreno, documenting two types of call. Male orangutans would produce one type of long-call without any obvious prompt, the other type would be in response either to another male’s call, or when an orangutan pushes over a tree. Known as ‘snag crashing’, orangutans push over trees to assert dominance. The calls in response to others calling or ‘snag crashing’ are distinct—faster and with more pulses—than the spontaneous calls.
By observing female orangutans’ response to the male’s long-calss, the team found that females could distinguish not only which male was calling but also the context of the call—whether it was spontaneous or in response to other calls or downing trees.
Females with offspring would move away when they heard spontaneous calls while available females appeared interested and moved closer, clearly indicating that the spontaneous calls were made largely to attract females.
However, both sets of females—those with offspring and those sexually-active—ignored long calls that were in response to other calls or ‘snag crashing’, since the calls were not directed at the females.
The Bornean orangutan (Pongo pygmaeus) is classifed as Endangered by the IUCN Red List, while the Sumatran orangutan (Pongo abelii) is considered Critically Endangered. Both species are threatened by habitat loss for plantations and logging.
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