The Orang Utan: It's repertoire
The males, as they grow old, develop immense pouches which hang down from the throat like gigantic double chins - not simply fat, but true pouches that can be inhaled with air. They extend far down the chest across into the armpits and right over the back to the shoulder blades. Although they may have been used by ancestral Orang Utans as resonators to amplify their voice like howlermonkeys, the modern Orang Utan does not sing. His most impressive sound is his 'long call', a lengthy sequence of sighs and groans which continues for two or three minutes. To produce it, he partly inflates his throat pouch and the call ends with a number of short bubbling sighs as the pouch deflates. But he makes this call infrequently, and most of his vocalisations consist of grunts, squeaks, hoots, heavy sighs and a sucking noise made through pursed lips. It is a varied repertoire but a quiet one that can only be heard fairly close by. The animal more often than not is alone and during these monologues he gives the impression of a recluse, mumbling and grumbling to himself in an absent-minded way. Males take up this solitary life as soon as they leave their mothers, travelling and eating by themselves and only seeking company when they briefly come together with a female to mate.