The calls of male orangutans contain information about the apes' identity and the context of the call, researchers say.

Azy, a 27-year-old male orangutan, sits outside and enjoys a piece of pineapple at the Great Ape Trust research centre in Iowa. (Steve Pope/Associated Press) An international team of researchers, led by Carel van Schaik of the University of Zurich, tracked the behaviour and calls of three male orangutans on a nature reserve in Borneo's Indonesian region.

While all orangutans have a wide variety of calls, only sexually mature male orangutans with enlarged cheek pads, or flanges, can make long-distance calls through the jungle.

Brigitte Spillmann of the University of Zurich described the calls as "a series of long, booming pulses and grumbles, which can be heard through over a kilometre of dense jungle."

The researchers wanted to know whether these "long calls" contain information about the identity of the ape and the reason for the call.

"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," Spillmann said in a release.

The scientists observed the orangutans and recorded their behaviours each time they emitted a call. Their results were published this week in the journal Ethology.

Calls may be response

Some of the apes' calls were spontaneous and not provoked by any obvious prompt. Other calls were in response to behaviours of other apes, such as another male's long call or a tree falling nearby.

Orangutans will sometimes push over trees as part of a noisy, dominant display called snag crashing, similar to chest pounding in gorillas.

The researchers found that the orangutans' pulsing calls in response to snag crashing or another male's call were faster and consisted of more pulses of shorter duration than calls that were spontaneous.

The scientists also observed the behaviour of some female orangutans who heard the male apes' calls.

Females with young offspring moved away from males making spontaneous calls, while sexually active females seemed to move toward them.

"This may be because in Borneo, females with offspring and rival males are not the target of the spontaneous long calls, but are eavesdroppers. However, the cost to the caller goes up if there is a more dominant male eavesdropper who may respond," said Spillmann.

The females who heard the calls that were in response to another male's behaviour ignored them.

"Long calls given in response to a disturbance are likely intended to repel rivals or potential predators, which accounts for the females' lack of reaction, compared with spontaneous long calls. Females are able to tell the difference between the types of long call and they react accordingly," said Spillmann.