But other times the helpers purposefully misunderstood the orangutans' requests and gave them only part of the desirable item or the yucky alternative.
When the entire portion of the treat was handed over, all but one of the orangutans ceased signaling entirely. Several of these seemingly satisfied individuals retreated into their cages, breaking off contact with the helper.
In cases where the animals were "misunderstood" and received only part of what they requested, they continued to signal to the experimenter, frequently repeating gestures already used.
When the apes were completely misread and didn't get any of the desirable food, they elaborated their range of movements and avoided repeating previously failed signals.
"The response showed that the orangutan had intended a particular result, anticipated getting it, and kept trying until it got the result," study co-author Cartmill said.
Although the study involved only six apes, scientists expect to find such communication patterns across the board in captive orangutans.
"There's no reason to assume that these six would have abilities that would be completely absent or completely unknown in other orangutans," said Robert Shumaker, director of orangutan research at the Great Ape Trust research center.
"If you document these capacities in even a small number of individuals, I think that's very, very meaningful," added Shumaker, who was not involved with the new study.
But researchers are unsure whether this type of gesturing occurs among orangutans in the wild or if it's simply a tactic used exclusively for communicating with humans.
"[It's] hard to detect [in the wild] without setting up a 'dumb audience' situation, as we did in the experiment," study co-author Byrne said.
"But we imagine it is very implausible that this whole subtle process developed in zoos and only for people. Let's be positive: This must be a basic part of orangutan communication."
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