Researchers have reportedly taught sign language to chimpanzees and other apes in the past. But the charadeslike pattern—observed in controlled tests of orangutans trying to communicate with humans—may better reflect the style of communication occurring in the wild as they struggle to convey their apish desires to one another.
"I think we're looking at orangutans' natural understanding of communication," says evolutionary psychologist Richard Byrne of the University of St. Andrews in Scotland.
Byrne's doctoral student Erica Cartmill videotaped six adult female Bornean and Sumatran orangutans from two different zoos and counted the number of visual, audible or other gestures each ape made when presented with two food options—tasty treats such as bread or a banana or less tempting fare such as celery or leek.
A human sat facing each ape silently for 30 seconds while the orangutan pointed, clapped, stared, smacked its cage or otherwise indicated its desire for the yummy snack. In some cases the human gave the animal the treat. But in others the experimenter forked over only half of it or the less desirable food instead, pretending to have partially or completely misunderstood the orangutan.
The apes typically stopped gesturing if they got the whole treat, but tended to repeat their original gestures if they only received a portion of it, whereas they more frequently switched to new signals if given the wrong grub, the researchers report in Current Biology.
The pattern is not simply a result of frustration, they wrote, because the orangutans tended to lose interest the longer they spent waiting for the food.
In a prior study, experimenters watched chimpanzees gesturing for food, but the results were more ambiguous because the humans had no way of indicating partial understanding, Byrne says.
He adds that the result supports the idea that orangutans and other apes do not gesture unintentionally, as some animal research assumes, but rather do so deliberately, guided by a simple theory of how other creatures' minds are working.
"If your audience is stupid and doesn't understand what you're saying," he says, "an intentional agent will think, 'Okay, what am I going to do next?'"