Gibbons Defend Against Predators With Song
A white-handed gibbon.
Credit: Esther Clarke, PLoS ONE

When a white-handed gibbon spots a lurking leopard, rather than high tailing it in the opposite direction, the furry ape will actually draw closer to its foe and belt out a song.

Scientists discovered that wild gibbons [image] in Thailand have crafted unique songs [click here to listen] as alarm calls to other gibbons, a discovery that might shed light on the evolution of spoken language.

The sounds that animals make are traditionally thought of simply as signs of their basic mood. At times, however, animal sounds are used to communicate specific details about the world to others.

For instance, vervet monkeys give one kind of call if they see a snake, prompting others to search the ground, and another type of cry if they see an eagle, leading others to watch the sky, explained study team member Klaus Zuberbühler, a psychologist and primatologist at the University of St. Andrews in Scotland. This is known as "referential signaling."

Relatively little evidence for such a level of communication was seen in more closely related primates in the wild. "It's been a puzzle," Zuberbühler said.

Gibbons are known for their loud, elaborate songs every morning, often coordinating in duets with their mates. These can be heard up to miles away through dense forest.

Primatologists led by Esther Clarke of the University of St. Andrews observed white-handed gibbons at Khao Yai National Park in Thailand. To see how these primates might respond to predators, the researchers built fake animals that resembled typical gibbon predators.

For example, the researchers wrapped a fake fur around a backpack to resemble a leopard. Python imposters were painted tubes more than 10 feet long, while the eagle dummy was made from chicken wire and papier-mâché covered in feathers and hoisted up 10 to 30 feet with a rope onto a branch. For the tigers, they covered a person in fake fur.

"The tough part was finding the gibbons everyday," Clarke recalled. Every morning before sunrise, the researchers went out and waited until the gibbons began their morning duet songs "and then ran to them," she said.

The gibbons spend much of their lives on tree branches 60 to 100 feet off the ground. But when they spotted the models, which were usually just a few feet off the ground, they descended to within 15 or 30 feet of the predators and sang at them.

"You might expect them to run away from the predator," Clarke told LiveScience. The gibbon's approach might be "to alert a predator to the fact that it's been seen, and thus there's no point in hunting anymore."

Gibbon songs are crescendos of up to seven sounds dubbed  “notes,” such as “wa,” “hoo” and “waoo." The scientists found that while the gibbons used the same notes in all of their songs, they arranged the notes differently for duets and those to alarm predators, particularly in the first 10 notes of the songs. This is the first time such communication has been confirmed in free-ranging primates outside humans. The finding could have implications for the development of human language.

"There are a number of reasons to believe that human speech is rooted in the primate lineage, so we're interested in other primate communication systems to shed light on what skills we all share and what skills are uniquely human," Zuberbühler said.

Their evidence suggests the gibbons also sing different songs depending on the kind of predator, Zuberbühler said, but further research is needed to confirm this.

The results are detailed in the Dec. 20 issue of the journal PLoS ONE.