Message songs: wild gibbons warble with a simple syntax.

Southeastern Asian forests harbor a small-bodied line of apes, known as gibbons, that sing like rainforest Pavarottis. These animals' full-throated refrains reverberate through dense vegetation.

A research team has now gone behind the music and gleaned the first evidence that singing gibbons rearrange notes to communicate with their comrades. This simple system, or syntax, for recombining sounds to convey messages represents a step toward human language that had not previously been demonstrated in apes, says psychologist Esther Clarke of the University of St. Andrews in Scotland.

Researchers have traditionally held that syntax arose only as the vocabulary of prehistoric people grew large and unwieldy. "We're finding the opposite in gibbons," says psychologist Klaus Zuberbuhler, also of the University of St. Andrews. "One way of escaping the constraints of their limited vocal abilities is to combine signals into more-complex sequences, which carry meaning."

Gibbons evolved complex vocal skills as a tool for finding long-term mates in a competitive social scene, the scientists theorize. In the December 2006 PLoS ONE, a new online journal, Clarke, Zuberbuhler, and a colleague outline basic rules for gibbon songs stimulated by a predator's presence versus those crooned with a mate.

From April 2004 to August 2005, the researchers studied 13 groups of white-handed gibbons living in Thailand's Khao Yai National Park. Each group consisted of two to six members--usually an adult pair, its offspring, and occasionally another adult male.

Clarke elicited predator songs by placing realistic models of threatening animals in trees where an entire group of gibbons could see them. Models included a fake fur-wrapped sack representing a leopard and a painted, papier-mache, crested serpent eagle covered in feathers.

The team recorded predator-induced songs, which began with series of soft "hoo" notes and included many instances of another note. Each predator tune lasted roughly 30 minutes.

Pairs of adult males and females that mate for life perform duets, often adjusting the tunes over time. In the new experiment, adult pairs of each group spontaneously produced duets that were captured by the audio recordings. These songs lacked introductory "hoo" notes and the repeated extra note of the predator songs, and duets lasted only 10 minutes.

Gibbons within earshot of singing comrades discriminated between duets and predator songs. Nearby females emitted a characteristic brief call after hearing any song, but they delayed this response for 2 minutes or more following predator tunes. All members of neighboring groups responded to predator-induced crooning by loudly repeating the sequence of notes.

Although a substantial gap separates human language from ape communication, the new study shows that "in gibbons, the difference in degree of vocal complexity and sophistication is not as large as some have been tempted to think," remarks biological anthropologist Barbara J. King of the College of William and Mary in Williamsburg, Va.

Biologist Dorothy Cheney of the University of Pennsylvania in Philadelphia recommends that recordings of the two song types be played to gibbons in the same setting. She adds that syntax in gibbon songs falls short of that in language, which uses words to serve specific functions in sentences as well as to refer to features of the world.