You've got to love gibbons. Part of a genus of primates known as small apes, they comprise seven species with such irresistible names as the northern white-cheeked gibbon, the southern buff-cheeked gibbon and the eastern black gibbon. They also sing conducting long-distance dialogues that primatologists call "duets." Now it appears those duets are also sung with what amounts to dialects or at least regional accents a decidedly human trait in a decidedly nonhuman critter.
All species of gibbons are jungle-dwellers, and all can be found across most unspoiled stretches of southeast Asia. Their calls and songs are ubiquitous throughout their range, forming a background accompaniment similar to bird songs though much, much louder. Like bird songs too, gibbon songs are packed with meaning, used to define territory, attract mates and, once those pair bonds are formed, strengthen them.
But even casual observers of gibbon songs have noticed that they're not all the same and that the farther you travel from any given spot, the more the songs seem to change. To determine how great those differences are and what drives the regional variations, a team led by primatologist Van Ngoc Thinh of the German Primate Center in Goettingen, Germany ventured into the jungles of Vietnam, Cambodia and Laos to listen to gibbons in 24 different areas.
The investigators set up listening posts in the jungle each morning throughout 2007 and 2008 and eventually collected 400 calls of male and female gibbons in 92 groups. They also extracted samples of mitochondrial gibbon DNA in 22 of the the 24 locations.
To untrained human ears, the songs the researchers gathered might not sound terribly hummable, but the scientists themselves heard the music in them. The female eastern black gibbon, for example, "produced fast up-down sweeps like a spiral spring with a vibrato sound on the first two notes," Thinh and his colleagues wrote. By contrast, "The first note of the male [western black gibbon] call had slightly ascending structure followed by notes with fast up-down modulation."
But while the species calls may have all differed, some were more different than others. The four species in the southern part of the survey region had very similar calls that were distinguished mostly by comparatively subtle lilts and dips like the accents of the residents of neighboring counties across the southern U.S. Two of the northern species, however, sang with twangs that were entirely different from each other as well as from the southern troops like adding a Boston and Chicago accent to the mix.
The investigators analyzed the frequency, pitch and other differences among all the songs reducing them to objective data as opposed to the subjective impressions of the listeners. They then plotted all of these on a scattergram, with each song's acoustical characteristics determining its position. Here too the gibbon calls clustered in three principal groups. Finally, the investigators conducted DNA analyses of their mitochondrial samples and found that those followed a similar drift, with the differences in genes widening as the differences in geography and song did.
"The high significant relation between acoustic similarity, geographic distance and genetic relatedness show that gibbon songs are a salient feature of their genetic relatedness," the researchers wrote.
That may not seem entirely surprising genes determine a whole lot of things, after all but it reveals a lot about singing gibbons, not the least being that they don't bring as much brainpower to their work as you'd think. Birds of the same or similar species tend to vary their trilling when they live close to one another the better to distinguish my song, my territory and my mates from yours.
Gibbon, as the scattergrams show, do just the opposite, with proximity driving musical similarity. That's because gibbons are pretty much born with their song software preloaded. It can change over time and over generations, but only at the speed the genes change. Songbirds must actually learn their tunes, making it possible to alter them in real time if too many competitors start singing along.
In that sense, birds may actually be a little bit closer to humans on the communications hierarchy than gibbons are. But our primate cousins are nonetheless a good deal more verbal than we ever knew and that makes the bonds of kinship a little tighter too.