Along the river’s edge, the dark forest rises like an impenetrable wall, behind which hidden insects buzz, rattle, and whine; birds chirp, whistle, and chide; unseen. Suddenly, the air is filled with what sounds like water drops resonating as they plunk into deep jungle pools, followed by a piercing, long, liquid note that spirals above the canopy and plummets back down to earth. It is swooping and sweet, as melodious as any bird, but mystifying and otherworldly. To hear a gibbon’s song is to listen to a sound as ancient as the forests from which it resonates.  

White-handed gibbons are known for their beautiful and unique vocalizations. As we will see, these vocalizations are not only delightful to hear, but they are complexly and subtly saturated with significance in ways that are reminiscent of human syntax.  

White-handed gibbons spend about 4 percent of their day vocalizing. This is high compared to other apes. Their neighbor in some regions, the orangutan, for instance, is relatively quiet, vocalizing infrequently except for long calling bouts by flanged males during mating season. Likewise, Africa’s chimpanzees only vocalize for about 1-2 percent of their day, and typically in short-range communication. Africa’s gorillas, on the other hand, spend roughly 3 percent of their day vocalizing.   

This isn’t the only way that gibbon communication is unique. In the animal kingdom, there are estimated to be only 14 species that “sing”. Among primates, these include a Madagascar lemur called the indri, the titi monkeys of South America, tarsiers, and, of course, our gibbons. But why call them songs; why not simply call them vocalizations and leave it at that? Have the scientists gone all poetic on us? Not exactly. “Songs” refer to a specific temporal pattern consisting of several notes repeating.  

There are two main “songs” in a white-handed gibbon’s repertoire: the great call/pair-bonded duet and the predator song.  As far as their note repertoire goes, both of these songs are identical, meaning they use the same sounds or “notes”, but rearranged in subtle and unique ways, and certain “notes”, as we will see, are more or less prevalent in each.    

Before noon each day, the family gathers together at the edge of their territory. There are a few introductory notes from both the male and female and then the dominant female of the family group produces a series of very loud notes, accelerating in tempo and ascending in pitch followed by trills that build into a bubbling crescendo. It is a part of the gibbon’s song, but not the song itself. It will be followed up by the male-female duet. The call lasts between 17 and 45 seconds and can exceed 100 decibels of sound pressure at close range, traveling over half a mile (1km) away. It needs to travel far to send a message. This great call is thought to mean something along the lines of “This is my territory, stay out!” To deliver this kind of message they have to be audible well beyond the gibbon’s territorial borders. 

This is because white-handed gibbon family groups have a home range they defend from intruders. The great call gives white-handed gibbons a sense of the borders, though these often overlap. To reach the largest number of bordering gibbons, the female chooses a high vantage point so that her voice will carry across the treetops. The great call preferentially takes place from tall trees in primary forests located on upper hill slopes, hill or ridge tops. 

Then something extraordinary happens: the male and the female sing together. This is the pair-bonded duet. It is loud and can last up to 11 minutes. The duet typically consists of multiple great call sequences, and each one of the singing pair has its own unique variation on the theme. Between the great call sequences in the duet, there is other singing known as “interlude songs”. These “interlude” songs actually make up the bulk of the singing in the duet, and compose the final stanza of the song.

Notably, each individual’s part varies slightly, suggesting a high degree of personalization. This fact, along with each individual’s unique tone of voice, likely plays a role in individual recognition among gibbons across territorial borders. 

Like the great call, this pair duet is a territorial message. However, it carries another message as well: the strength of the pair’s social bond. Essentially it means something along the lines of “I’m a male gibbon and I’m with her” and “I’m a female gibbon and I’m with him”. A listener will be able to tell how long a pair has been together simply by judging how well they sing together. Newly formed pairs go through a lengthy process of adjusting their vocal contributions so that their songs complement one another. This process can take months to learn properly. As their relationship matures, their coordination improves. The male must pay attention to the female’s vocals, so he can adjust his vocals accordingly.      

But these pair duets aren’t all about announcing undying love or defense of their favorite hill or treetop; it’s also about listening to others. Distant pairs will respond to their song with their own pair-bonded duets with their own information. The duets involve rapid, clear notes from both the male and the female known as climax-coda sequences. Once the climax-coda sequence is heard from a distant pair, the male and female stop and listen. The structure of the gibbon’s great call is such that it allows gibbons to detect and prepare to listen to the climax-coda sequences of distant gibbons, further honing this long-distance communication tool. In the moments of listening, the pair is able to identify the territory of nearby gibbons and assess the bond strength of distant pairs. It may also help with natal dispersal by giving mature offspring an idea of where potential mates, rivals, or other groups might be.    

Predator songs are exactly what they sound like—vocalizations used to identify predators to the family group, recruit group members in defense, alert family members in adjacent territories, and let the predator know they have been seen. Interestingly, white-handed gibbons use these songs strictly for ground predators, such as tigers, clouded leopards, and reticulated pythons, and not for aerial predators such as raptors. Predator songs have a longer duration than duet songs and are produced by almost everyone in the family group.   

Songs also reveal various attributes about the singer, such as gender and age. Males sing at a higher pitch than females. Counterintuitively, a family of hormones, known as androgens, the most famous of which is testosterone, is what produces higher voices among male gibbons. One reason for this may be that females select males with higher-pitched voices, suggesting that male calls carry with them messages about their sexual status and virility.  

Males also have their own phrases in gibbon songs. These phrases are characterized by lip-smacking and modulation of sounds at rates similar to those observed in human speech. Male gibbons, for example, are the only ones to produce “trills”, a type of modulation that contains many notes per exhalation. Like human speech, these vocalizations are produced exclusively during exhalation and exhibit a kind of rhythm. This is unique in the primate world where vocalizations can occur both through exhalation and inhalation, and tend to fit fewer sounds into a breathing cycle. 

On the other hand, dominant females tend to use more of the basic 6 notes than others in the group, possibly because they are issuing instructions. The mother is also the one who teaches both sons and daughters how to sing. Gibbon mothers and daughters will co-sing female-specific songs which help the child learn how to sing. Interestingly, up until at least 5 years of age, juvenile and adolescent males also produce female-like great calls when co-singing with their mothers. As these males reach sexual maturity, they gradually transition to producing male-specific vocalizations.  

The song also reveals age-related information about the singer. Younger adult males, for example, will produce longer calls than senior males. Likewise, aging females produce weaker great calls with a notable decline in tempo and frequency. Both of these examples likely signal a change in reproductive status.   

Songs are elaborate acoustic structures made up of a variety of “notes” or “units” that alone or in conjunction with other “notes” convey a variety of context-dependent meanings. Researchers have isolated 7 basic vocal notes:

(1) The “hoo” note stands out for its wide range of versatility and its central role in communication. Acoustically, it is a low-frequency quiet note that consistently spans a much narrower frequency range than “wa” notes. It is used while feeding, when separated from group members, when encountering predators when interacting with neighbors, and also within the elegant duet songs of mated pairs. While both male and female gibbons use the “hoo” note, the frequency of this note is different between the sexes when used in different calls. While it is hard to know, researchers think this might indicate that males and females emphasize different messages during communication or that there exist different roles between male and female gibbons. Already this “hoo” note has been diced into six separate subunits (with more subunits being likely). The versatility of the “hoo” note demonstrates that white-handed gibbons modify their calls to convey messages or emotions based on specific situations. 

(2) The “wa” note is a loud and penetrating note that is short and rises steeply, consistently spanning over 100 Hz in frequency range. On a spectrogram, it appears as more or less a straight or sometimes slightly concave line. The “wa” takes center stage in the duetting song between pair-bonded gibbons. The male solo song consists of a sequence of different note types, including repetitions of the “wa” note, organized into distinct phrases. During the duet, the male and female combine their respective “wa” notes together (and other notes in what is called an antiphonal pattern in which they take turns vocalizing in a coordinated way). Being loud and high-pitched, the “wa” note travels long distances, advertising its presence to neighbors. In this context, it contributes to the strengthening of the pair bond and signals a joint territorial defense between mated partners.  

(3) Like the “wa” note, the “leaning wa” is also a loud and penetrating note, reaching amplitudes of 100 decibels, only the “leaning wa” is longer in duration than the “wa” notes. On a spectrogram, it appears to lean or slant to the right, and may also have a slight bump or inflection in the middle of it. It is one of the building blocks used in duets and predator songs. It is also used in the male solo song during the “great call” together with other note types such as “wa”, “hoo” and “oo” to make up distinct phrases.    

(4) The “oo” note is of a relatively flat note, of even pitch and varying duration. At times, it may rise slightly at the start, the significance of which is currently unknown. The “oo” note is another ingredient in the duet songs or mated gibbon pairs and occurs typically in the early stanzas of the song. Notably, the “oo” note is rarely used if ever during predator songs. This suggests that whatever the precise meaning of “oo”, it doesn’t convey meaning about predators, but rather may play a role in communicating social information.

(5) The “sharp wow” note is a loud and penetrating note. It rises steeply at first then falls steeply to produce a concave curve. It invariably spans more than 700 Hz in the frequency domain. It is associated with conveying information about predatory threats or perceived social threats. As the name might suggest, the “sharp wow” is used in response to predators, and is most commonly incorporated towards the ends of “predator songs”. Interestingly, the “sharp wow” is incorporated into duet songs when a social threat is detected as well, for example when a neighboring group is counter-singing or when the singing occurs at relatively close distances. 

(6) The “waoo” note is highly variable and spans a much lower frequency range than the ‘sharp wow’. It always rises steeply at first, but then may hold pitch at an even level or fall in pitch to create a convex curve. The wahoo appears early in the duet songs between bonded pairs. Though it occurs in “predator songs”, it is rather rare, suggesting it has less to do with predators. 

(7) The “other” group consisted of notes that didn’t quite fit into the shapes and definitions of the other notes. The researchers believe these may warrant their own unique note category at a future date. They were often highly variable. 

Of course, notes are rarely, if ever, produced in isolation. Rather, they are strung together into structures called “phrases” or “figures”. These phrases then make up “songs”. So while many of the notes above are used in similar situations, such as predator calls or the duet song, they carry subtly different meanings in different sequences.

To make things even more complex, sometimes it’s not what you say, it’s how you say it. Gibbons will introduce a wobbly or trembling quality to one of the six notes mentioned above. Researchers are still deciphering what these variations might mean, but they are getting closer to breaking the code.