Javan Silvery Gibbon (Hylobates moloch)

Chapter 6

Female Song Bout

© 1981 Markus Kappeler

6.1. General

The behaviour of the silvery gibbon comprises, on certain occasions, loud vocalizations which can be heard by man over distances of 500-1500 meters. Among them are:

1. harassing calls (chapter 5),
2. border-conflict screaming (chapter 7), and
3. the female song bout.

The female song bout is a solo peculiar to the adult female (although sometimes accompanied by the subadult daughter) and - contrary to the other loud vocalizations - occurs very frequently.

Within the female song bout highly conspicuous and formally more or less rigid climaxes occur consisting of an acoustic and a locomotory component: the so-called «great calls» [MARSHALL & MARSHALL, 1976] combined with «bursts» of locomotory activity in the branches.

In the following, a description is given of the female song bout; subsequently, the frequency and the circumstances of its occurrence are examined, the observed effects on conspecifics are presented, an attempt at determining its function is made, and finally, a short characterisation of the particularities of the song bout of the female H. l. moloch - as compared to those of other hylobatids - is given.

6.2. Structure of the female song bout

6.2.1. Rough structure of the acoustic part

a. Acoustic components

The song comprises two main acoustic components:

1. Single calls (figure 4, line 1). They all show similar sound characteristics, but differ with regard to sound intensity.

2. Great calls (figure 4, line 2). They consist of approximately 25 single calls and, as concerns structure and varying sound characteristics, show a stereotyped pattern (for details s. 6.2.2.); the intensity of great calls uttered by one individual hardly varies.

Since great calls are often broken off before the end, the song also comprises great-call fragments (of varying length; s. 6.2.2.).

b. Standard pattern of the song

The acoustic components are not inserted into the song at random, but certain groupings can be recognized. Accordingly, the song generally has three sections:

1. Song introduction. It consists exclusively of single calls which are mostly uttered at irregular intervals but partly also in series.

2. Great-call phase. It consists of a series of great calls and great call fragments in irregular sequence.

3. Interphases. In the intervals between the great calls, single calls can be interspersed in small numbers. As in the song introduction, these single calls are uttered partly at irregular intervals, partly in series.

c. Variations

Variations of the standard pattern mainly concern:

1. Breaking off the song at any place. The song may consist, in extreme cases, only of introductory single calls, or comprise only a single great call in the great-call phase.

2. Duration of and number of calls in the song introduction. Song introductions varied in duration between 0.4 and 5.1 minutes (average 2.2 minutes); the minimum number of single calls was 7, the maximum 226 (average 75; n = 37 introductions).

3. Duration and regularity of the call intervals in the introduction. In 37 introductions, the intervals between calls varied from 0 up to a maximum of 12 seconds; on the average, 34 calls per minute were uttered.

4. Number, duration and number of calls of the interphases. There are songs without interphases, and others in which single calls are uttered between almost all great calls; as a rule, interphases occur more frequently in the first half of the great-call phase - especially between the first 3-5 great calls. Their duration corresponds to the intervals between great calls (s. below). The number of single calls in the interphases is small (1-12).

5. Sound intensity of single calls.

6. Duration of and number of great calls in the qreat-call phase. Great-call phases varied in duration between 0.2 and 46.7 minutes (average 7.1 minutes); between 1 and 87 (average 15) great calls and great-call fragments (resp. 1-52 (average 9) complete great calls) were recorded during one song (n = 68 songs).

7. Duration and regularity of great-call intervals. In 68 songs, the intervals between the great calls were 1-105 seconds (average 18 seconds); on the average, 2.1 great calls (including great-call fragments) per minute were uttered.

8. Proportion of qreat calls to qreat-call fraqments. The proportion of great calls to great-call fragments varied from song to song - even in the same individual; it lay between 1 : 0.3 and 1 : 2.4 (average 1 : 0.8).

d. Duration

From the above mentioned variation it follows that the total duration of the individual song varies considerably: In 392 songs, it lay between 1.0 and 48.0 minutes (average 9.3 minutes).

Frequency distribution:

<10 minutes: 67% of all sonqs,
10-19 minutes: 24% of all songs,
20-29 minutes: 8% of all songs,
30 and more minutes: 1% of all songs.

e. Subadult females

Subadult females may accompany the song of their mother during the song introduction and also during the great-call phase.

In the great-call phase, the accompaniment is always rhythmically coordinated (i.e. synchronized great-call singing (figure 4, line 3); immediate interruption of the great call when the mother does not finish her great call).

6.2.2. Acoustic structure of the great call

On the average, the great call of the female silvery gibbon consists of 24.5 (18-33) single calls and lasts 15.5 (11-19) seconds (n = 87 great calls of 29 females).

a. Standard pattern

The standard pattern of this clearly defined vocal unit shows the following 7 phases (s. figure 4, line 2):

A: 1 (maybe 2) soft and very short introductory calls,
B: a series of about 8 calls, lasting approx. 4 seconds,
C: 2 longer calls within approx. 2 seconds which form the transition to
D: 2 long soaring sounds each approx. 2 seconds in duration,
(Immediately following the first of the two soaring sounds - and longest call of the great call - a gasping sound (x), resulting from voiced inhalation, is often heard.)
E: 2 calls of decreasing length within approx. 2 seconds which form the transition to
F: a series of about 8 very short calls, lasting 2-3 seconds,
G: often a soft single call of almost stable pitch which closes the great call.

As can be seen from figure 4, the structure of the great call shows a certain symmetry:

- introductory call - series of short calls - 2 calls of increasing length,
- 2 long soaring sounds,
- 2 calls of decreasing length - series of short calls - final call.

This symmetry however is not perfect; the single calls, on the one hand, are not symmetric regarding sound frequency, while the rapid series of calls, on the other, are clearly arranged asymmetrically.

b. Great-call fraqments

The frequency distribution of the timing of break-offs in 61 great calls which were not completed is as follows:

49 (80%) during the great-call phase B
(namely: -15 (25%) after 1-5 calls, -34 (55%) after >5 calls),
7 (11%) during the great-call phase C,
4 (7%) during the great-call phase D,
0 (0%) during the great-call phase E,
1 (2%) during the great-call phase F.

Thus, when great calls are broken off, it usually happens in the great-call phase B and 3-5 seconds after the beginning of the great call.

c. Variations

The described standard pattern of the moloch great call is constant throughout the species' total area of distribution; however, variations can be observed concerning

1. the duration and sound quality of the single calls (especially of the C-, D- and E-phase),
2. the intervals between the calls (especially of the B- and F-phase),
3. the number of calls in one great-call phase (especially in the B- and F-phase).

These variations of the great call characterize the different individuals; in the great call of the single individual - apart from the possibility of breaking off the great call - variation is very low.

Examples: In the Turalak study area, the observer was able to recognize 5 different females by distinct characteristics of their great calls:

female A: especially long and rapid call series B,
female D: extremely long first soaring sound D,
female H: narrow sound spectrum in all calls (lack of upper sound frequencies),
female K: extremely long, but very slow call series F,
female M: impure (broken) calls in the series F.

The individual characteristics, which can even be distinguished by man, should allow the local gibbon population to recognize individual females by their voices, i.e. to establish from which female a certain song emanates. This mode of recognition is reinforced by the fact that, for each sector of the forest, only 1 female is in residence.

d. Subadult females

The standard pattern of great calls uttered by subadult females is identical with the above, and the variations are of the same kind as in the great calls uttered by adult females. However, in general, the great calls of subadults are 2-3 seconds shorter than those of their mothers, whereby mainly the sounds in the D-phase are shortened (s. figure 4, line 3).

Subadult females can be recognized by their softer and higher-pitched voices.

6.2.3. Behavioural components of the female song bout

a. Adult females

An adult female, about to sing, moves to the periphery of the crown of one of her singing trees (s. section 6.3.), uttering single soft hoo notes.

There she stays for the whole duration of the song introduction which now follows. The calls are uttered - sitting or standing - with the head slightly inclined upwards (figure 5) and are not directed to a certain place or a special sector of the forest.

As a rule, the female also stays in the crown of this tree during the qreat-call phase. Occasionallv, however, it was observed that females moved to other singing trees, either (1) between two neighbouring singing trees (one or several changes), or (2) - in rare cases - along fixed «singing routes» over several singing trees (s. figure 8B). Movement itself was silent, i.e. was undertaken during the great-call intervals, and was interrupted for singing great calls.

Independent of whether the great-call phase is uttered on one single tree or whether movement takes place inbetween, each great call is combined with a burst of locomotion within the crown. (These bursts can be omitted by females carrying an infant.)

At the beginning of the great call, the female remains sitting or standing still; then, during the E- and the first part of the F-phase (the great-call climax as regards sound intensity and extent of the sound spectrum), she brachiates with swift and far reaching swings through the crown to another point on the periphery. There she ends the great call, mostly in a sitting position. Frequently plant parts (mainly withered or rotting branches) break off during these vigorous movements and fall noisily to the ground.

It seems that breaking off plant parts is deliberate on the part of the female and, accordingly, belongs to the behavioural repertoire when singing, because (1) a female was never observed to «stumble», i.e. lose her balance, and (2) the gibbons normally avoided withered or rotting branches, i.e. obviously recognized them in time.

The following observations of CARPENTER [1940] for H. l. carpenteri and ELLEFSON [1968] for H. l. lar support this assumption:

CARPENTER reports «vigorous shaking of branches» associated with the singing of great calls.

ELLEFSON mentions «presumably intentional breaking off dead branches, which come crashing to the forest floor,» by adult males during territorial battles (s. 7.5.1.).

If the female does not move to another singing tree ~during the great-call intervals, she looks out, more or less horizontally, over the canopy.

At the end of the song bout, the adult female usually leaves the singing tree immediately and moves silently to another part of her home range, followed by the rest of the group. Very often foraging ensues.

b. Subadult female

When a subadult female accompanies the song of her mother, she is mostly in the same singing tree, but does not, as a rule, move to its periphery. If the mother moves to another singing tree during the song bout, the daughter usually follows her; otherwise, she sits in the same place the whole time.

Bursts of locomotion as shown by the adult female during the great calls were never observed in subadults.

c. Non-singing group members

Non-singing group members usually take up positions at a lower level in the closer vicinity of the singing tree - seldom on the singing tree itself - and behave inconspicuously. If the female moves to another singing tree during the song bout, they follow her at a distance.

Before the beginning of the song bout, the adult male - together with the female - often utters hoo notes, but lapses into silence during the song introduction. During the great-call phase, he mostly stays in the same place the whole time, observing (s. section 5.2.).

Subadults (males and non-singing females) and younger group members pursue various quiet activities such as feeding, grooming and resting.

6.3. Singing places

Female gibbons select special trees in their territory for singing. These singing trees have partially, if not completely, free-standing crowns which - either because of the outstanding height of the trees, or because of their topographical location (e.g. on a ridge) - protrude above the forest canopy.

Each female uses several singing trees; as a rule, if she sings several times on the same day, a different singing tree is chosen each time.

Female D (Turalak) used a total of 15 different singing trees (of which 6 were also used as sleeping trees; figure 8B). They were all between 40 and 50 meters high and belonged to 7 different species; apparently various tree species can be used as singing trees.

Concerning the location of singing places in the home range, it was observed that singing trees were mainly situated in central parts of the home range (s. section 7.3.), although some were also in peripheral parts, i.e. in areas overlapping with home ranges of neighbouring gibbon groups (s. figure 8B). The latter were mainly used - by both groups - when the general food abundance was low and a rich food source was located in the respective border area. Singing routes were all located in the central area of a group's home range (figure 8B).

6.4. Frequency and conditions for the occurrence of female song bouts

6.4.1. Frequency

Songs of the 5 females of groups A - E, living in the central part of the study area (Turalak), were registrated on 89 (= 68%) of a total of 130 observation days spread over the year; on 41 days (= 32%) no songs were heard.

Not necessarily all 5 females sang on «singing days», but:

on 8 days (= 6% of all observation days) 1 female,
on 15 days (= 11% of all observation days) 2 females,
on 26 days (= 20% of all observation days) 3 females,
on 23 days (= 18% of all observation days) 4 females,
on 17 days (= 13% of all observation days) 5 females.

The highest number of singing days (70, = 54% of all observation days) was recorded for female B, the lowest number (38, = 29%) for female E. On the average, 59 singing days (= 45%) out of 130 observation days were registrated per female.

A female may sing more than once per day; on the average, each of the 5 females sang:

lx on 44 singing days (= 74%),
2x on 11 singing days (= 19%),
3x on 4 singing days (= 7%).

On the average, 78 songs on 59 singing days were registered per female, i.e. 1.3 songs per singing day (resp. 0.6 songs per observation day). The 5 females sang a total of 392 songs during the 130 observation days.

6.4.2. Influence of weather

Female gibbons do not usually sing when it rains. In Turalak, 50% of all songs were noted when the sun was shining, 48% when the sky was overcast, and only 2% in the rain (equal observation time (15 days) per weather category; since the audibility of songs was greatly reduced on rainy days, the frequency of songs during rainfalls might have been somewhat higher).

Approaching thunderstorms also seem to influence the singing activity of female gibbons. It was noted repeatedly that, shortly before the beginning of heavy rain, several songs were started nearly simultaneously, then broken off when the rain actually began.

6.4.3. Influence of the time of day

Observations indicate that the occurrence of songs is closely correlated with the day-night cycle.

It can be seen from figure 6 that only 3% of all songs were started before sunrise, but about 50% in the first hour (resp. 90% in the first three hours) after sunrise.

Some low singing activity (5% of all songs) was observed again over noon; these noonday songs occurred as a rule after resting phases in the gibbon groups.

Day-time also influences the duration of songs (s. 6.2.1.d): Songs begun before sunrise lasted on the average 11.1 minutes, those begun in the first three hours after sunrise 9.5 and the others 5.8 minutes.

6.4.4. Seasonal variations

The frequency of songs was subject to seasonal variations. The annual average was 0.60 songs per day and female, the average for the months June/July/August 0.72, while that for the months February/March/April was 0.53.

Thus, in the months June - August 1.4 times as many songs occurred as in the months February - April.

These variations could be correlated with:

1. the seasonal variation of food abundance; the food abundance in the months June - August was markedly higher than in the months Feb. - April (figure 2);
2. the reproductive cycle of the silvery gibbon; new-born gibbons were observed more frequently in the months June/July.

Since females, who had not given birth to an infant during the observation period 1975/76, also sang more frequently during the months June - August than during the other months, it can be assumed that the seasonal variations of song frequency are (positively) correlated with the variations of food abundance.

6.4.5. Singing as a reaction to conspecifics

a. Singing as a reaction to singing neighbouring females

When a female sings in her territory, other - neighbouring - females will often immediately start to sing as well («chorussing», TENAZA [1976]). This reaction, however, is not obligatory; in many cases songs remain «unanswered».

On several occasions when two neighbouring females sang simultaneously, they coordinated their songs rhythmically, the one singing her great calls in answer to the great calls of the other as a companion voice («countersinging», TENAZA [1976]).

In most cases where two females sang in rhythmical coordination, they were only approximately 100 meters apart from each other.

Such «dialoque songs» usually lasted longer than «normal songs». The average duration was:

6.7 minutes (n = 119) when no neighbouring females sang,
17.2 minutes (n = 35) when two females countersang,
10.2 minutes (n = 107) when other gibbon females in the vicinity sang (chorussing), but no countersinging occurred.

b. Singing during territorial border conflicts

During confrontations between neighbouring gibbon groups in the border areas of their territories, border-conflict screaming (s. 7.5.1.) often occurred. During this screaming the adult females involved occasionally (i.e. at long, irregular intervals) uttered single great calls. In these cases, however, it was not possible to speak of «female song bouts», since the song introduction as well as the visual display were missing, and between the great calls the females screamed in unison with, and in the same manner as, the other group members.

c. Singing when detecting intruders

Visual detection. In four cases it was observed that a resident gibbon group, while foraging, met strange conspecifics inside their territory. Each time the intruders immediately fled; in two cases they were pursued by the resident adult male over a short distance.

In all four cases the resident female immediately moved into one of her singing trees in the vicinity and began to sing. These songs were rather intensive, i.e. relatively long with a short introduction. In one case the subadult female joined her mother.

Acoustic detection. Strange songs inside, or at the periphery of, her territory also cause the resident female immediately to climb a singing tree in the vicinity of where the song is coming from and to sing there.

In three observed cases the strange songs originated from females of gibbon pairs who were new to the area and apparently did not possess a territory of their own. The intruders became quiet instantly and fled. In each case they were pursued by the resident adult male; in one case a subadult group member (probably male) also made an aggressive advance towards the intruders.

In several cases a resident female could be induced to sing by playing a tape-recording inside her territory.

6.4.6. Summary

Singing, as a rule, occurs - more or less corre lated with the time of day - spontaneously.

The singing activity is almost regularly blocked by rain; before thunderstorms, however, it is stimulated.

The social situation (in the widest sense) - inside the territory as well as in its vicinity - can induce female song bouts; in this connection conspecifics intruding into the territory must be especially mentioned. Singing neighbouring females, however, only seem to enhance the motivation of females who are disposed to sing anyway.

6.5. Effects of female song bouts on conspecifics

6.5.1. Singing induces singing

1. Singing as a reaction to song bouts of neighbouring females. See 6.4.5.a.
2. Singing as a reaction to strange songs inside the territory (intruders or tape-recordings). See 6.4.5.c.

6.5.2. Singing initiates conflicts

1. Singing on the border. In the season of low food abundance it could be observed, several times, that a resident female sang near a rich food source in the border area between her own territory and that of a neighbouring gibbon group. In 6 of 7 observed cases the neighbouring group immediately appeared. In 4 cases the female of the neighbouring group also began to sing (alternately) at a distance of 50-100 meters, and the two groups retreated into their respective territories after the end of the song bout. In 2 cases a border conflict, combined with screaming and chasing, developed between the two groups (s. 7.5.1.).

2. Singing in a strange territory. See 6.4.5.c.

6.5.3. Singing induces flight

In more than 10 cases it could be observed that non-resident individuals (single animals or pairs), which were present in the territory of a gibbon group, reacted to a song of the resident female with immediate flight from the forest area, without actually having been detected by the resident group.

6.6. Attempt at determining the function of the female song bout

The female song bout has the character of an acoustic / dynamic-visual demonstration of the group's presence, performed by the adult gibbon female as the «representative» of the family.

Occupancy of the respective part of the forest is advertised to all conspecifics in the area - established neighbours or wandering strangers. Furthermore, the female song bout expresses a latent threat towards conspecifics; the flight reaction of strange animals and the fact that song bouts may initiate border conflicts both point to this.

The song bout of the adult female might therefore serve in the first place to manifest the attachment of space and resources to the family qroup, i.e. serve to maintain the stability of the territorial system.

Moreover, the female song bout undoubtedly has an intrasocial significance. The adult male and also - in many cases - the subadult daughter seem to be impressed by the song bout of the adult female and participate in it in a certain way. The female song bout therefore might also serve to manifest and strengthen the family bond.

6.7. Comparison with other Hylobatids

The female song bout is a homologous feature of the whole family Hylobatidae [CARPENTER,1940; MARSHALL,1972].

However, considerable interspecific resp. intersubspecific differences can be recognized mainly concerning (1) the acoustic and locomotory contribution of the male to the female song bout, and (2) the acoustic structure of the great call sung by the female.

The role of the sexes in the female song bout as well as its acoustic standard pattern are however stereotyped throughout the whole range of distribution of each gibbon taxon; the female song bout has therefore significance as a taxonomic feature [MARSHALL & MARSHALL,1976; DEMARS & GOUSTARD,1978].

6.7.1. Contribution of the male

In H.l.moloch, H.l.mülleri and H.klossii the male does not contribute, either acoustically or locomotorily, to the female song bout; the female song bout in these gibbon taxa is a solo of the female [MARSHALL & MARSHALL,1976; TENAZA,1976].

In all other gibbon taxa, however, the male participates with a fixed role in the female song bout:

1. In H.syndactylus, the song bout is a highly organized duet between the partners; the climax is - acoustically and locomotorily - mainly an effort of the male [LAMPRECHT,1970; TEMBROCK,1974; CHIVERS,1974].

2. In H.hoolock, both partners utter, more or less simultaneously, similar call sequences; during the great call climax, both show a burst of vigorous locomotion in the branches [MCCANN,1933; MARLER & TENAZA,1977].

3. In H.concolor, H.pileatus, H.l.lar, H.l.carpenteri and H.l.agilis the acoustic part is mainly a female effort; the male utters single calls in the intervals between the great calls of the female, as well as a short fixed sequence of calls («coda», MARSHALL & MARSHALL [1976]) at the end of each great call.

While in H.l.lar, H.l.carpenteri and H.l.agilis the male and the female show a burst of locomotion, in H.concolor this is performed by the female alone; in H.pileatus: ? [DELACOUR,1933; CARPENTER,194O; ELLEFSON,1967; BROCKELMAN,1977; DEMARS & GOUSTARD,1978; GITTINS,1979].

6.7.2. Acoustic structure of the great call

The differences between the standard patterns of the great calls of the different gibbon taxa are considerable. It is possible, however, to make a rough classification of the different great call types according to the occurrence, arrangement and number of the short (uttered in series) and the long (soaring) calls. (The classification is based on the papers of the above mentioned authors.)

1. Characteristic of the female great call in H.syndactylus is the lack of soaring calls; the great call consists exclusively of relatively short calls.

2. In H.concolor, H.hoolock and H.klossii the beginning of the great call consists of soaring calls.

In H.concolor, there are only 2-3 such calls; they are followed by a series of short calls which terminate the great call.

In H.hoolock and H.klossii, there are several long calls, followed by a series of short calls; the great call ends as it began with several long calls.

3. In H.pileatus, H.l.mülleri, H.l.moloch, H.l.lar, H.l.vestitus and H.l.agilis the great call begins with a series of short calls, progresses to soaring calls in the middle part, and returns at the end to a series of short calls.

In H.pileatus and H.l.mülleri, the final sequence of calls is longer, and the calls are uttered in extremely rapid succession («bubbling», MARSHALL & MARSHALL [1976]).

In H.l.moloch, the number and frequency of calls is approximately the same as in the series at the beginning of the great call.

In H.l.lar, H.l.vestitus and H.l.agilis, there is - with regard to length and frequency of the calls - only little difference between the series of calls at the end of the great call and the soaring calls.

Duration of the great call. The duration of the qreat call in the different gibbon taxa is - apart from 2 exceptions - rather uniform: 15-18 seconds (H.l.moloch: approx. 16 seconds).

The two exceptions are (1) H.concolor, whose great call is rather short (approx. 10 seconds), and (2) H.klossii, whose great call - with a duration of 30-40 seconds - is by far the longest.

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