Markus Kappeler Vocal Bouts and Territorial Maintenance in the Moloch Gibbon in H. Preuschoft et al. «The Lesser Apes. Evolutionary and Behavioural Biology», Edinburgh University Press, 1984, S. 376-389

1984 hatte ich die Gelegenheit, aufbauend auf meiner 1981 abgeschlossenen Dissertation drei Kapitel im Buch «The Lesser Apes» zu veröffentlichen. Das über 700 Seiten starke Werk war im Anschluss an die internationale Konferenz «The Lesser Apes» entstanden, welche im Juli 1980 auf Schloss Reisensburg bei Ulm durchgeführt worden war und an welcher ein Grossteil der damals aktiven Gibbonforscher teilgenommen hatte. Hier das Kapitel «Vocal Bouts and Territorial Maintenance in the Moloch Gibbon» im Wortlaut:

This chapter describes and evaluates the four types of loud vocal bouts observed in the moloch gibbon of Java. The female solo song bout, the most common type of vocal bout, plays an important role in the defence of space and resources claimed by the family group. It is often given as a direct response to territorial intrusion. The male solo song bout, heard very rarely, is given by single males and probably serves to attract mates. Border conflict call bouts consist of communal screaming by all group members. Such bouts occur during the rare encounters of neighbouring groups and might help in reinforcing the common territorial boundary. They are accompanied by aggressive behaviour between the adult males. Harassing call bouts, also consisting of communal screaming, are given in response to leopards and, possibly, man. They seem to be threat displays to impress and ultimately drive away predators.

Territory maintenance in the moloch gibbon is based on a distinctive sex-specific division of labour between the adult pair; the female engaging in a non-directed expression of intolerance, the male in actual directed aggression.

 

Introduction

 

The behaviour of the moloch gibbon, Hylobates moloch, comprises on certain occasions loud vocal signal - short simple notes or stereotyped complex phrases - which can be heard by humans over distances of 500-1500 m. In contrast to soft signals which typically occur singly, these loud signals are usually uttered in bouts of several minutes' duration.

There are four kinds of these bouts, which differ with regard to acoustic components, structural organization and the vocal contribution of different sexes and age classes. According to the definitions of Haimoff (1984), two of them would be considered «song bouts», the other two «call bouts». I will describe all four types briefly, because during many call bouts the adult female occasionally gives «great calls», which are more typical of song bouts. The four types of vocal bouts are as follows:

1) The female song bout is a solo peculiar to the adult female (although sometimes accompanied by the subadult female). As in Müller's and Kloss gibbons, but in contrast with all other types, the male makes no acoustic or locomotor contribution to the female's song bout. The female song bout contains conspicuous and rather stereotyped climaxes consisting of acoustic and locomotor components: the repeated «great calls» (Marshall and Marshall, 1976) combined with bursts of swinging and jumping in the branches. A description of the female song bout is given here, with information on the frequency and circumstances of its occurrence and effects on conspecifics.

2) Male song bouts are rare, and mated males were not observed performing song bouts at all during the study. This rarity of male singing is one of the most unusual features of this species. What little information was obtained on male singing is presented in one brief section.

3) Harassing call bouts and (4) border conflict call bouts are performed by all members of the gibbon group, including adults and young of both sexes. These bouts consist of incessant loud single screams without any coordination between individuals or temporal patterning being evident. During border conflicts the adult female, in addition to uttering screams, may occasionally give single great calls. Harassing call bouts and border conflicts are treated in greater detail in Kappeler (1981b).

The data in this chapter were collected from August 1975 to October 1976 in Ujung Kulon/Gunung Honje Nature Reserve in West Java, Indonesia. The reserve harbours Java's western-most gibbon population, which is estimated at 400-1400 individuals (Kappeler, 1981a).

Turalak, within the reserve, was chosen as the main study area. It lies approximately 50 m above sea level and is uniformly covered with primary tropical lowland rain forest, providing optimal gibbon habitat.

As is true of other gibbon taxa, the moloch gibbon lives in monogamous family groups which limit nearly all of their daily ranging to their territories ­ fixed areas from which neighbouring groups and other intruders are repelled. The mean home range size of the six groups resident in the study area was 17.4 (range 11.9-22.2) ha, and their mean defended territory size was 16.4 (range 11.2-20.9) ha. The mean size of thirty-one repeatedly observed resident gibbon groups in and around Turalak was 3.3 individuals (see Kappeler, 1981a for more details).

Structure and Duration of the Female Song Bout.

The female song bout comprises two main acoustic components: (1) Single wa notes (Fig. 31.1, line 1). These all show similar acoustical characteristics, but differ in sound intensity. (2) Great calls ( Fig. 31.1, line 2 ), which consist of approximately twenty-five notes in a highly stereotyped pattern. The intensity of the great calls of an individual female varies little. Since great calls are often broken off (aborted) before the end, the song also contains great-call fragments of varying length.

These acoustic components of the song bout are not inserted into the song at random: a definite overall pattern is present. The song bout can be divided into three types of sections or «sequences» (Haimoff, 1984). First, the introductory sequence consists exclusively of single wa notes which are mostly uttered at irregular intervals but partly also in series. Secondly, the great-call sequences consist of great calls or great-call fragments, which are given at fairly regular intervals, but in irregular order. Thirdly, so-called organizing sequences are given in the intervals between the great calls. This type of sequence consists of single wa notes as in the introductory sequence, uttered partly at irregular intervals, partly in series.

Variations in structure of the female song bout may consist of:

1) Breaking off the song bout at any place. The song may consist, in extreme cases, of only introductory wa notes, or comprise only a single great call in the great-call sequence.

2) Duration of the introductory sequence. Introductory sequences varied in duration between 0.4 and 5.1 minutes (mean 2.2 minutes); the minimum number of wa notes was 7, the maximum 226 (mean = 75; n = 37 introductory sequences).

3) Total duration of great-call and organizing sequences. This varied between 0.2 and 46.7 minutes (mean = 7.1 minutes). An average of 15 (range = 1-87; n=68 bouts) great calls and great-call fragments were given during a single song bout. The average number of complete great calls given was 9 (range = 1-52; n = 68 bouts).

4) Duration of the organizing sequences. In 68 songs, the intervals between two great calls lasted 1-105 seconds (mean 18 seconds).

5) Proportion of great calls to great-call fragments. The proportion of great calls to great-call fragments varied from song bout to song bout, even in the same individual; it averaged 1:o.8 (range 1:0.3-1:2.4).

With these variations in structure of the female song bout the total duration of the bout varies considerably. In 392 song bouts, it ranged between 1.0 and 48.o minutes (mean 9.3 minutes); 67 percent of the song bouts lasted less than 10 minutes, 24 per cent were 10-19 minutes, 8 per cent were 20-29 minutes, and 1 per cent 30 and more minutes.

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

The standard pattern of this clearly defined vocal unit shows the following seven phases (Fig. 31.1, line 2):

A. One (sometimes two) soft and very short introductory notes.

B. A series of about eight notes, lasting approximately 4 seconds.

C. Two longer notes within about 2 seconds which form the transition to:

D. Two long soaring notes, each approximately 2 seconds in duration. Immediately following the first of the two soaring notes (and longest note of the great call) a gasping sound (x), resulting from voiced inhalation, is often heard.

E. Two notes of decreasing length within approximately 2 seconds which form the transition to:

F. A series of about eight very short notes, lasting 2-3 seconds.

G. Often a soft single note of nearly constant pitch which closes the great call.

The great call thus shows a certain symmetry, with short notes at beginning and end and longer notes in the middle.

When great calls are broken off, this usually happens in phase B, 3-5 seconds after the beginning of the great call (80 per cent of sixty-one great-call fragments recorded).

The standard pattern of the moloch great call is constant throughout the species' total area of distribution. Variations can be heard, however, in the duration and sound quality of the notes (especially of the C-, D- and E-phases), in the length of the intervals between the notes, and in the number of notes in particular great-call phases, especially in the B- and F-phases. Variations in the great call characterize different individuals as variation within individuals is relatively low. In the Turalak study area the observer was able to recognize five different females by distinct characteristics of their great calls. One female had an especially long and rapid note series B, another an extremely long first soaring note D, a third a narrow sound spectrum in all notes (lack of upper sound frequencies), a fourth an extremely long but very slow note series F, and a fifth female had broken notes in the F series. These individual characteristics doubtless allow the local gibbons to recognize individual females by their voices. This mode of recognition is facilitated by the fact that in any given sector of the forest, only one female is normally in residence.

Subadult females may accompany their mothers during all sequences of the song bout. In the great-call sequence, the accompaniment is always rhythmically coordinated or synchronized (Fig. 31.1, line 3). If the mother does not finish her great call the daughter immediately interrupts her own.

The pattern of great calls uttered by subadult females is the same as that uttered by adults, and the variations are of the same kind. The great calls of subadults average 2-3 seconds shorter than those of their mothers, however, mainly because of a shortening of the notes in the D-phase (Fig. 31.1, line 3). Subadult females can often be distinguished from their mothers by their softer and higher-pitched voices.

Non-vocal Behaviour Accompanying Singing.

To initiate a song bout, the adult female moves to the periphery of the crown of one of her singing trees (see next section) uttering single soft hoo notes. There she stays for the duration of the introductory sequence. The wa notes are uttered sitting or standing with the head slightly inclined upwards (Fig. 31.2) and are not directed to any particular sector of the forest. Usually the female remains in the crown of the tree during the great-call sequence. During some bouts, however, the female moves to another singing tree or, in rare cases, travels along a fixed «singing route» over several singing trees (Fig. 31.3). Movement itself is silent and is undertaken during the organizing sequences between great calls.

Regardless of whether movement takes place during the great-call
sequence, each great call is accompanied by a burst of locomotion within the crown of the tree. At the beginning of the great call, the female remains sitting or standing still; then during the climax - the E- and the first part of the F-phase - she brachiates swiftly through the crown to another point on the periphery. There she ends the great call, usually in a sitting position.

Frequently, rotting branches or other plant parts break off during these vigorous movements and fall noisily to the ground. Such breaking of branches is not obviously deliberate but nevertheless seems to be a part of the display. Gibbons are extremely agile and virtually never «stumble» on dead branches during normal rapid brachiation. Observations by Carpenter (1940) and Ellefson (1968) on the lar gibbon support the idea that branch breaking is intentional. Ellefson mentions «presumably intentional breaking off dead branches, which come crashing to the forest floor», by adult males during territorial conflicts.

During the organizing sequences the female often looks out over the canopy. At the end of the song bout she 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.

When a subadult female accompanies the song of her mother, she usually sits in the same singing tree and 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. Bursts of locomotion as shown by adult females during great calls were never observed in subadults.

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

Before the beginning of a song bout, the adult male - together with the female - often utters hoo notes, but lapses into silence during the introductory sequence. During the great call sequence, he usually stays in one place the whole time, observing. Subadults (males and non-singing females) and younger group members pursue various quiet activities such as feeding, grooming and resting.

Singing Trees.

Female gibbons select special trees in their territory for singing (Fig. 31.3). These singing trees have partially, if not completely, free-standing crowns which either because of height or topographical location 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 fifteen different singing trees, of which six were also used as sleeping trees. They were all between 40 and 50 m high and belonged to seven different species; apparently various tree species can be used as singing trees.

Singing trees were mainly situated in central parts of the home range, although some were also in peripheral parts, even in areas of overlap with the home ranges of neighbouring gibbon groups. Such peripheral trees were used by females of more than one group when general food abundance was low and a rich food source was located in the border area. Singing routes were all located in the core area of a group's home range.

 

 

Timing and Functions of Female Songs

Frequency and Timing of Singing.

In the Turalak study area with five resident females, 392 female song bouts were noted. They were heard on 89 out of 130 observation days spread over the year; on 41 days (32 per cent) no song bouts were heard. Each female sang on an average of 45 percent of days (range 29-54 percent), and sang an average of 1.3 bouts per singing day. This represented, on average, singing once on 44 days, twice on 11 days and three times on 4 days.

Weather influences singing behaviour. Female gibbons do not usually sing when it rains. In Turalak, 50 per cent of all song bouts were noted when the sun was shining, 48 per cent when the sky was overcast, and only 2 per cent in the rain (equal observation time of fifteen days per weather category; since the audibility of song bouts was greatly reduced on rainy days, the frequency of song bouts during rainfalls might have been somewhat higher). Approaching thunderstorms also seemed to stimulate singing, however. It was noted repeatedly that shortly before the beginning of heavy rain, several song bouts were started nearly simultaneously, then broken off when the rain actually began.

The incidence of singing during the morning is shown in Fig. 31.4. Only 3 per cent of all song bouts were started before sunrise, but about 50 percent were begun within one hour after sunrise, and 90 per cent within the first three hours. Some singing activity (5 per cent of all song bouts) was observed again during midday, usually after the groups rested.

Time of day also influenced the duration of song bouts. Song bouts 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.

The frequency of song bouts was subject to seasonal variations. The annual average was 0.60 song bouts per day per female; the average for the months June/July/August 0.72, and that for February/March/April was 0.53. These variations were correlated with seasonal variation of food abundance; the food abundance during June-August was markedly higher than during February-April (see Kappeler, 1981b).

Social Contexts and Effects of Female Singing.

When a female sang in her territory, neighbouring females often immediately started to sing as well, or «chorus» with her (Tenaza, I976). I define chorusing as occurring when the song bouts of two females of different groups overlap for at least half of their duration. Usually, when chorusing occurred, one female began her song within 0.5-3.0 minutes after a neighbouring female had begun, and ended shortly after the neighbour ended. Reactions by neighbours were not invariable, however; in many cases song bouts were not answered. Song bouts in chorus with other females usually lasted longer (mean 11.9 minutes; n = 142) than bouts sung alone (mean 6.7 minutes; n = 119).

In more than ten cases, non-resident gibbons (single animals or pairs) present in the territory of a group reacted to a song bout of the resident female with immediate flight from that area of the forest, without being detected by the residents.

Occasionally intergroup confrontations were initiated by singing. Several times during the season of low food abundance a female began singing near a rich food source in the border area between her territory and that of a neighbouring group. In six of the seven observed cases the neighbouring group immediately appeared. In four cases the female of the neighbouring group also began to sing, only 50-100 m away. The two groups retreated into their respective territories after the ends of the song bouts. In two other cases, however, a border conflict with screaming and chasing developed between the groups.

Songs from strange individuals inside or at the periphery of a territory provoked the resident female to climb a singing tree in the vicinity and begin singing. 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 upon hearing the resident and fled. In each case they were pursued by the resident male, and in one case a subadult group member (probably male) also made an aggressive advance toward the intruders.

In one of these cases of intrusion, the strange animals were seen well enough to be sure that they were a mated pair of adults. I detected them as they started to flee upon hearing the song of the female of resident group D. I followed them until they stopped in a cluster of trees at the periphery of D's home range (group D had not seen them). There they remained quiet until dusk. They stayed in that cluster of trees (of less than 1 ha) for two days. Early in the morning on the fourth day, the female started to give great calls, after a short introductory sequence of only nine hoo notes, from a tall tree. After the second great call, resident group D, which had not visited that part of its home range for several days, suddenly appeared, and the strange pair fled. I lost contact with them and did not see them in the area again. This incident is of particular interest because it shows, first, that pairs can form before they obtain a territory, and secondly, that the females of such pairs can perform song bouts, although they cannot persist in this behaviour without a territory of their own.

In several cases a resident female could be induced to sing by playing a tape recording of strange great calls inside her territory.

On four occasions a resident gibbon group was observed to encounter conspecifics roaming within its territory. In two cases the intruders were single animals (of unknown sex) and in the other two cases they were pairs. Each time, upon the appearance of the resident animals, the intruders instantly took flight. In the cases of the two pairs, the resident adult male pursued them over a short distance (less than 100 m). In all four cases the resident adult female immediately moved into one of her singing trees in the vicinity and began to sing. These song bouts were rather intensive, being relatively long and with a short introduction. In one case the subadult female joined her mother.

 

 

Male Singing

 

During 130 full days of listening in the Turalak study area none of the five resident mated males within audible range ever performed a song bout. It thus appears that territorial male moloch gibbons do not sing. The only male song bout heard was by an unmated individual on the border between two territories. The male sat still during the bout at a relatively low level (15 m) in the trees. Another similar bout was heard from a distance in another part of the nature reserve. Two such song bouts were tape recorded from a solitary male caged in the Jakarta Zoo; this male sang about every two days. The two bouts heard in the wild lasted from 0755 to 0813 hours and 0519 to 0603 hours, respectively.

Three types of notes are given in the male solo:

1) wa notes, variable in length; they are like the wa notes produced by the adult female during the introductory and organizing sequences of her song bout.

2) wa-oo-wa notes, doubly inflected in pitch; rather variable but usually short.

3) wa-ooo notes, the oo sound at the end of varying length. These are characteristic male notes never produced by females. They are also produced by males during «border conflict screaming» and as harassment calls in reaction to predators or man.

No phrases or higher order patterns of notes are recognizable in the male solo song bout, but the wa-ooo notes seem to increase in frequency during the bout. Wa-oo-wa notes are relatively rare.

The function of the male solo is probably to attract a female or to help in setting up a territory.

 

 

Call Bouts

Border Conflicts between Neighbouring Groups.

In comparison with the lar gibbon (Ellefson, 1968) and the agile gibbon (Gittins, 1979), where family groups were observed to take part in border conflicts two or three times a week, the Turalak population of the Javan gibbon rarely engaged in such conflicts. Only ten encounters between neighbouring groups were observed. These all occurred in the overlap area of the home ranges of three groups, around the north-west corner of the range of group D (Fig. 31.3).

During most of the year all three groups roamed occasionally through this part of the forest without encountering each other. In spring (February-March), however, when the availability of food was generally low, each of the groups gathered there repeatedly to feed in a large fruiting Ficus benjamina. Several encounters between pairs of groups occurred at this time. It seems, therefore, that the groups normally avoid each other in the overlap areas of their home ranges except when these areas contain important food sources.

In four of the ten encounters observed, countersinging between the two adult females occurred, but no directed aggression. The animals separated after the ends of the song bouts. Six cases, however, had a distinct conflict character; these will be described in more detail. Five occurred in the morning between 0730 and 1130 hours and one around 1600 hours. They varied in duration between 5 and 40 minutes (mean 26 minutes).

With one exception, the behaviour of the gibbons during border conflicts comprised: (1) directed aggressive behaviour by the adult males; (2) single screams uttered by all animals; and (3) single great calls of the adult females. The shortest of the conflicts included only aggressive chasing between the adult males and screams uttered by them; the females as well as the younger group members remained silent. In all other cases, as soon as the two neighbouring groups became aware of each other all members of both groups began to utter loud screams, and immediately the two adult males swiftly swung over to within 20-30 m of each other. For a moment they remained still and stared at each other. Presumably they were now at the common boundary between their territories. The other members of each group followed their respective leaders at a distance and dispersed roughly 20-50 m behind them in the branches. All animals continued screaming almost without interruption. Sometimes one or the other adult male moved to and fro at short distance. Suddenly, one of them advanced with vigorous swings upon the other who turned and fled deeper into his territory. Shortly afterwards both males reappeared, the former attacker being pursued. The chase now went across the border and into the other territory. Chasing back and forth continued for most of the conflict bout, but usually it was interrupted several times by periods in which the males paused at the border and stared and screamed at each other while hanging or sitting. The pursuits occurred in nearly vertical as well as horizontal directions and occupied all levels of the forest.

The other group members showed no directed aggression during border conflicts. They usually remained in one place the whole time; sometimes individual animals moved slightly. They contributed only noise to the conflict, but all the while attentively observed the behaviour of the males. In one 19-minute conflict involving seven animals in all, I estimated that 2500 screams were given.

The adult females, in addition to uttering single screams, occasionally gave single great calls. In the 19-minute conflict mentioned above, one female uttered ten, the other six, great calls. These calls were given after irregular intervals, not after the measured intervals that characterize great call sequences of regular song bouts. The structure of the great calls themselves, however, was in no way different. In contrast to song bouts, the females did not show any conspicuous locomotion during great calls, but remained in one place.

The younger members of the groups generally became silent when a female sang her great call and resumed screaming after the end of the call.

The females usually gave their great calls while the males were sitting opposite one another. In almost all cases the males then proceeded to chase one another again.

Border conflicts usually ended rather abruptly. After one of the fast chases, both males stopped near the starting point, but instead of sitting opposite one another again, they wandered away quietly, without paying any further attention to each other. As soon as the males separated, the other group members calmed down and likewise retired into their respective territories.

Border conflicts between neighbouring groups are a constant feature of the family Hylobatidae, but the behaviour of gibbons during these bouts varies among different taxa. In all taxa, the adult males show directed aggression and utter single calls, though not always loud calls (e.g. «battle hooing», Ellefson, 1968). Communal screaming by all group members seems to occur only in the Javan gibbon and in the siamang (Chivers, 1974). Great calls by females are associated with intergroup conflicts in several species, however (Brockelman and Srikosamatara, 1984). Neither great calls nor screaming by the whole group during conflicts have been reported in Kloss or agile gibbons (Tenaza, 1975; Whitten, 1980; Gittins, 1979).

 

Harassing Call Bouts.

Harassing call bouts constitute the final major category of loud calls. They consist of short loud screams emitted by all group members mixed with bursts of agitated movement. This behaviour was witnessed on three occasions in response to a leopard, and on several occasions in response to man. It is not an invariable response to man, however; on many occasions the group may observe silently from the canopy, withdraw quietly, or flee through the trees rapidly and conspicuously. Harassing call bouts have none of the characteristics of singing bouts: spontaneity, temporal organization, complex acoustic patterning and coordination among participants.

 

 

Conclusions

 

The female song bout has the character of a dynamic acoustic-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 therefore serves in the first place to manifest the group's claim on space and resources.

Broadcasting intolerance towards non-group conspecifics, however, does not guarantee maintenance of the territory per se. The observations show that floating strangers who intrude or attempt to settle in the territory and neighbouring groups who try to expand their territories may nevertheless contest the group's exclusive claim. It is the adult male who repels such challengers from the territory, and thus acts as «representative» of the family group in actual conflict situations.

Thus in the moloch gibbon, territoriality seems to be based on a neat sex-specific division of labour: the female engaging in non-directed broadcasting of potential aggression (towards all other conspecifics in the area), the male in actual directed aggressive behaviour (towards animals which have entered the territory). Such an arrangement may reduce the total amount of energy expended by the group in maintaining the territorial system.

In addition to its clear role in territoriality, the female song bout undoubtedly has intrasocial significance. The adult male and also, in many cases, the subadult daughter, seem highly attentive to the song bout of the adult female and participate in it in a certain way: the male by quietly remaining in one place for the whole duration of the song bout and surveying the surroundings, the daughter by vocal accompaniment. The male also seems to play a role in initiating song bouts. Thus, although the adult male contributes neither acoustically nor with locomotion to the female song bout, it is an «event» in which the adult pair and the subadult daughter participate. The female song bout might therefore also serve to manifest and strengthen the family bond in spite of appearing as a solo. This could be true as well for the noisy border conflicts.


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