Chapter 7


Home Range and Territory

© 1981 Markus Kappeler

 


7.1. General

Silvery gibbon groups have fixed excursion areas, i.e. home ranges: Each group limits its foraging to a certain part of the forest, uses certain trees in this area for sleeping and singing, and keeps to essentially settled routes for fleeing.

Accordingly, the use of habitat is based on the exact knowledge of a home range.

This attachment of the group to a home range is combined with intolerance towards non-group conspecifics and - except in certain border zones of the home range - with dominance over them.

Apart from these border zones, the home range is equivalent to a territory.

Border zones: The group home range can overlap at the boundary with home ranges of neighbouring groups. The intolerance of the group towards conspecifics extends also to these overlap areas; in the outlying parts of them, however, the respective neighbouring groups turn out to be dominant. These peripheral regions of the group home range («border zones») therefore belong to the territories of neighbouring groups and may be termed «infrinqement areas».

The following sketch serves to illustrate the territorial field structure in the overlap area of the home ranges of neighbouring gibbon groups. (...)

This chapter will consider:

1. size of home range and territory,
2. distribution of main activities in the home range,
3. non-directed territorial behaviour,
4. behaviour during encounters with non-group conspecifics, and
5. function of the territory.

 

 

7.2. Size of home range and territory

 

7.2.1. Home range


a. Area inside the perimeter

During the observation year, the extent of the home ranges of the 6 neighbouring groups A - F in the Turalak area could be determined fairly accurately.

Indications as to the delimination of the home ranges were given in the first instance by the routes chosen by the individual groups during foraging. Additional information was gathered from (1) the routes taken by the groups after disturbance by the observer (they always fled towards central parts of their home ranges; s. 5.4.4.), and (2) countersinging of neighbouring females (s. 6.4.5.a).

The mean size of the 6 home ranges was 17.4 ha (11.9 - 22.2); on the average 2.1 (1.4 - 2.6) ha per home range lay in the overlap area with neighbouring home ranges.


b. Actually usable area

In general, the area inside the perimeter of the home range can not wholly be used by the gibbons, since the home range usually comprises certain parts where the vegetation does not meet the animals' needs [concerning needs s. KAPPELER,1981].

Data concerning the actually usable area were only noted for the home range of group D:

The area inside the perimeter of this home range was 13.5 ha; of this, an area of approximately 8.5 ha was covered with trees which were at least 20 meters in height and accessible to the animals (i.e. not standing isolated; s. figure 7).

The area which could actually be used by group D was therefore only around 60% of the total area of its home range.


c. «Excursion space»

Gibbons use their home range three-dimensionally: Basically, the canopy region is used upwards, completely, and, downwards, to a level of approximately 10 meters above ground.

In the home range of group D (area covered with trees) the average height of the canopy was approximately 30 meters, resulting in a «vertical home range» of around 20 meters and, correspondingly, a volume of 1.75 million m3 excursion space inside the perimeter.

As with the area inside the perimeter of the home range, the volume inside the perimeter of the excursion space cannot be used entirely as ranging or feeding space; however, because of lack of data, no assessment can be made concerning the actually usable space.

 

7.2.2. Territory


The boundaries of the territories coincided in some places - i.e. where vegetation unsuitable for the gibbons separated the home ranges of neighbouring groups from one another - with the borders of the home ranges.

In the overlap areas of the home ranges of neighbouring groups, information on the location of territorial boundaries could basically be obtained from territorial disputes (s. 7.5.1.) between the groups. Such disputes however were observed only very seldom so that the territorial boundaries in the overlap areas could generally not be determined.

Since in the 6 observed cases the disputes were carried out roughly in the middle of the overlap areas, the size of the territories was determined on the assumption that one half of each overlap area of the home range of a group with neighbouring home ranges generally belonged to the group's territory.

In this way, an average size of 16.4 (11.2 - 20.9) ha was assessed for the 6 territories in Turalak. The territories of the 6 groups were therefore, on the average, 6% smaller than their home ranges.

Territory size observed for other gibbon taxa varies between approx. 7 ha (H.klossii; TENAZA [1975]) and approx. 40 ha (H.l.lar; ELLEFSON [1967]).

In his investigations of a population of H.l.lar, ELLEFSON [1967] mentions a difference of approximately 10% between territory and home range.

The actually usable area inside the perimeter of the group D territory was approximately 8 ha, i.e. approximately 60% of the 12.7 ha constituting the total area of the territory; the volume was around 1.6 million m3.

 

 

7.3. Intensity of use of the different parts of the home range
(Distribution of the activities in the home range)


The different parts of the home range are used with different intensity.

To begin with, there may be - as mentioned above (section 7.2.) - areas inside the perimeter of a home range which are not used, since their vegetation does not correspond to the needs of the gibbons.

Apart from this, however, it can be seen that, generally, the more or less central parts of the home range are used more intensively than the peripheral zones.

This fact will be elucidated in the following, based on the distribution of the main activities in the home range.

To investigate the distribution of the activities of a gibbon group in its home range, a grid of l/4 ha squares was laid over the range map of group D, and the occurrence of the activities sleeping, singing and feeding in each specific square was determined. The findings were then compared with one another to establish whether there was a correlation between the three distributions.

 

7.3.1. Sleeping

Silvery gibbons do not sleep at random on any tree in their home range, but visit particular trees for this purpose. These «sleeping trees» are the starting and end points of their daily foraging excursions. As a rule, the whole group sleeps in the
same tree, and usually two different sleeping trees are visited on two consecutive nights.

Group D used a total of 19 trees to sleep in. Figure 8A shows their distribution in the home range. The sleeping trees are situated in 11 (= 18%) of the total 62 squares.

 

7.3.2. Singing

Special trees are also visited for singing (s. section 6.3.).

The adult female in group D used a total of 15 such singing trees; 11 of them were used regularly during the whole year, the other 4, however, only sporadically during two months (s. section 6.3.).

Figure 8B shows the distribution of the singing trees in the home range of group D. The singing trees used during the whole year are distributed among 9 (= 14.5%), those used seasonally among another 4 (= 6.5%) of the total 62 squares.

 

7.3.3. Feeding

To determine the distribution of the activity «feeding» in the home range, the length of the foraging route in each square of the grid was measured.

Stationary feeding bouts were not given special consideration; they occurred during all foraging excursions repeatedly and in more or less regular intervals and were always of relatively short duration (s. section 4.3.). Their influence on the picture
resulting from the distribution of route length of the foraging excursions should therefore be insignificant.

The distance which group D travelled daily on its foraging excursions varied between approximately 500 and 2500 meters; on the average it was around 1200 meters (n = 11 days; these observation days were distributed over the whole year and can be - according to the observer's experience - considered as a representative sample of the group feeding behaviour in the course of the year).

Smaller detours by the individual group members were not considered. Only the length of the general route of the foraging excursion was measured.

In figure 7, a typical example of a daily route is shown (length: 1350 meters).

On the 11 observation days, group D covered a total of 13.600 meters during its foraging excursions; it thereby moved through 49 (= 80%) of the total 62 squares, which means that it fed in most parts of its home range.

The distance covered per square, however, varied considerably: Whereas the average value of the 11 daily routes was approximately 220 meters per square, the actual distance covered per square varied between 0 and 960 meters:

in 40 squares (65%) it lay below the average (<220 meters),
in 2 squares (3%) it corresponded to the average (= 220 m),
in 20 squares (32%) it lay above the average (>220 meters).

In figure 8C, the distance travelled in each specific square is given. In 13 squares, a distance higher than twice the average value (>440 meters) was covered; in the figure these squares are shaded (60% of the daily feeding activity occurred in these 13 squares).

 

7.3.4. Summary

If one compares the findings on the distribution of the three activities sleeping, singing and feeding in the home range of group D, the following pattern emerges (figure 8D):

1. The 11 squares visited for sleeping, the 9 used during the whole year for singing, and the 13 most frequently visited for feeding are largely congruent; they make up only 16 (= 26%, i.e. approximately 1 quarter) of the total 62 squares.

2. These 16 squares are not distributed at random over the home range, but form a coherent block.

3. This block, while comprising the geometric centre of the home range (s. figure 8D), lies mainly in the eastern half, bordering on the north-east periphery of the range.

Because of the high density of use - and there fore its «central» meaning for group D - this block will be termed «core area».

A measure for the varying intensity of use of the core area as opposed to the rest of the home range is obtained from the proportion of time spent by the group in each of the two parts of the range on the one hand, and the proportion of area sizes on the other:

Group D spent 20.5 hours - i.e. 85% of the 24 hour cycle, or 68% of its 11 hour activity period - in the core area; this area, however, was 2.8 times smaller than the rest of the home range.

Therefore the intensity of use of the core area was 16 times, resp. 6 times, larger than that of the other parts of the home range.

Obvious reasons for the intensive use of the core area were not apparent: It had neither an especially high density of trees resp. food trees compared to the rest of the home range, nor was it distinguished by any special topographical features.

It is probable, therefore, that the high density of use sprang primarily from an endogeneously based relationship of the animals to this area. Presumably it was the part of the forest with which they were most familiar and, consequently, in which they preferably stayed.

A tendency to use the core area of a home range intensively has also been observed in other gibbon taxa: ELLEFSON [1967] and CHIVERS [1974], investigating groups of H.l.lar resp. H. syndactylus, mention concentrated activity in a small, more or less centrally situated part of the home range, combined with excursions into the other, more peripheral parts of the range.

 

 

7.4. Non-directed territorial behaviour

 

7.4.1. Surveillance of the surroundings

As mentioned earlier (section 5.2.), the silvery gibbon often interrupts its daily activities for a short time and looks around. It remains quietly in one place, scans the surroundings and is also very receptive to acoustic stimuli.

This periodical surveillance of the surroundings is functionally polyvalent. Questions such as «Where is food?», «Which are the best passages?», «Where are the other group members?», «Are there predators close by?» are answered in this way.

Undoubtedly, it also serves to detect non-group conspecifics which may have entered the territory.

 

7.4.2. «Patrolling effect» of foraging

The silvery gibbon moves through considerable parts of its home range every day in search of food (s. 4.3.1. & section 7.3.3.).

Foraging, therefore, serves not only to procure food, but at the same time gives the gibbon a continuous orientation concerning the actual and prospective supply of food in the home range.

Moreover, foraging has a functional aspect with regard to territoriality: It provides the gibbon with the opportunity for an early encounter with trespassers and so of driving them off.

 

7.4.3 Singing

The female song bouts are a demonstration of the animals' presence with the character of a non-directed «threat display» (s. section 6.6.).

 

 

7.5. Behaviour during encounters with non-group conspecifics

 

7.5.1. Border conflicts between neighbouring groups


a. Occurrence and duration

In comparison to the populations of H.l.lar respectively H.l.agilis investigated by ELLEFSON [1968] and GITTINS [1979], where family groups took part in border conflicts 2-3 times a week, the Turalak population of H.l.moloch rarely engaged in such conflicts. During the whole observation year only 6 border conflicts were observed in the 6 groups residing in the study area.

This, however, roughly corresponds to the observations of CHIVERS [1974] and TENAZA [1975] on the frequency of border conflicts in H.syndactylus resp. H.klossii.

A total of only 10 encounters between neighbouring silvery gibbon groups was observed. They all occurred in the same part of the forest, namely the overlap area of the home ranges of three groups (NW-corner of the home range of group D (s. figure 7)).

In the course of the year all three groups roamed occasionally through this part of the forest during foraging, but no encounters between the groups were observed.

In spring (February/March), however, when - during a generally low supply of food (s. figure 2) - each of the three groups gathered there repeatealy for stationary feeding bouts in an especially large and, at that time, fruiting Ficus benjamina, several encounters between 2 groups occurred (encounters between all 3 groups were not observed).

It seems, therefore, that the groups normally avoid each other in the overlap areas of their home ranges; however, in exceptional cases, i.e. when the overlap areas are especially attractive (due to a special supply of food), rule does not apply.

In the 10 observed encounters, in 4 cases countersinging between the two adult females (s. 6.4.5.a), but no directed aggressive behaviour of the two groups, occurred; the animals separated after the end of the song bouts.

6 cases, however, had distinct conflict character; the behaviour observed in these cases is discussed below.

Of these border conflicts 5 took place in the morning between 7.30 and 11.30 and 1 around 4 p.m.

The border conflicts varied in duration between 5 and 40 minutes (average 26 minutes).

 

b. Observed behaviour

With one exception (s. below), the behaviour of the gibbons during border conflicts comprised:

1. directed aggressive behaviour of the adult males,
2. single screams uttered by all animals, and
3. single great calls of the adult females.

One - the shortest - of the 6 border conflicts comprised only directed aggressive behaviour of the two adult males and single screams uttered by them; the adult females as well as the younger members of the groups remained silent. However, since the behaviour of the males did not deviate from that observed in the other border conflicts, this instance will not be treated separately.

As soon as the two neighbouring groups became aware of each other, all the animals in both groups began to utter loud screams, and immediately the two adult males approached one another with swift swings. Some 20-30 meters apart they stopped, remained still and stared at each other. Presumably they were now at the common boundary of their territories.

The other members of each group followed «their» respective male at a distance and dispersed roughly 20-50 meters behind him in the branches.

All the animals still screamed almost uninterruptedly.

The adult males now sat opposite one another for several minutes screaming; sometimes the one or the other moved shortly to and fro.

Then, suddenly, one of the males advanced with vigorous swings upon the other who turned and - pursued by the attacker - fled deeper into his territory. Shortly afterwards both males reappeared, although with the former attacker in the role of the pursued now.

Usually the chase continued past the starting point straight into the opposite territory from where the animals reappeared a little later - again in reversed «sequence». Sometimes, however, the chase was interrupted near the starting point, and the males sat, hung or stood opposite one another again for a few minutes.

Chasing and sitting opposite one another alternated several times.

The pursuits occurred not only in a horizontal, but frequently almost vertical, direction and took place therefore at all levels of the forest.

The pursuits went only approximately 100-150 meters from the starting point into the territories of the two males, but their speed was so high and the vegetation so dense that details could not be observed, Therefore it remained uncertain

1. how the «sequence» was changed: whether the retreating male stopped at some point, offered resistance to the other male and chased him back, or whether the advancing male stopped at some point of his own accord and returned, and was then pursued by the other male. (Rather in favour of the latter case are the observations of ELLEFSON [1968] on border conflicts between H.l.lar groups; the males, after having pursued their opponent for a maximum of 100 meters, returned of their own accord to the starting point, i.e. independently of whether they were pursued or not.);

2. whether acts of fighting between the males occurred, as have been observed by ELLEFSON [1967] for H.l.lar and also by TENAZA [1975] for H.klossii.

While sitting opposite one another both males uttered single screams almost constantly. The pursuits, however, were not necessarily accompanied by constant screaming, although mostly they were reinforced by single screams.

The other group members (females and younger animals) showed no directed aggressive behaviour during the border conflict. They usually remained in the same place the whole time (sometimes individual animals moved slightly) and took part only by screaming. For the rest they attentively observed the behaviour of the males.

Occasionally the screaming - during the pursuits of the males - diminished in strength, only to return to full force as soon as the males sat opposite one another again. Apart from this, no coordination of screams within the group could be recognized.

A border-conflict screaming, 19 minutes in duration, in which 7 animals (including adult males) took part, comprised approximately 2500 single screams (= average 130 screams per minute).

The adult females, in addition to utterinq single screams, occasionally, i.e. in long irregular intervals, gave single great calls (s. 6.4.5.b).

During the already mentioned 19-minute border-conflict screaming, one female uttered 10, the other 6 great calls.

Contrary to the behaviour during female song bouts (s. 6.2.3.), the females did not show any conspicuous locomotion while uttering the great calls, but also remained in one place. Furthermore, in many cases, shortly after one of the females had begun a great call, the neighbouring female began to sing, too. In border conflicts, therefore, they often uttered their great calls almost simultaneously, and not alternating, as was generally the case in song bouts of two neighbouring females near the border (s. «countersinging», 6.4.5.a).

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

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

Suppression of calling by other group members during great calls of the female has also been noted for other gibbon taxa (H.l.carpenteri, H.l.ssp.; «prepotency of the great call over other kinds of vocalizations» CARPENTER [1940]; TEMBROCK [1975]).

Border conflicts were wont to end rather abruptly. After one of the fast chases, both males stopped near the starting point, but instead of sitting opposite one another again wandered quietly - without paying any further attention to each other - to the more central parts of their home ranges.

As soon as the males separated, the other group members calmed down and likewise retired.

c. Comparison with other Hylobatids

Like the female song bout, the border conflict between neighbouring groups constitutes a homologous feature of the whole family Hylobatidae.

In all gibbon taxa, the adult males show directed aggressive behaviour and utter single calls («battle hooing», ELLEFSON [1968]).

However, the participation of females and younger group members in border conflicts varies in the various gibbon taxa:

1. Communal screaming by all group members and sporadic utterance of single great calls by the female seem to occur - except in H.l.moloch - only in H.syndactylus [CHIVERS,1974].

2. In H.l.lar no such border-conflict screaming exists; the females, however, may occasionally utter single great calls [ELLEFON,1967].

3. In H.klossii and H.l.agilis, neither does common screaming by the whole group nor great calls on the part of the females occur during border conflicts. Possibly the females and the younger group members utter single hoo notes [TENAZA,1975; GITTINS,1979].

 

7.5. 2. Reactions to intruders

On 7 occasions a resident gibbon group was observed to encounter roaming conspecifics - in two cases single animals, in five cases pairs - which had entered its territory.

Each time, upon the appearance of the resident animals, the intruders instantly took flight.

In 5 cases the resident adult male pursued them over a short distance <100 meters) and returned to his group again, without any fighting having taken place.

In 1 of these 5 cases a subadult (presumably male) accompanied its father during the short pursuit of the fleeing strangers.

In all 7 cases the resident adult female moved immediately after the encounter to a nearby singing tree and began to perform an intensive song bout (s. 6.4.5.c).

 

 

7.6. Functional aspects

 

Group territoriality signifies intolerance towards non-group conspecifics bound to a fixed area, and dominance of the «owners» over «strangers».

The social environment is polarized, positive social relationships within the group standing opposite potential aggression towards non-group conspecifics; this is specially clearly demonstrated by the joint behaviour of group members during encounters with non-group conspecifics.

The function of territoriality is undoubtedly the attachment of vital resources (especially food; s. below) to the group, i.e. to the reproducing pair and its offspring as long as they are still dependent.

In the home range of group D, all functional places (sleeping/resting trees, singing trees, drinking places) were severally represented. However, the food supply seemed to be low in spring: During this season the animals undertook more distant foraging excursions, roamed more often making more use of the peripheral zones of their home range, and took part in territorial border conflicts.

Since the young emigrate, or are expelled, on becoming adult (= derangement of the social polarization) the population density and, consequently, the stress on the biotope remain almost constant. In this way overexploitation of the resources (especially of food) is avoided.





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