Long duets deafeningly performed by both sexes
are also the main form of territorial proclamation in the gibbons.
These long-limbed apes live in monogamous family groups of a male
and female along with several of their offspring. The juveniles
span several years of reproductive effort, constituting a type of
social system which probably stems from the gibbons' likely habit
of pairing for life. Their song is remarkably sophisticated and
elaborate, as well as being extremely loud. The 'great call' of the
large siamang overcomes the smothering effect of the billions of
leaves in the forest canopy so successfully that it can be heard at
a distance of 1 km (1000 yds) or more. During the elaborate duets
generated by Mueller's gibbon (Hylobates muelleri) the males and
females produce their own individual songs which interact in an
intricate interlacing of sounds. These show a distinct sequential
organization in which the female always takes the lead. Solo
singing is restricted to the males, who give vent to lengthy and
involved bouts of song which lacks the sequential arrangement heard
in the duets. Each species of gibbon has its own particular song,
which is recognized by opposing groups and helps stave off boundary
disputes.
However, singing alone may not always adequately
serve this purpose and all too often a border encounter
deteriorates into blatant displays of aggression or even physical
combat. When this happens it is always the males who go in for the
rough stuff, while the females sit and watch or even indulge in a
spot of mutual grooming. It seems that, as in human arguments, the
initial 'war of words' between conflicting gibbon groups may not
always suffice to avoid the outbreak of battle. Gibbons need large
territories to provide them with the wherewithal to survive through
the leaner seasons. These are around 54 ha (133 acres) in the lar
gibbon and around 48 ha (119 acres) in the siamang which, though
larger, tends to cover less ground during each day. With the
omnipresent chance that pushy neighbours may try their luck at
trespass and the possible annexation of a slice of territory, it is
not surprising that boundary defence occupies a remarkable
percentage of a gibbon family's daily routine. It involves singing
on most mornings (and sometimes in the afternoons as well) plus
fairly regular border skirmishing, although this is very rare in
the siamang whose earth-shattering outbursts of afternoon singing
generally seem to be enough to keep any rivals at a respectful
distance.
The song of a gibbon family echoing through the
dripping groves of stately tree ferns and moss-draped trees in some
Malaysian rainforest is undoubtedly both impressive and memorable,
but for that special quality of spine-tingling eeriness calculated
to raise the hairs on the back of the neck it cannot compare with
that master among primate songsters, the indri. When an indri
family's wailing chorus erupts suddenly and without warning from
just a few metres above one's head in some damp Madagascan forest
the effect is heart-stopping.
The indri is the largest of the extant lemurs,
looking surprisingly like a big black and grey teddy bear. Indris
have a remarkably laid- back lifestyle, rising late, retiring early
and not moving very far or very fast in between. They live in
monogamous family groups of two to five individuals, usually
comprising a male and female along with their offspring. These may
occasionally be joined by one or more subadults from another group,
their presence apparently being tolerated mainly because their
eventual intention is to pair off with the resident juveniles when
they become mature. The family territory usually occupies 15-30 ha
of upland rainforest and has a perimeter band around 30-50 m wide
which is shared reasonably amicably with neighbouring groups.
Border disputes are rare, probably because each indri family's
regular song sessions serve adequate notice not only of their
continued ownership of that patch of forest but also of the sex
ratio within the group, its size and possibly even the sexual
condition of its members. The latter possibility has been inferred
from the considerable increase in singing noted during the breeding
season in midsummer.
For the indri song represents a particularly
useful method of social communication. With its rather low- quality
leafy diet and fairly cool habitat the indri would find regular
aggressive defence of the territorial boundaries a most tiresome
and energy-wasting affair. Song is less energy-demanding, and yet
by penetrating a long way it has the advantage of conveying
information to several groups at the same time without the need
even to move a limb - let alone start a fight. Indeed, the indri's
song is truly sensational in its power, carrying for up to 2 km
under favourable conditions, during which it may reach the ears of
as many as twenty different groups round about. Song also virtually
eliminates the need for regular demarcation by scent marking, a
habit much less common in the indri than in almost any other
lemur.
It is the male who first strikes up the indris'
weird chorus, this being just about the only occasion when he takes
the lead in anything. In all other respects he is thoroughly
subservient to the female, to whom he meekly and obediently gives
way in everything - he will even give up a favourite feeding spot
to her, usually without the slightest sign of hesitation or
resentment. Such placid reactions are typical of the indri, and
female-led groupings are not unique to them; in fact they are so
common in the lemurs as to be more or less the rule. This is an
important aspect of lemur social organization which sets them apart
from the mainly male- dominated monkey and baboon societies.
Watching indris for hour after hour in a
Madagascan forest can be a rather patience-stretching and motive-
questioning experience. If the weather is wet, droves of ravenous
leeches eagerly loop their way across the damp leafy carpet to
latch on to the tasty human. Unfortunately the human is rendered
easy and very static meat through being forced to sit there for
long periods just waiting for the typically somnolent indris
actually to do something in the trees above. However, even baby
indris are imbued with the playfulness natural to most infants, so
if a youngster is present in the group it may at least try to liven
things up by trying to instigate a bout of play with its parents,
scrambling around over their unresponsive bodies as it attempts to
elicit some reaction. Alas, all too often such adolescent high
spirits are doomed to failure in the face of the uncompromisingly
unresponsive constitution of the average adult indri. Even mutual
grooming, the delight of so many primates, is rare among indris, in
whom any kind of physical contact seems in general to be
shunned.
In some parts of its range the indri may live
alongside another species of lemur boasting considerable stentorian
powers, the beautiful ruffed lemur. Smaller and lighter than the
indri, and considerably more agile, ruffed lemurs live in
pair-bonded groups of a male and female with their offspring. The
'bark-bray' call, somewhat reminiscent of a cross between a dog and
a donkey, suddenly erupts from the foxy snouts of these impressive
animals with a startling unexpectedness which is particularly acute
when it happens at night, in marked contrast to the exclusively
day-singing indri. For an animal no bigger than a large domestic
cat, the volume of sound produced is really staggering. Every other
sound in the forest is drowned out as a calling round between
several neighbouring groups is set in train. Such rounds of song
initiated by one group and taken up by others, often far into the
distance, are also typical of the indri, certain gibbons, titis,
howlers and the guereza.
The ruffed lemur has a wide vocabulary of calls
for use in a variety of situations, from the bellowing 'roar-
shriek' which dominates the forest in communication between groups
to a rapid chatter signifying submissive status and a soft mew
normally exchanged between a mother and her infant; in all there
are thirteen different sounds. As in the similarly loud-mouthed
indri, ruffed lemurs seldom if ever use scent to demarcate
territory. The small family groups of the red-bellied lemur from
the same rainforests are strictly territorial, fiercely defending
the family boundaries in rowdy fights with neighbouring groups.
Both sexes delineate the perimeter of their territory by scent
marking with the ano-genital region, although males also deploy a
secondary marker from a gland situated on top of the head.