Song
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.