In the red howler there is one context in which verbal abuse serves no purpose - when several males from a rival group gang up to try and take over another group's females by kicking out the males. During such invasions the generally placid demeanour of the average howler is completely cast off and a savage bout of fighting may result in severe injury or even death. Just who wins generally seems to depend on the extent of mutual cooperation between either the invading or defending males: aggressive or defensive pacts between males on either side are seemingly vital in deciding the outcome. If successful, the raiders may consolidate their newly won position of sexual dominance over the group's females by killing one or more of the vanquished males' offspring. Such instances of infanticide are probably far more common among primates than had been thought, even being recorded in such generally peaceful animals as Milne-Edwards diademed sifaka, and it may be standard procedure under certain circumstances in Hanuman langurs.

In howler monkey society males rank above females and young adults above their elders, who gradually descend the status ladder as they grow older; immature juveniles, however, rank lowest of all. Normally there are few outward signs of this dominance hierarchy except when a subordinate gives way to a superior in matters such as access to food. The muriqui of Brazil's remnant Atlantic coast rainforests - the largest of the South American monkeys - also seems to reinforce the view that large size goes hand in hand with a placid lifestyle in arboreal primates for which leaves are the main form of sustenance. Muriqui groups may contain up to thirty-four members of both sexes. The males seem to get on particularly well with one another - any one male seldom allows himself to drift away from another by more than 5 m or so, and male-male relations are closer in this species than in any other primate. In fact a general feeling of harmony seems to pervade the entire group and belligerence of any kind is extremely rare, even when access to drinking or feeding sites is at stake. Even lengthy mating sessions (up to eighteen minutes at a time) between group members seem to generate none of the annoying curiosity or even outright interference which can make the love life of some other primates, especially baboons, so vexacious and difficult to consummate in peace. This is not the only unusual aspect of muriqui society, for it is the females who habitually leave the group of their birth and join another, leaving the males behind to constitute the stable nucleus of the group. Such a system is rare in primates; a further example is the red colobus, although in that species the males also show some roving tendencies. Female emigration is also found in the gorilla: a female may move home of her own accord, lured away when a neighbouring group comes close enough to effect the switch. She may possibly move on again later, trying several groups before finally settling down. In chimpanzee society, however, the females are generally coerced into leaving the group by pressure from other members. Generally, though, in most primate societies it is the males who are the emigrants.

In some multi-male societies the dominant males employ eye- catchingly explicit methods of demonstrating their superiority to a subordinate without having to prove the point by picking a fight. In the savannah-living vervet monkey the alpha male flaunts his genital region as a kind of 'badge of office'. In this species the scrotum is a conspicuous powder blue, against which the scarlet penis shows up particularly well when fully extended. To complement this vivid display there is a patch of red skin at the base of the tail, and below that a patch of white fur. A high-ranking male of a mind to reduce a subordinate to a state of cringing submission needs merely to stand up high on his rearlegs and strut in a cocksure manner around the inferior, who adopts a suitably hunched posture in response. As the alpha male circles, the grovelling object of his visual put-down also turns, but at a slower rate, constantly keeping the brilliant genital display in sight. The subordinate also keeps his own genitals well out of harm's way by retracting the scrotum into his body and keeping his rear end pointed well away from his circling tormentor. This protective attitude towards his vulnerable genitals may be crucial, as alpha males have a habit of eliminating once and for all a subordinate's future prospects as a sexual competitor by dashing in and taking a quick and conclusive bite at a carelessly exposed scrotum.

This little drama is the so-called 'red, white and blue display', which has been seen being given (with various small differences in detail) by vervets over a wide area. For example in Natal the displayer holds his tail in a curve in order to avoid spoiling the impact by covering his genitals; in Kenya the tail is held erect like a pleased cat. There are also variations from area to area, in the actual mode of giving the display, indicating that social behaviour in the vervet, as in most primates, is not set in tablets of stone but is able to respond to local conditions. During the red, white and blue display the recipient is normally thoroughly intimidated and cringes, giving vent to submissive wails and casting rapid nervous glances at the strutting performer. The latter uses his performance to reinforce his right of access to the females, a vital consideration in any multi- male group in which constant militant competition for mating rights could be potentially lethal for the participants and destructive of group cohesion. Indeed, when the females actually come into oestrus mere visual exhibitions alone may no longer be powerful enough to prove superiority. When this happens a real and very nasty fight may be the only way of sorting out the winners and losers; it can result in some unpleasant injuries, especially to such vital equipment as the genitals.

In wedge-capped capuchins (Cebus olivaceus) the dominance hierarchy among the females can lead to a rather unusual technique for 'pulling rank' by some of the more dominant individuals: parasitic nursing behaviour. Infant primates are often born hustlers and will routinely try and hijack a meal of milk from a 'foreign' nipple if one happens to be handy and there is a chance of getting away with it, and there is often a good chance of that. However, in the wedge-capped capuchin it is high-ranking adult females who poach milk under duress from their low-ranking companions. The latter do not appear to take kindly to such an infringement of their privacy and exhibit distinct signs of distress, but they have to sit and suffer as they are in no position to do anything about the situation without risking punishment, though thankfully such forced nursing episodes are normally short-lived. The reason for this strange behaviour is unclear; it may simply be a rare but regularly practised method of emphasizing status among the females.

The desire to avoid retaliation when approaching a superior has led to the evolution of a peculiar kind of behaviour in both barbary macaques and savannah baboons. In most primate societies the young juveniles are sacrosanct and can behave in the most outrageous way with even the most senior members of the group without fear of reprisal. A special spot of colour or brightly coloured pelage may be the key which switches on this tolerant response; or, more likely, switches off the normal aggressive reaction. Males of three subspecies of Papio cynocephalus have frequently been observed using infants as 'passports' to approach another male, often one in consort with a female, without fear of being beaten up. The infant is treated casually or even quite roughly by its kidnapper, being picked up and treated as a necessary object with a useful role to play in allowing a close approach in safety. The precise reason for this infant-napping is difficult to establish, as the guilty party may use his reluctant safe- conduct pass merely to approach the superior male and not make any physical contact.

Similar behaviour is seen in the barbary macaque, although in this species the young are quite closely involved with the adult males, even to the extent of sometimes being given lifts on their backs. However, when exploited as an aggression deflector a young juvenile in his clutches allows a subordinate male to approach a superior without risk of attack, and such baby borrowing is quite common in this species.

With few exceptions an infant is generally only treated with tolerance by males within its own group. One or more of these will probably have fathered all the resident juveniles, so showing tolerance makes good sense. However, there are records of infanticide within a group, although the circumstances which bring this about are seldom clear. For example, up to 1984 infanticide had been observed in chimpanzees once in the group at Budongo Forest, Uganda, seven times at Gombe Stream in Tanzania and six times at Mahale National Park in the same country. These episodes fall into three categories: the killing of an infant in one group by males from another; the killing of an infant (almost always a male) by males of its own group; and the killing of an infant by a female of its own group. Just why males should kill infants in their own group is a mystery, as chimpanzees' social arrangements decree that all or most of the infants within a group will have been sired internally. This suggests that the murderers are almost certainly killing their own offspring, a most peculiar act from a genetic point of view. In one case at Mahale, however, the attacking males may have been under the impression that the slaughtered infant had been sired by a male from another group, as the mother had been absent for a time, but observers' records suggest that this was incorrect and the most likely father was among the killers. The actual act in this instance was rather gruesome, and the mother suffered a considerable mauling in her efforts to protect her offspring by using her body as a shield. In most cases of chimpanzee infanticide the triumphant killers end up by eating their grisly trophy. In one well- observed instance the victim's brother begged for and received some of the meat from his brother's corpse- just part of a general feast in which several females also enjoyed various bloody titbits from the still warm body of the murdered infant.

Infanticide in species exhibiting the 'one-male' form of social system is perhaps a little easier to explain. In one-male groups a single male tries to defend access to a varying number of females against invading males from other groups intent on supplanting him. This form of society is found in species as varied as Geoffrey's spider monkey, agile mangaby, guereza and gorilla. In the widespread and highly successful Hanuman langur, one-male groups occur alongside multi-male and all-male groups. In multi- male outfits there may be a gradual and peaceful leadership succession which does not involve the death of any of the group members. By contrast, lone males trying to defend their exclusive property may enjoy little success against the kind of determined cooperative offensive which seems typical of this species. In one encounter which was closely watched a group of males attacked another group containing a single male accompanied by nearly thirty females and immatures. Despite their best efforts to defend their babies three females lost their offspring to the invading males, one of whom subsequently supplanted the resident male who had been ineffectual in countering such a concerted onslaught. Bereft of their progeny the bereaved mothers soon offered themselves to their new master and mated, giving birth to his issue a few months later. To us the females' actions may seem incredibly heartless - but after all, they do not want a cissy for a mate who may waste their reproductive potential by fathering a succession of equally gutless offspring lacking the ruthless determination necessary to depose a group leader and sire a line of offspring for themselves. So it is logical to make overtures to a male who exhibits all the signs of being one of life's winners.

A new leader normally harasses some of the other infant-bearing females soon after his conquest, but his baby-killing endeavours require skill and cunning as he has to reckon with the rest of the females who will often gang up to beat him off. Such attempts to weed out a predecessor's offspring after a takeover seem to be more common than pre-takeover infanticide. However, the latter situation has a greater chance of success if several males act in consort to overwhelm the opposition. This makes it easier to track down the mothers, who normally quickly see what is coming and do their best to hide away from the would-be killers of their babies. Some of the older females may have seen it all before and know the mortal danger to their offspring posed by the invading band of males. After all, their current lord and master probably won his job in the same way, and their new leader can probably only look forward to a tenure of around two years before he in turn is toppled in a cycle of takeovers which seems to typify Hanuman langur society.

The reasons for such acts of infanticide have been disputed, but seem to involve an acceleration of the coming into oestrus of the bereaved females. They would otherwise not be sexually receptive while still nursing babies which to the new master are not only superfluous to requirements but potential competitors. Similar takeovers accompanied by infanticides also occur in several other monkey species, including blue- and red-tailed monkeys in Africa and silvered and purple-faced langurs in Asia.

Hanuman langur society eventually settles down after such a traumatic episode and apart from this the females lead very stable lives, forming the nucleus of the group over long periods. Although there is an age- related dominance hierarchy among them (young mature animals gain dominance over their older colleages, and then drop down through the hierarchy as they too age) they spend a lot of time grooming one another in matronly harmony. This friendly accord also extends to the great indulgence shown towards handling one another's young, even when newly born. This is a common trait among the colobines and quite the opposite of most cercopithecine mothers, who exhibit extreme protectiveness of their offspring until they reach quite an advanced age.

Gorilla family life generally seems to be very amicable: the silverback group leader may groom his females as well as enjoying their attentions. He also shows extreme tolerance towards the youngsters in his group as they playfully clamber over his body. In fact his giant figure seems to be a positive attraction for juveniles of varying ages, who seem irresistibly drawn towards him in a bid to sit near him or even groom him. On the other hand, lone males intent on wife theft will bulldoze their way into a group and abduct a female after first getting her infant out of the way by killing it.

Patas monkey males seem to lead rather lonely lives, despite being the head of a group of females and offspring. The females appear to be notably anti- male for much of the year, actually threatening their leader if he comes close. They only reveal any clear signs of friendliness when they come into oestrus, when it is they who solicit his attentions. The group leader's wives, by contrast, lead very gregarious and affable lives featuring long grooming sessions and a great deal of mutual baby-sitting. Patas males too form single-sex groups in which they enjoy a much richer social life, forming close bonds with certain selected companions. What a marked difference from the isolated life of the leader in a mixed group, who seems to pay the price for sexual success by becoming almost a social outcast. Patas monkeys roam over enormous ranges in their dry savannah environment - up to 38.5 sq km (15 sq miles) has been reported - and when neighbouring groups make the occasional chance meeting they try to avoid any contact. The only exception happens during the breeding season, when rival males may raise the temperature somewhat by making aggressive chases and postures towards the other side.