Growing Up in a Monkey Group

Thelma E. Rowell

Methodological Problems

Primates as an order have an exceptionally long generation interval. Compared with other mammals of equivalent weights, gestation is long, lactation is long, and the juvenile period is long. Thus the minimal generation time is about 20 months for a marmoset smaller than a laboratory rat, 4 years for a typical Old World monkey the size of a dog, perhaps 10 or 12 years for a great ape. Nor does maturation cease with sexual maturity: increasingly, it is becoming clear that interaction patterns, the "social role," of adult primates of either sex continues to change for many years (e.g. Sackett and Ruppenthal 1973). This is not necessarily a peculiarity of primates, however; social maturation probably continues beyond sexual maturity in other social mammals.

We can, of course, make cross-sectional descriptions of social development, obtaining means and ranges of such measures as time spent in contact with mother, or frequency of maternal intervention in interactions between infants and others at particular ages. This method can be extended relatively easily to infants develop-

ing in widely different situations, including highly controlled and modified laboratory situations, with the effect of extending the range of the readings at a particular age. Development, however, is a process whose interest largely derives from the resultant end product, so that longitudinal studies are more satisfying. Yet even with enormous resources the answer to a simple question like "is there an effect of mothering received on maternal behavior towards first offspring" is nearly a lifetime research commitment on apes, and involves a time span far longer than the average project funding even for monkeys.

We have then two distinct types of information: first, descriptions of social behavior at successive ages, with a measure of variation to be encountered between individuals in the same general environment, and the extension of variation to be expected if environmental conditions are changed. Second, there are, or should be studies of what amount to causal processes, correlation of early variation in social experience with later variation in adult behavior at maturity. Because of the time needed to obtain the latter type, discussion of social development in monkeys must be augmented with the inspired reading of cross-sectional data as a series in time (cf. Kummer 1968), the extrapolation of laboratory data to natural situations with quite different social environment, and other forms of inspired guesswork. In this article I aim towards a consideration of social development in undisturbed groups of monkeys in their natural habitat, about which little data are available, because wild monkeys are shy and most live rather invisibly in dense vegetation. Data from provisioned wild troops in Japan provide some information, but social behavior has undoubtedly been altered by feeding if only because it allowed the numbers of animals in a troop to increase enormously (up to 700 in a troop). Individuals have, however, been followed through their whole lives, and genealogies are known. A step further removed are the rhesus macaques free ranging on islands off Puerto Rico. These also have a very dense population, are dependent on localized provided food, and are in an alien habitat; but known genealogies have been followed for long periods. Other sites are being developed which should provide valuable longitudinal information, such as wild populations of baboons at Gilgil in Kenya and Gombe Stream in Tanzania.

Kinship

Except for these studies, field students have guessed about the kinship of their animals, in fact very often individual identification was not possible. Captive groups typically start with strangers and studies are rarely long enough to see lineages develop from scratch (Kaufman's studies, below, are a notable exception). In both these circumstances there is a tendency to describe social interactions as between age/sex classes. This is better than nothing as a first approximation, but there is a danger of extrapolating from this an expedient to handle incomplete data to the assumption that monkeys actually interact with examples of an age/sex class, whereas in reality they interact with individuals.

Where kinship groups exist in a captive group, or are known in a wild group, it is clear that a major part of an individual's identity depends on its relations. In a captive vervet group that has been followed into the third generation, Bramblett (1970) found that both sexes were ranked according to their maternal lineage. Subadult males defended their mothers and only their own mothers if the senior, adult male who normally defended the group was absent; if all adults were removed, each infant was defended by its oldest male sibling only. Grooming, spacing, and other behavior could also be best described in terms of kinship links. Loss of kinship information can result in gross oversimplification. For example, a catalogue of the types of interaction seen between, say, adult males and infants under six months, which might perhaps be published under a title like "the role of adult males in the socialization of infant monkeys" would in fact be the sum of several different relationships with varying degrees of overlap in the type of interactions they include. Ransom and Ransom (1971) avoided this type of oversimplification in an illuminating way. In their study of interactions between male baboons and infants in a troop at Gombe Stream, four very different types of relationship were analyzed.

Most studies of infant development in captivity have explored the relationship of mother and infant in the first months, and the way in which the infant's dependence on the mother lessens. This interest stems in large part from the current stress on maternal care and the development of independence in Western society, but it also is a simple-seeming system that lends itself to experimental

manipulation and is reliably duplicated. In contrast, in wild or large, long-standing captive groups of monkeys, a mother and her new infant appear not so much as interacting individuals but as a single unit, with very different social properties from the pregnant female that preceded it. Social development becomes not so much a matter of leaving the mother but of entering new relationships, a gradually growing, but never complete separation of the interaction patterns of mother and infant.

Special Status of Young Infants

The infant monkey is not usually threatened or attacked, nor does it threaten or attack others. As a badge of this immunity it has distinctive markings: its fur is a different texture, and usually a different color from that of adults or juveniles. Thus dark brown stumptail macaques have blond infants, while light grey hanuman langurs have dark brown infants. Distinctive adult patterns are lacking—the brilliantly colored de Brazza's monkey (Cercopithecus neglectus ) with orange browband and long white beard has an allover brown infant, the talapoin infant has a pale pink face, lacking the dark cheek-lines and yellow side-whiskers of the juvenile and adult.

Infant monkeys all look much more alike than do adults of different species, and their infancy is recognized and responded to by other species. I have seen a captive baboon female lipsmack and grunt appreciatively at a newborn vervet infant in the next cage, to the alarm of its mother who probably knew that baboons are predators of vervets. Occasionally we find monkeys in the wild living with troops of the wrong species—an adult male vervet lived in one of the baboon troops I studied in Uganda, and Aldrich-Blake (1968) reported a female coppertail monkey that lived with a blue monkey troop (Cercopithecus ascanius with C. mitis ) and even had a hybrid infant. It is probable that these were the result of infant theft and adoption at an interspecies encounter. The infants were reared successfully because not only do infants look alike, but behavior towards an infant appropriate to ensuring its survival is much the same for all monkeys, and the behavior of the infants which helps to elicit such responses is also very similar.

There are differences in rates of increase of locomotor independence—Chalmers (1972) has found a correlation between habitat and developmental rates, with infants growing up high in trees be-

ing slower to go out of mother's arm's reach, reasonably enough, than infants of terrestrial species, which first run safely along the ground. In other respects the arboreal infants developed as fast as closely related terrestrial species, and the arboreal mothers were not more restrictive.

Social Experience of Young Infants

All infants in the first three months or so need suckling, carrying, grooming, protecting. During this time they are learning about their companions. Sackett (1970) presents evidence that there is an inborn recognition of, and preference for adults of own species. Yet the evidence of animals living with the wrong species in the wild, even though their own species uses the same range and the troops must on occasion meet, and the evidence of numerous hand-reared pet monkeys that continue to prefer human company even after being placed with groups of their own species suggests that this innate tendency may be to a greater or lesser extent overruled by early experience. Presumably it is normally reinforced by the early experience. The young monkey also learns to recognize the members of his troop, and the social interaction patterns to be expected between them and his mother and himself. An infant patas monkey, for example (personal observations) develops decided preferences among the would-be caretakers in the group which seem to be the result of experience of the type of handling it may receive from them. It tries to avoid caretakers that are rough or restrictive. The type of care given depends partly on the relationship of the caretaker to the mother and partly on the maturity of the caretaker. Juvenile females gradually come to treat infants less as playthings and more maternally, possibly as a result of learning which types of caretaking allow her to keep the infant for longer periods of time (Lancaster 1971). Thus the infant may aid in the social development of the older animal as well as vice versa. The infant's preference among alternative caretakers does not always coincide with that of its mother. In a cage situation mothers are reluctant to allow females ranking higher than themselves to take the infant because they have trouble in taking it back; yet the high-ranking female is often the most relaxed and permissive caretaker, and so is favored by the infant. This is the first point at which the mother's and infant's interaction patterns begin to diverge.

Besides the infant's special and generally recognized status as an

infant, it is also responded to from the moment it leaves its mother as an individual with known kinship. A juvenile female will glance at its mother before picking it up, even though she is not the closest other monkey. An adult female in a captive group will unerringly interfere in an infant play group if her friend's child is, in her opinion, being treated roughly, and cuff away the others (Rowell, Hinde, and Spencer-Booth 1964). Occasionally one sees a juvenile female retrieve an infant and carry it back to its own mother, perhaps to the other side of the group (e.g. Breuggeman 1973). An infant that is known to have influential protectors will be treated with great respect at least if any of its protectors are in sight. An adult male vervet will carefully avoid walking directly up to an infant because if one happens to scream at his approach a pack of females will instantly attack him, assuming him to have harmed it (Lancaster 1970). In this way an infant learns its own status including its rank, which is basically that of its mother. Differences between infants in the way they behave gradually become apparent, and are probably in large part the result of their different social experience.

Interspecific Variation in Social Experience

Although the caretaking needs of the infant and the social stimuli it provides are similar in different species of monkey, there is great variation between species as to who provides the care, and hence great variation in the social experience of the infant. At one extreme, the young infant is at first almost exclusively the concern of its mother, who fends off interested adults and juveniles by threats or by continually avoiding or turning away so they must look over her shoulder at the infant, according to her rank. As the infant gets older it is permitted to interact first with siblings and maternally related females, later with peers. But the pattern established very early, of interacting mainly with maternal relations, continues throughout life or until a male leaves his natal troop. Although all ages and both sexes do some caretaking, females of all ages do much more than males. Something like this pattern is typical of most of the species of monkey where infant development has been extensively studied—the rhesus and the pigtail macaques and the baboon and the vervet. It is by no means, however, the only social environment that the infant monkey may experience. In the

Barbary ape (Macaca sylvana ), studies of two separate populations (Deag and Crook 1971, Burton 1972) have emphasized the frequent interaction of males of all ages with infants. Burton gives the impression that the majority of the infant's social interactions other than with its mother are with males. In a mangabey (Cercocebus albigena ) Chalmers (1968) found that yearlings were cared for mainly by adult males, and similar behavior is reported from some Japanese macaque troops (M. fuscata ), though in both these species adult males interact only cursorily with small infants. All the species mentioned live in groups containing several adults of both sexes. Other species live in groups of one male and several females, obviously offering a different social environment for infants. The patas (Erythrocebus patas ) male is so peripheral to his group of females that infants must scarcely be aware of his existence until they are several months old. At the other extreme, marmosets and tamarins live in pairs, and the infant (usually one of twins) is cared for by father, mother, and juvenile siblings almost equally, though after the first three weeks the infant is returned by father and juveniles to the mother only for nursing (Nicoll, personal communication).

Multiparous female baboons are interested in a new infant, in that they investigate and groom it and occasionally pick it up if the mother permits, but their behavior towards the infant may also be ambivalent. They may behave as if they were jealous of the infant taking the mother's attention, and tweak at it maliciously behind her back (Ransom and Rowell 1972). Care is given more by nulliparous females. In contrast, once a patas infant is about three weeks old and starts to leave its mother other adult females frequently take it so that in the field it is very difficult to tell which infant belongs to which female (Gartlan, personal communication). In a caged group of patas, we have seen a female regularly take and suckle an infant younger than her own. In langurs the infant is taken and shared among all females in the group from birth, so that it is even more difficult to discover which infant belongs to which mother.

This catalogue of diversity could be continued, but the point has, I hope, been made that the environment for social development varies a great deal from one monkey species to another. The only generalization that can be made is that never is the mother the in-

fant's only caretaker. We would expect such different experiences to produce different "personalities" among the adults. Monkey species do have very different and characteristic temperaments, as any zoo keeper would agree, but we have no study yet which relates child-rearing practices to later adult behavior in different monkey species.

A Zoologist's View of Caretaking

A zoologist must always ask "what is the selection pressure that has caused this behavior to evolve?" Monkeys other than mothers spend time and energy caring for infants, and such expenditure must be selected against unless it is advantageous, in the strictly limited sense that the animal that does it must thereby be enabled to leave more offspring than one that does not.

For the first part of the answer we should return to our opening statement. It takes a lot of time to produce a primate infant, and a lot of energy goes into it, so that each individual already at birth represents considerable parental investment. It will probably be more efficient to rear this one successfully than to give up and start again. (Compare for example a newborn mouse, one of a litter of ten needing only three weeks gestation, thus a much more expendable individual in terms of maternal energy output.)

Monkeys live in relatively closed groups, so there is some chance that a male protecting an infant is protecting one of his, or his father's or his son's offspring. It may be since paternity is not known in multimale groups at least, that a blanket approach is the most economical—giving care to all infants ensures that own progeny receive it; providing the proportion of own progeny is sufficiently high, such behavior has selective value.

Hamilton (1964) pointed out that strictly speaking the problem is not one of leaving as many progeny as possible, but of causing the birth and survival to maturity of as many individuals of the most similar possible gene combinations. A full sibling has more similar genetic constitution than do offspring, and the offspring of a full sibling are likely to have a genetic constitution very nearly as close as that of own offspring. Thus there may be as much selective advantage in ensuring the survival of siblings or the offspring of siblings as there is in reproducing oneself. This is particularly true where the infant is expensive to produce, as are infant monkeys. "Altruism" is closely akin to enlightened self-interest.

I have suggested elsewhere (Rowell 1972) that it may be advantageous for protection to be extended from younger to older members of a monkey group as well as vice versa. Old monkeys have long memories that may enable the group as a whole (including their own offspring) to circumnavigate some infrequent environmental crises—to find the remaining water in a drought of a severity that only occurs once every twenty years, for example. In exchange for such rarely used information it may well be selectively advantageous for a group to include aged individuals even if they must occasionally wait for them in progressions or defend them against predators they no longer have the agility to avoid.

If the evolution of caretaking behavior towards infants by animals other then the mother can be explained in general by the selective advantage of ensuring the survival of closely related young carrying a high proportion of the protector's genes, we must then ask the significance of variation, within the order, of the extent and intensity of nonmaternal parental care. One possibility is that the variation simply represents "play" in the system—the differences between species in who gives care, and how much, are by-products of other selection pressures, and are not large enough to affect infant survival. Alternatively, different habitats may give selective advantage to different patterns of child care, so that the variations we see may represent adaptations to specific environmental pressures. We are now entering the field of pure speculation, but one general point can still be made: it is unlikely that the answer will be found by considering a single set of behavior patterns out of the context of other interspecific behavioral differences. For example Deag and Crook (1971) point out that infant barbary apes are unusual in that they wander a long way from their mothers as soon as they become mobile. If infants were retrieved by their own mothers when the troop is alarmed, as is usual for vervets or rhesus monkeys, such errant infants would surely fall easy prey to dogs or jackals. At least we can say that the combination of behavioral characteristics of wandering infants and strongly parental males is fortunate for the species, though whether one trait had a causal relationship to the other in evolution, we cannot of course know.

There remains the problem of the freeloader. A monkey that did not waste energy looking after infants might live in a troop of normal caretakers and actually be at a slight advantage—his offspring would be protected by others and he could put his saved energy

towards leaving more offspring, some of which would carry his genetic tendency towards nonaltruism. Presumably a group that came to carry too high a proportion of freeloaders would be unable to protect its infants successfully, and a balance between these conflicting selection pressures would be achieved. It ought to be to the advantage of altruistic monkeys to develop behavior to exclude or coerce nonaltruistic animals in the group, since such animals ultimately pose a threat to the survival of their own offspring. Bramblett (1970) suggests that female vervets expected protective behavior of the adult male in his captive group, and that some of their punitive behavior directed toward the adult male occurred when he was not adequately fulfilling this role. Rhesus males will attack a female whose infant is screaming, usually because she is preventing it from suckling or otherwise rejecting it; and when a mother baboon finally accepts her weanling on the nipple and his tantrum ceases there is typically a chorus of greeting grunts from nearby animals, apparently indicating approval, which perhaps might also be taken as an attempt to influence the behavior of a troopmate towards a more parental attitude.

Enculturation? Learning How to Make a Living

Infant monkeys learn about their surroundings as well as their society from group companions. Some of their earliest coordinated movements are attempts to intercept food the mother's hand is conveying past the infant to her mouth. For several weeks the infant talapoin only attempts to eat food that older animals are already eating, picking up scraps and taking fragments from mouth or hand, long after he has the coordination to get food for himself. In the cage such behavior may persist for several months, the infant waiting on a shelf while others go to the tray to fetch food, but in the wild infants forage for themselves earlier, by about three months (personal observation). Juvenile monkeys often smell the mouth of an animal that is eating and then go and fetch the same sort of food for themselves (Gartlan 1969, on vervets). This behavior has been especially noticed in the genus Cercopithecus, which are mostly forest living; far more plant and animal species are available as food in tropical forest than in any other environment, and many of them are relatively rare. Thus a young forest

monkey has a formidable learning task to acquaint itself with the whole range of foods used and how they are gathered and eaten.

Monkey troops do not always utilize all available foods in their home range, and adjacent troops with similar resources may use different items. Thus Harding (1972), observing predatory behavior of baboons in Kenya, had one troop that frequently killed antelope kids but showed no interest in guinea fowl, flocks of which could feed through the troop unmolested. A neighboring troop included guinea fowl in its diet. Such differences in behavior must be the result of troop traditions, juveniles learning specific food habits from their parents.

Establishing New Traditions

It is of great interest to consider how these traditional differences between troops might arise, and a unique opportunity to study this phenomenon has been most elegantly exploited by workers of the Japan Monkey Centre. It is to be expected that new techniques for obtaining food will develop most frequently in a period of major habitat change. Such a change was produced for the monkeys of Koshima islet: food began to be left for them regularly so they would come and spend more time on the open beach where their behavior could be studied. They quickly accepted the offerings of sweet potato and wheat, but had some problems with sand adhering to them, which they attempted to rub off with their hands in the usual Old World monkey fashion. One juvenile female discovered that sand could be removed better by washing sweet potato in water, and, a year or so later, that sand could be separated from wheat by floating the wheat on water. Kawai and his co-workers were able to follow the spread of these new behavior patterns through the group. Mothers learned from offspring, juveniles learned from juvenile peers. Adult males were much slower to learn than females probably because they had little contact with the innovative juvenile subgroups, and the older an animal the less likely he was to learn a new habit (Kawai 1965). The new environment generated other new habits, such as swimming for sport, which later included fetching possible food items from under water, walking on hind legs, and using a begging movement of the hand towards the observers. All these behavior patterns were first developed by juveniles and spread slowly up the age scale through the popula-

tion. Itani (1958) observed the acquisition of new diet items in another troop (Takasakiyama). He suggests five probable routes of acquisition of such new habits (in this particular case, eating candy), which depend on the particular social organization of Macaca fuscata :

1. Mothers learn from their infants.

2. Subadult and adult males learn from infants in their care.

3. Adult males learn from females during consort pairing in the mating season, or from females with whom they have strong grooming relationships.

4. Mothers learn from their adult offspring.

5. Elder brothers and sisters learn from younger siblings.

Again, these transmission routes (except perhaps number 3) are from younger to older members of the group.

Learning about the environment from group companions is thus a two-way process. The young learn the established traditions of their troop concerning food, and also the paths and limits of the home range and possible sources of danger from the older members of the troop, which also maintain a memory store of appropriate responses to infrequent but recurring environmental changes. The young, in their exploratory behavior develop new techniques, especially during periods of environmental change, which older animals then learn and which are incorporated in the tradition of the troop.

Although in the Koshima group it was possible to identify a single author of the innovations, when watching young monkeys playing and exploring new objects I have been struck with how several heads are better than one. In a group of juvenile talapoins investigating a sink stopper, for example, one animal handled the object while one or more looked over his shoulder. To begin with, that others were waiting made him retain interest and work harder on the problem. Eventually he left it for a while and the next monkey took it and tried some of the same manipulations he had just been watching, but added a few variants of his own; this process continued until collectively they dismantled the sink stopper, posted one part through the wire of the ceiling, and hung the other on a nail in the wall, after using it as a hat. One juvenile playing alone would not have had the persistence, and is unlikely to have had all

the required insights to make so much of such unpromising material.

Cultural Differences in Social Interaction Patterns

Monkey troops differ not only in their ways of interacting with their environment, but in their pattern of social interactions. Again the best known example comes from Japan. Itani (1959) described care of yearlings and two-year-olds by adult males especially during the birth periods when the mothers were turning their attention from the yearlings to the newborn infants. Males of three troops showed this behavior extensively. In eight other troops it has never been seen, while in seven more occasional isolated incidents have been observed. One may speculate whether males learn the behavior by imitation of other males, or whether infants, being cared for by males, are conditioned to care for infants in their turn. Presumably the paternal behavior of the barbary ape, now a species characteristic, arose as such a cultural variant.

Social behavior of rhesus monkeys from an urban environment is different from that of forest-reared monkeys, and the difference continues when both are maintained in the laboratory (Singh 1966). Urban monkeys are more aggressive, and also more active and manipulative, which characteristics are appropriate for the highly competitive urban environment. While the environment must have some direct effect on the individual's behavior, we are probably dealing here with another cultural difference as well.

The hamadryas baboon has an entirely different social organization from the common baboon, troops consisting of several separate one-male harem units. This difference is based on a single interaction pattern, the herding of females by males: a male goes to a female who is not following him closely enough and bites the back of her neck. She responds to this by following him more closely. By studying animals in a natural zone of interbreeding, Nagel (1973) was able to show that this herding behavior is genetically determined in the males, but is learned by females as juveniles. Hybrid males show partial, and rather ineffective herding and so are at a disadvantage in breeding. Females, whether pure hamadryas, pure common, or hybrids can successfully adapt to the prevalent social organization of a troop. The behavior is innate for males, culturally determined for females.

Conclusions

Social development of primates is a process that continues throughout life, in which younger animals can alter the behavior of older ones as well as vice versa. For this reason I have avoided using the term "socialization," which to me implies a process applied "from above" by older animals to originally asocial infants until they conform to some standard known as "adulthood." Not only is the infant monkey the least asocial creature in existence, but its social development does not resemble "socialization" as I arbitrarily describe it. Infants bring highly specific infantile stimuli to an interaction, and while they are learning about older animals, they may themselves be contributing to the social development of their partners. Juveniles learn the traditional customs of their troop concerning diet, routes, and dangers. In return they contribute to the tradition as during their characteristic exploratory play they discover new properties of the environment which may be learned from them by older kin. Young adults behave deferentially towards older ones although they may have become stronger than their seniors, and in return they gain from the older animal's store of experience. It is in terms of such reciprocal relationships that the selective advantage of living in close-knit permanent troops may be understood.

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