Introduction (Dutch)

Teasing in young chimpanzees (Adang, 1984)

Exploratory aggression in chimpanzees (Adang, 1985)

Exploring the social environment:
a developmental study of teasing in chimpanzees (Adang, 1986a)

Samenvatting (Adang, 1986b, Dutch)
Summary (Adang, 1986c, this page)

Acknowledgements (Dutch)

Resume (Dutch)

(drawing by Rene Spijkerman based on photos by Otto Adang)
Young chimpanzees in the Arnhem Zoo chimpanzee colony are frequently observed throwing sticks and stones at adult groupmembers, slapping or punching them. This behaviour becomes noticeable at the age of two and continues until adolescence, at about the age of nine. Even occasional punishment does not discourage the youngsters from persisting in this type of annoying behaviour which has also been observed among free-ranging chimpanzee groups and among other primate species  although sporadically (see e.g. van Lawick-Goodall, 1968; Bogess, 1982; Rowell, 1967; de Waal, 1977). A variety of terms are used to refer to these activities of youngsters, such as teasing, annoyance, pestering, harassment, provocative behaviour, etc. I chose to use the term "quasi-aggressive behaviour" to designate the behaviour. I was interested in the factors that determine the occurrence of the behaviour, and how quasi-aggressive behaviour can be related to "aggressive" and "play" behaviour.

Whilst aggression can be regarded as an attempt to exert influence on another individual by using, or threatening to use, violence, social play seems to be an important feature in the development of locomotor and social skills. For detailed references on aggression see Angst (1980), and on play see Fagen (1981). But what function does quasi-aggressive behaviour serve for developing youngsters?

Over a period of four years, potentially bothering acts of ten chimpanzee youngsters living in the large, semi-free ranging chimpanzee colony in Arnhem were systematically recorded, irrespective of the responses evoked. Descriptions of behaviour patterns mentioned in this paper are to be found in van Hooff (1973) and de Waal & van Hooff (1981). The recorded acts were those which were consistently recurrent and which seemed to be found, at least in some instances, irritating by some groupmembers, excluding acts performed with expressions typical to either "aggression" or "play". Only acts directed at adult targets were recorded. The results of the quantitative research summarised below give a mainly qualitative picture of the development of quasi-aggressive (QA-)behaviour of male chimpanzees up to the age of eleven and female chimpanzees up to the age of eight.

Quasi- aggressive behaviour

In general, the number of QA-initiatives performed by a youngster varied between one and two per hour. Of all the recorded acts, object throwing was by far the most popular, followed by hitting, kicking and similar provocations. Less frequently, youngsters were observed aiming (without actually throwing) objects at adults or displaying "bluff-like" behaviour. QA-initiatives occurred unexpectedly, and were performed from a distance or from behind; it was seen that the youngsters were ready to run away, as if anticipating possible retaliations. The film "The Family of Chimps" (Haanstra et al, 1984) contains some very good examples of QA-behaviour. Analysis of the contexts in which the acts occurred indicated that QA-behaviour tended to be "spontaneous" with no identifiable external cause.

None of the adult groupmembers were exempt from the provocative actions of the younger chimpanzees. Both adult males and females served as targets, no matter their positions in the dominance hierarchy. Youngsters refrained, however, from teasing their mothers or other members of their own subgroup. Nearly half of all agonistic interactions between youngsters and adults could be attributed to preceding provocations by the youngsters. However, the most common reaction of adult groupmembers was to ignore youngsters performing QA-behaviour. Other responses included fear (marked by teeth baring), evasion, friendliness (touching or grooming), and more neutral reactions (approaching or holding out a hand). The analysis of the interactions was concentrated on establishing whether or not the responses had a stimulating effect on the continuation of the behaviour. As with the study of any behaviour pattern, effects can give clues about possible adaptive consequences.

The absence of play signals (also in the responses of targets) and the tense rather than relaxed attitude of the youngsters indicated that QA-behaviour could not be identified with social play; this was supported by the fact that the interactions were not accompanied by the reciprocal friendliness typical of play. Similarly, the absence of distinct motives justified the conclusion that QA-behaviour is not simply a cautious, infantile form of reactive aggression. Furthermore, the possibility that QA-behaviour was a method for a youngster to attain a position in the hierarchy just below its own mother (dependent rank) was rule out by the observation that high-ranking groupmembers were targeted as well.

The comparison between QA-behaviour, aggressive behaviour and playful behaviour led to the hypothesis that QA-behaviour was a form of social exploration, a means for youngsters to learn what social restrictions they were subject to and to explore the possibility of reducing these reductions (cf. Hassenstein, 1973). If this hypothesis was correct, it might be expected that QA-behaviour would occur primarily in relationships in which the responses of the recipients were relatively unpredictable, or variable. As a measure of response-variability the information theoretical uncertainty measure H was used (Shannon & Weaver, 1949). The observations confirmed that the predictability of responses varied with the partners in the relationship. The degree of predictability depended on both animals of the dyad; there was no evidence that the responses of one animal tended to be systematically more predictable than those of another animal. The degree of predictability in a relationship proved to be closely connected with the frequency in QA-behaviour in that particular relationship. Predictable responses were related to a low frequency of QA-behaviour, whereas unpredictable responses were tied up with a high frequency. However, these correlations were not found as strongly in all types of relationships. The data required that a distinction be made between two forms of exploratory provocative behaviour.

Adang, O.M.J., J. A. B. Wensing, J. A. R. A. M. v. Hooff (1987), The Arnhem Zoo colony of Chimpanzees development and
management techniques. International Zoo Yearbook (26): 236-248
Angst, W. (1980): Aggression bei Affen und Menschen. Springer-Verlag Berlin
Archer, J. & L. Birke (1983): Exploration in animals and humans. Van Nostrand Reinhold, Wokingham
Bandura, A. (1973): Aggression: a social learning analysis. Prentice-Hall, Englewood Cliffs
Bogess, J. (1982): Immature male and adult male interactions in bisexual langur (Presbytes entellus) troop. Folia Primatol.,
38: 19-38
Dollard, J., L.W. Doob, N.E. Miller, O.H. Mowrer & R.S. Sears (1939): Frustration and aggression. Yale Univ. Press, Yale
Fagen, R. (1981): Animal play behavior. Oxford University Press, New York
Goodall, J. (1971), In the shadow of man. Houghton Mifflin, Boston. In het Nederlands: In de schaduw van de mens. Elsevier,
Goodall, J. (1986), The chimpanzees of Gombe. Patterns of behavior. The Belknap Press of Harvard University Press
Haanstra, B., O.M.J. Adang & J. A. R. A. M. van Hooff (1984), Chimps onder elkaar/ The Family of Chimps. Bert Haanstra
Films bv, Laren
Hassenstein, B. (1973): Verhaltensbiologie des Kindes. Piper, Munchen
van Hooff, J.A.R.A.M. (1973): A structural analysis of the social behaviour of a semi-captive group of chimpanzees.
       In: Expressive movement and non-verbal communication (M. von Cranach & I. Vine, eds). Academic Press, London
Van Lawick-Goodall, J. (1968): The behaviour of free-living chimpanzees in the Gombe Stream area. Anim. Behav. Monogr.1:
Noë, R., F.B.M. de Waal & J.A.R.A.M. van Hooff (1980): Types of dominance in a chimpanzee colony. Folia Primatologia,
24: 90-110
Pusey, A.E. (1978): The physical and social development of wild adolescent chimpanzees (Pan troglodytes schweinfurthii).
       PH. D. thesis, Stanford Universty
Rowell, T.E. (1967): A quantitative comparison of a wild and a caged baboon group. Anim. Behav. 15: 499- 509
Seligman, M.E.P. (1975): On depression, development and death. Freenman, San Francisco
Shannon, C.E. & W. Weaver (1949): The mathematical theory of communication. Univ. Illinois press, Urbana
de Waal, F.B.M. (1977): The organisation of agonistic relations within two captive groups of Java-monkeys (Macaca
       fascicularis) Z. Tierpsychol. 44, 268-312
de Waal, F.B.M. & Hoekstra (1980): Contexts and predictability of aggression in chimpanzees. Animal Behaviour, 28: 929-37
de Waal, F.B.M. & J.A.R.A.M. van Hooff (1981): Side-directed communication and agonistic interactions in chimpanzees.
Behaviour 77, 164-198.

QA-behaviour and response variability

All adult-youngster relationships taken together were ranked in two ways:
* frequency of QA-behaviour performed by the youngster 
* amount of variablity shown by the adults in response to QA-behaviour
Subsequently Spearman's rank correlation was performed, with the following results:

                                 male targets  female targets

YEAR            N                   rS              p                                    N                 rS               p
                      -----------------------------------                                 ------------------------------------         
1981               17                -.30             ns                                   55               .64             <.001
1982               21                 .27              ns                                   61               .57            <.001
1983                21                 .30              ns                                   69               .52            <.001
1984               13                 .52           <.05                                  77               .57            <.001

                                                  N:       number of relationships
                                                  rS:       Spearman's rank correlation coefficient
                                                  p:        one-sided probability (significance level .05)
Female targets: reducing uncertainty
The observations revealed the existence of a strong correlation between response-variability and QA-behaviour performed by young males and females towards female targets (see table). Relationships in which females showed a great var in responding were marked by a high frequency of QA-behaviour. Another correlation was found to exist between changes in response-variability and changes in the frequency of QA-behaviour. As soon as a female's responses became less variable, a decrease was observed in the frequency of QA-behaviour directed at the female in question: more variable responses, on the other hand, led to a higher incidence of QA-behaviour. An increase in the predictability was particularly noticeable in relationships marked by a high frequency in QA-behaviour. These observations confirmed the impression that QA-initiatives actually generate changes in predictability and that youngster attempt to reduce theuncertaintywith regard to the reactions they evoke.uncertaintyreduction is a well-known concept in psychological literature (cf. Seligman, 1975). The correlation between QA-behaviour and the predictability of responses remained unchanged as the youngsters grew older.

Although the incidence of QA-behaviour in a particular relationship seemed to be determined by the predictability of the responses, the course of the interactions seemed to depend on the nature of the responses evoked. The effects of the various response patterns were measured in three ways: the percentage of initiatives that led to prolonged interactions, the termination of these extended interactions and the percentage of initiatives repeated within fifteen minutes. Aggressive responses appeared to have a significantly stimulating effect on the prolongation or repetition of QA-behaviour. Submissive (fearful) responses were stimulating in the short term only. Friendly responses  with females making contact with troublesome youngsters  were not stimulating at all, and the same applies in instances where the target animal ignored the provocative youngster. Other response patterns produced no systematic effects.

The relative differences in the effects produced by the various response patterns were maintained throughout a youngster 's development. As a youngster grew older, considerable changes did occur, however, in the type of response evoked from the target adults. Teasing infants of about two years old were nearly always ignored. With increasing age, youngsters more and more managed to elicit more definite responses. Aggressive responses were frequently observed at this stage. On reaching adolescence, youngsters received predominantly submissive responses. At the same time they succeeded  in an increasing number of relationships in reducing the variability in responses.

The effects of submissive responses (stimulating in the short, but not in the long term) indicate that youngsters try to maximise this response type. Apart from reducinguncertainty,their behaviour seemed to be increasingly aimed at securing control over the response of their targets (for the distinction between predictability and controllability see Seligman, 1975). Adolescent males seemed to concentrate their efforts on attaining dominant positions, and therefore directed their activities towards the lowest raking females first. Acknowledgement of the adolescent's superior status by the target females enables the adolescent to move up in rank, a phenomenon that is also known from wild living chimpanzees (e.g. Pusey, 1978).

As youngsters grew older, notable changes occurred in the nature of their QA-behaviour. Instead of engaging in provocation at a distance and running off quickly, they tended to approach their targets and stay nearby. In an increasing frequency, the acts were preceded by bluff displays, in which the youngsters , with their hair erect, stamped on the ground. This sort of adultlike bluff behaviour could result in a brief attack the female, who was not seldom treated roughly. Gradually, the behaviour displayed by adolescents could no longer be qualified as quasi-aggressive.
Although superficially similar, QA-initiatives directed at adult males were determined by other principles than those directed at adult females. Males showed far more predictable response patterns than females did, yet the correlation between predictability of responses and frequency of behaviour was less clear. In over 70% of the interactions, males responded by ignoring provocative youngsters; aggressive responses proved to be rare, and submissive responses were not observed at all. The various types of  responses had no noticeable effect on the prolongation or repetition of QA-behaviour. Youngsters , nevertheless, appeared to be more afraid of males than of females. Occasionally, they even showed "submissive greetings" (Noe et al, 1980) while performing QA-behaviour directed towards adult males. QA-behaviour directed towards male targets was significantly less frequently prolonged than that directed towards female targets, it was of shorter duration, and was less quickly repeated.

One notable finding was that the occurrence of QA-behaviour showed a clear correlation with the rank of the male target in the dominance hierarchy. The alpha male proved to be a highly popular target. QA-behaviour towards males seemed to be performed particularly during a bluff display or another conspicuous social interaction, e.g. when males were at play, when they were in the process of reconciliation after a conflict, or when a male was copulating with an oestrus female. Such contexts were more and more preferred as youngsters grew older; youngsters also increasingly showed signs of excitement and submission during the performance of QA-behaviour.

Another salient point was that the relationship between youngsters and adult males, in a few respects, changed to a marked degree well before the youngsters reached puberty. When youngsters were involved in a conflict, males tended to support infants, but were inclined to give support to the opponents of juveniles (over five years of age) and adolescents. The youngsters themselves showed the opposite tendency. As infants they tended to side with the opponents of adult males, but once they had become juveniles they took the part of the adult males.

Juvenile males who were engaged in a conflict increasingly directed non-agonistic behaviour at adult males not involved in the conflict. This behaviour seemed to be aimed at preventing the adult male from supporting the juvenile's opponent (side-directed appeasement, de Waal & van Hooff, 1981). The presence in the group of oestrus females often appeared to stimulate adult males to enter into conflicts with juvenile males  but not with younger animals -, even though these juveniles, who are not yet sexually mature, could not be regarded as rivals. The significance of this behaviour undoubtedly lay in its conditioning function. The youngsters were in this way trained to keep their distance before becoming virtual competitors.

Male targets: investigating authority
Two functional types of quasi-aggressive behaviour

     Reducing uncertainty             Investigating authority
Actor            male & female youngsters            male youngsters
Action          throwing, no submissive greetings                  bluff & bluff-like with submissive greetings
Target           especially females                                                          high-ranking males
Reaction      variable: much aggression, submission, evasion               mainly ignoring
Context        no immediate context                                                    male activities
Function      reducing uncertainty, later gaining control                       investigating "authority"
In both functional categories QA-behaviour serves as a means for youngsters to acquire information about social relationships. Youngsters performing QA-behaviour learn about (social) effects of their own behaviour. The QA-behaviour directed at adult females seems to be aimed at uncertainty reduction and gaining control. Adult males seem to have a clear and predictable relationship with youngsters. Therefore social learning can be quicker in relationships with males. Before the onset of puberty the relationship between young males and adult males changes markedly, and the QA-behaviour directed at adult males, although also exploratory in nature, seems to be aimed at learning other kinds of things than the QA-behaviour directed at adult females; viz. at learning about the nature and constituency of "dominant" behaviour.

Quasi-aggressive, or provocative, behaviour seems to be a means for young chimpanzees to acquire information from their targets. The information they obtain helps them to function properly in their social environment, just as exploration of the physical environment is indispensable for learning how to make use of it effectively (see Archer & Birke, 1983 for a treatment of exploratory behaviour). The special value of this study lies in the deeper insight it provides into the way in which chimpanzee youngsters gather information about their social environment. Social learning does not only occur through observation, imitation and conditioning, but also through a youngster's own exploratory activities and the ensuing effects. In as far as this social exploration is harmful to other groupmembers, it can be labelled as a form of aggressive behaviour. Social play with its fixed "rules" and reciprocal character lacks the conditions necessary for obtaining the kinds of information referred to above. As youngsters grow older, gradual changes occur, not only in the nature of the behaviour displayed, but also in the objectives, especially regarding QA-behaviour towards adult females. At first the behaviour of the youngsters seems to be aimed at uncertainty reduction, but more and more, male youngsters concentrate their efforts on gaining control over female groupmembers. At this stage, the behaviour of the youngsters can be regarded as the precursor of dominance-oriented aggression  also relatively spontaneous in nature  that is performed by adult males (de Waal & Hoekstra, 1979).

Most studies of (quasi-)aggressive behaviour of human children claim that frustration is the root cause (starting with Dollard et al., 1939). Another prominent element seems to be imitation (e.g. Bandura, 1973). However, very little is known about the possible functions of aggressive behaviour performed by humans (see e.g. Angst, 1980). The results of the present study may point to factors controlling the occurrence of the behaviour and to functions that might also apply to the (quasi-) aggressive behaviour of human children. Only comparative research may reveal if this is indeed the case. 

Teasing, Harassment, Provocation
The development of quasi-aggressive behaviour in chimpanzees

Ph. D. Thesis by Otto M.J. Adang, Utrecht University, 1986
(for a list of all publications in English, click here)