Conflict management in wild chimpanzees (Pan troglodytes) - Qucosa

Conflict management in wild chimpanzees (Pan troglodytes)

Von der Fakultät für Biowissenschaften, Pharmazie und Psychologie der Universität Leipzig genehmigte DISSERTATION zur Erlangung des akademischen Grades doctor rerum naturalium Dr. rer. nat.

vorgelegt von Diplom-Biologe Roman Wittig geboren am 23.07.1968 in Karlsruhe

Dekan:

Prof. Dr. Kurt Eger

Gutacher:

Prof. Dr. Christophe Boesch Prof. Dr. Peter Kappeler Prof. Dr. Frans B. M. de Waal

Tag der Verteidigung: 19.12.2003

Preface

to Fynn and Cathy

ii

Preface

This thesis is partly based on the following publications and manuscripts: 1. Wittig, R. M. & Boesch, C. (2003a). Food competition and linear dominance hierarchy among female chimpanzees of the Taï National Park. International Journal

of Primatology. 24: 847-867. 2. Wittig, R. M. & Boesch, C. (2003b). 'Decision-making' in conflicts of wild chimpanzees: An extension of the Relational Model. Behavioral Ecology and

Sociobiology. 54: 491-504. 3. Wittig, R. M. & Boesch, C. (2003d). The choice of post-conflict interactions in wild chimpanzees (Pan troglodytes). Behaviour. 140: 1527-1559. 4. Wittig, R. M. & Boesch, C. (in review). How to repair relationships in wild chimpanzees (Pan troglodytes). Ethology.

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Table of contents

TABLE OF CONTENTS NATURE OF CONFLICTS AND CONFLICT MANAGEMENT IN ANIMALS - AN INTRODUCTION

1

1.1 THE NATURE OF CONFLICTS 1.2 CHANGING THE VIEW OF AGGRESSION 1.3 COSTS AND BENEFITS OF AGGRESSION 1.4 BENEFIT OF CONFLICT MANAGEMENT – COST REDUCTION 1.5 CONFLICT MANAGEMENT IN WILD CHIMPANZEES OF THE TAÏ NATIONAL PARK

2 2 3 5 6

BIOLOGY OF COMMON CHIMPANZEES (PAN TROGLODYTES)

8

2.1 DISTRIBUTION OF CHIMPANZEE 9 11 2.2 SOCIAL STRUCTURE AND MATING SYSTEM OF CHIMPANZEES 2.3 DIET AND ASPECTS OF FEEDING ECOLOGY OF CHIMPANZEES 13 2.4 RECONCILIATION (AND CONFLICT MANAGEMENT) IN CHIMPANZEES – A BRIEF HISTORY 14 GENERAL METHODS OF THE STUDY

16

3.1 DATA COLLECTION 3.1.1 STUDY SITE 3.1.2 OBSERVATION PROCEDURE 3.1.3 ETHOGRAM 3.2 OPERATIONAL DEFINITIONS 3.2.1 BEHAVIOURS 3.2.2 CONFLICT VARIABLES 3.2.3 CONTEXT VARIABLES 3.2.4 DOMINANCE VARIABLES 3.2.5 SOCIAL VARIABLES 3.2.6 VARIABLE ADAPTATIONS FOR STATISTICAL COMPARISON 3.3 MULTIVARIATE DYADIC APPROACH 3.4 STATISTICS 3.4.1 MULTIVARIATE ANALYSIS 3.4.2 DYADIC PERMUTATION TESTS 3.4.3 PROGRAMS

17 17 17 18 20 20 20 21 21 22 23 23 23 24 25 26

FOOD COMPETITION AND LINEAR DOMINANCE HIERARCHY AMONG FEMALES 27 4.1 INTRODUCTION 4.1.1 HIERARCHIES IN CHIMPANZEES 4.1.2 SOCIO-ECOLOGY OF DOMINANCE RELATIONSHIPS 4.1.3 RELATIONSHIPS IN CHIMPANZEES 4.2 SPECIFIC METHODS 4.2.1 DATA AND TEST CONDITIONS 4.2.2 OPERATIONAL DEFINITIONS 4.2.3 STATISTICS

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28 28 28 29 31 31 31 32

Table of contents

4.3 RESULTS 4.3.1 FEMALE HIERARCHY 4.3.2 FOOD COMPETITION 4.3.3 CORRELATES OF THE LINEAR HIERARCHY 4.3.4 SOCIAL RELATIONSHIPS 4.4 DISCUSSION 4.4.1 SUMMARY OF RESULTS 4.4.2 DOMINANCE RELATIONSHIPS 4.4.3 CONTEST COMPETITION 4.4.4 REASONS FOR LINEARITY 4.4.5 COMPARISON AMONG STUDY SITES

33 33 34 37 37 39 39 39 40 40 41

DECISION-MAKING IN CONFLICTS: EXTENSION OF THE RELATIONAL MODEL 43 5.1 INTRODUCTION 5.1.1 THE RELATIONAL MODEL (RM) 5.1.2 EXTENSION TO THE RELATIONAL MODEL 5.1.2.1 Likelihood of winning 5.1.2.2 Conflict duration and cost of aggression 5.1.2.3 Creation of social costs 5.1.3 TESTING THE EXTENDED RELATIONAL MODEL 5.2 SPECIFIC METHODS 5.2.1 DATA AND TEST CONDITIONS 5.2.2 ADDITIONAL OPERATIONAL DEFINITIONS 5.2.3 SPECIAL ANALYSIS 5.3 RESULTS 5.3.1 STEP 1: BENEFIT 5.3.1.1 Benefit of the resource 5.3.1.2 Likelihood of winning 5.3.2 STEP 2: COST OF AGGRESSION 5.3.2.1 Conflict intensity 5.3.2.2 Conflict duration 5.3.3 STEP 3: SOCIAL COSTS 5.3.3.1 Creating social costs 5.3.3.2 Reducing of social cost 5.4 DISCUSSION 5.4.1 SUMMARY OF RESULTS 5.4.2 GRABBING THE BENEFIT 5.4.3 ECONOMIC HANDLING OF THE COSTS OF AGGRESSION 5.4.4 MINIMISING SOCIAL COSTS 5.4.5 THE EXTENDED RELATIONAL MODEL

44 44 45 45 46 46 46 48 48 49 49 51 51 51 52 53 53 54 55 55 57 59 59 59 60 61 62

CHOICE OF POST-CONFLICT INTERACTIONS

64

6.1 INTRODUCTION 6.1.1 ADVANTAGES AND DISADVANTAGES OF POST-CONFLICT INTERACTIONS 6.1.2 POOL OF PCIS 6.1.2.1 Reconciliation 6.1.2.2 Consolation 6.1.2.3 Redirected aggression

65 65 66 66 66 67

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Table of contents

6.1.2.4 Renewed aggression 6.1.2.5 Avoidance of interactions (no PCI) 6.1.3 TESTING THE CHOICE OF PCI 6.2 SPECIFIC METHODS 6.2.1 DATA AND TEST CONDITIONS 6.2.2 ADDITIONAL OPERATIONAL DEFINITIONS 6.2.3 SPECIAL ANALYSIS AND STATISTICS 6.3 RESULTS 6.3.1 ARE PCIS DEPENDENT ON THE CONFLICT? 6.3.2 THE EFFECT OF CONFLICT DURATION 6.3.3 AVOIDANCE OF INTERACTIONS (NO PCI) 6.3.4 NORMAL INTERACTIONS – BUSINESS AS USUAL 6.3.5 CHOOSING A POST-CONFLICT INTERACTION 6.3.5.1 Reconciliation 6.3.5.2 Offered consolation 6.3.5.3 Renewed aggression 6.3.5.4 Redirected aggression 6.3.5.5 Third party aggression 6.4 DISCUSSION 6.4.1 SUMMARY OF RESULTS 6.4.2 BUSINESS AS USUAL 6.4.3 AVOIDING FURTHER INTERACTIONS 6.4.4 RECONCILIATION 6.4.5 CONSOLATION 6.4.6 RENEWED AGGRESSION 6.4.7 REDIRECTED AGGRESSION 6.4.8 THIRD PARTY AGGRESSION 6.4.9 CHOICE OF PCI: GENERALISED RULES

67 68 68 69 69 70 70 71 71 72 73 73 74 74 77 77 77 77 79 79 80 81 81 83 83 84 84 85

HOW ARE RELATIONSHIPS REPAIRED ?

87

7.1 INTRODUCTION 7.1.1 WHY REPAIR RELATIONSHIPS? 7.1.2 WHAT DAMAGES RELATIONSHIPS? 7.1.3 HOW TO REPAIR THE DAMAGE? VARIATION IN RECONCILIATION 7.2 SPECIFIC METHODS 7.2.1 DATA AND TEST CONDITIONS 7.2.2 ADDITIONAL OPERATIONAL DEFINITIONS 7.2.3 MEASURING RECONCILIATION 7.3 RESULTS 7.3.1 OCCURRENCE OF RECONCILIATION 7.3.2 FUNCTION OF RECONCILIATION 7.3.3 VARIATION WITHIN LATENCY AND DURATION OF RECONCILIATION 7.3.4 VARIATION WITHIN COMPLEXITY OF RECONCILIATION 7.3.5 INITIATOR OF RECONCILIATION 7.4 DISCUSSION 7.4.1 SUMMARY OF RESULTS 7.4.2 FUNCTION OF RECONCILIATION 7.4.3 THE VARIATION WITHIN RECONCILIATION 7.4.4 REPAIRING THE DAMAGE

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88 88 88 89 91 91 92 93 94 94 94 97 99 99 100 100 101 101 103

Table of contents

OPEN QUESTIONS AND POSSIBLE ANSWERS - A GENERAL DISCUSSION

106

8.1 CONTRIBUTION OF THIS STUDY TO OUR KNOWLEDGE IN CONFLICT MANAGEMENT 8.2 AVOIDANCE AND PREVENTION OF CONFLICTS 8.3 CONFLICT REGULATION IN A WIDER SOCIAL CONTEXT 8.4 EFFECTIVENESS OF THE MECHANISMS OF CONFLICT MANAGEMENT 8.5 SIMILARITY OF CONFLICT MANAGEMENT IN HUMANS AND CHIMPANZEES 8.6 PROSPECTS

107 108 110 111 113 114

SUMMARY

116

9.1 SUMMARY 9.2 ZUSAMMENFASSUNG

116 118

ACKNOWLEDGEMENTS

123

BIBLIOGRAPHY

125

APPENDICES

149

CURRICULUM VITAE

159

DECLARATION OF INDEPENDENCE

160

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Chapter 11: Nature of conflicts and conflict management in animals - an introduction

Figure 1.0 Male Marius is sitting on a tree-fall after fighting a competitor over a female in oestrous (Photo by R. Wittig).

1

The present study focuses on social (group-living) animals

Chapter 1: Nature of conflicts and conflict management – an introduction

1.1 The nature of conflicts

Living in groups holds many advantages. Social partners may increase protection against predators (Hamilton, 1971; Alexander, 1974), they can provide support in competition (de Waal, 1978), they facilitate food acquisition (Janson, 1988a; Boesch & Boesch, 1989) and they may help in caring for ones’ offspring (O'Brien, 1993; Muroyama, 1994). To increase these advantages individuals build cooperative relationships (Wrangham, 1980), that usually reflect long-term investments and can change over time (Kummer, 1978; Hinde, 1979). Social partners, however, can also be disadvantageous, as they compete for the same resources or they pursue different, even contrary, goals. An individual’s mating success, food acquisition and spatial patterns are often influenced by the behaviour of the social partners (Hamilton, 1967; Eisenberg et al., 1972; Parker, 1974; Clutton-Brock & Harvey, 1978). When competing for the same resource social partners face a problem: both claim the same resource, but often only one can possess it (e.g. food competition: Hand, 1986). When pursuing contrary goals social partners face another problem in as much that both try to achieve different aims. In order to do so, however, they may need to prevent the other from realising their aim (e.g. weaning conflict: Trivers, 1974, conflict of the sexes: Dawkins, 1976). Such situations are defined as conflicts of interest (McEnery, 1985; van der Dennen & Falger, 1990). A conflict of interest often escalates and results in aggression. Aggressive interactions are then called open conflicts, or for the sake of simplicity, just conflicts.

1.2 Changing the view of aggression

The traditional view of aggression implied a spontaneous, uncontrollable and antisocial nature to conflicts. This idea derived from the concept that aggression is an independent instinct, which is driven by a number of internal and external variables (Lorenz, 1963; Hinde, 1967; Tinbergen, 1968). This concept suggested that the motivation for aggression could increase up to a threshold where aggression would erupt spontaneously. The conditions for opening such a pressure relief valve would be situations in which aggression had not been vented for some time. Lorenz (1963) postulated that

2


the function of intra-species aggression, in plain terms, is a spacing mechanism. The repulsive reaction of opponents to aggression would produce an even distribution of individuals over the habitat, which would serve the survival of the species by guaranteeing an optimal exploitation of all resources. Over the last 30 years, however, the view of aggression has changed slowly as it became apparent that aggression seems to be primarily an adaptation to cope with competition (Maynard Smith, 1982; Huntingford & Turner, 1987; Moynihan, 1998). Competition appears to be ubiquitous and inevitable, as an animal’s environment does not consist solely of food supply, sleeping places and shelter from predators, but also of social partners. Therefore, each individual competes with others (e.g. kin, friends, rivals, mating partners, neighbours, other species) over the resources that contribute positively to their inclusive fitness. As resources are limited, mechanisms are required to regulate access to resources. Aggressive interactions, for example, regulate the control over resources. However,

besides

the

advantage

of

controlling

resources,

aggression

carries

disadvantages. One of these drawbacks is that in attacking or fighting individuals run the risk of injury or even death. Recently the view of aggression has changed again and aggressive behaviour has been recognised as a tool of negotiation in conflicts of interest among group-living animals (de Waal, 1996a; Silk, 1996; Aureli & Smucny, 1998; Aureli et al., 2002). Aggression – besides tolerance and avoidance of the social partner – is seen as one possible strategy to regulate a conflict of interest. Conflicts of interest, therefore, can be settled by sharing the resource among the competitors (tolerance towards the partner: de Waal, 1989a), or by the subordinate partner leaving the resource to the dominant (avoidance of the partner: Janson, 1985). A conflict of interest, however, can also escalate and result in aggression.

1.3 Costs and benefits of aggression

Whether or not a conflict of interest escalates into aggression depends on three variables of a cost-benefit estimation: (1) each competitor’s motivation to fight, which depends on the value the resource has for each of them, (2) each competitor’s likelihood of winning, which means each competitor’s likelihood of accessing the resource, and (3) 3


the costs of aggression that each competitor is prepared to accept when the conflict of interest escalates into aggression (Maynard Smith, 1982; Huntingford & Turner, 1987; Dunbar, 1988; Preuschoft & van Schaik, 2000). The value of a resource is related to how much benefit the resource provides for a competitor in terms of fitness increase. The limiting factor for fitness increase appears to be different for each sex. While in males the limiting factor is the accessibility of mates, food access seems to be the limiting factor in females (Crook & Gartlan, 1966; Emlen & Oring, 1977; Wrangham, 1980; Janson, 2000). Direct fighting over these resources is one possible way to access the benefit. However dominance relationships often regulate the access to resources. The dominance rank of a male, for example, may improve his access to fertile females. Many studies in the wild have found a positive correlation between dominance rank and reproductive success for male primates (Pope, 1990; de Ruiter & van Hoof, 1993; Dixson et al., 1993; Ohsawa et al., 1993; Altmann et al., 1996; Gerloff et al., 1999; Constable et al., 2001; Launhardt et al., 2001; Boesch et al., in prep.). Injury and death are obvious risks of fighting. Injuries can have tremendous costs leading to infection, infirmity and disability, which can have detrimental consequences for food acquisition, travelling and predation risk (Jaeger, 1981; Tuttle & Ryan, 1981; Nishida et al., 1985; Mesnick & Le Boeuf, 1991). Despite injuries and death, there are other costs of conflicts. Aggressive interactions induce acute stress in opponents (Maestripieri et al., 1992). Stress is costly, as it may distract the attention of opponents from beneficial activities and it may be energetically expensive. Furthermore aggression disturbs the relaxed coexistence between opponents and thus interferes negatively with their relationship (Cords & Aureli, 1996). Relationships reflect the history of many interactions among social partners and negative consequences will be mirrored in all of their interactions (Kummer, 1978; Hinde, 1979). Thus, an out-competed cooperative partner may for example withhold support from the aggressor in the future. In short, the basic dilemma of group-living competitors may be that in initiating a fight the aggressor risks losing a cooperative partner. Such costs will be referred to as social costs. As soon as individuals have several possibilities to resolve a problem situation, such as a conflict of interest, they are viewed as going through a decision-making process (Krebs & Kacelnik, 1991). The term decision does not necessarily imply any conscious reasoning process, but rather that behaviours can switch between options. The causal

4


mechanisms underlying such switching as well as the functional significance are of main interest for understanding the evolution of decision-making. The theoretical perspective of decision-making is based on the assumption that individuals maximise the net benefit of a conflict of interest and, therefore in the long term, increase their fitness (Hamilton, 1964). Thus the decision can be analysed in terms of costs and benefits of alternative courses of action, while measuring costs and benefits in terms of a more immediate metric, such as access to food, energy expenditure, risk of injury, or number of copulations. When an individual decides to engage in aggression, which is not cost-free, the benefits that are accessible through the conflict must prevail over the costs of conflicts. In order to maximise the net benefit of the outcome of a conflict, opponents could either increase the benefits or decrease the costs of the conflict. Because benefits gained are fixed by the value of the resource, only costs of the conflict are variable. Therefore adaptive strategies are usually recognised when they decrease disadvantages (e.g. dominance relationships: Bernstein, 1981). However another possibility for an adaptive strategy would be finding an alternative way of gaining the advantages (e.g. food sharing: de Waal, 1992).

1.4 Benefit of conflict management – cost reduction

By conflict management one usually understands all actions, strategies and social agreements that serve to reduce the costs of conflicts to one or both opponents (Mason & Mendoza, 1993; Cords & Killen, 1998). Many actions of conflict management are postconflict interactions, which are interactions of one or both conflict partners after their aggressive interaction. Reconciliation, the friendly post-conflict interaction between former opponents using affiliative behaviour, can repair the disturbance of the relationship of conflict partners in order to allow for future cooperation between them (Cords, 1992; Cords & Thurnheer, 1993), and seems to be able to reduce acute stress (Maestripieri et al., 1992; Aureli, 1997). Consolation, a friendly post-conflict interaction between a conflict partner and a third party using affiliative behaviours, and redirected aggression, when a conflict partner attacks a third party after an aggressive interaction, are both supposed to reduce the stress of conflict partners (Das, 2000; Watts et al., 2000). In contrast to post-conflict interactions, dominance relationships manage conflicts of interests before any escalation by preventing a conflict of interest from escalating into aggression (Preuschoft & van Schaik, 2000). On the other hand, ritualised aggression 5


enables aggression to be used in a conflict of interest whilst incurring minimal costs, as ritualised aggression reduces the risk of injuries. Thus strategies that regulate the outbreak or the level of escalation are included in conflict management. Although conflict management reduces the costs of conflicts it does not necessarily resolve the conflict of interest. Dominance relationships, acceptance of ownership or ritualised aggression, for example, regulate the access of a resource with low or no costs of aggression, but they do not eliminate the incompatibility of the opponents’ goals. On the other hand, sharing resources can end the conflict of interest over sharable resources, since both opponents obtain their goal (reviewed in: Aureli & de Waal, 2000a).

1.5 Conflict management in wild chimpanzees of the Taï National Park

This study was conducted in order to understand the economics behind conflict management and the causes and functions of the decision-making” of wild chimpanzees in the Taï National Park (Pan troglodytes verus). Given the complexity of the topic, many detailed questions are of importance for the understanding of the conflict management, which are mentioned within the result chapters (4, 5, 6 and 7). Chimpanzees are of special interest for the study of conflict management (see chapter 22), since they are our closest living relatives (Ebersberger et al., 2002), and they were the first non-human species where reconciliation was observed (de Waal & van Roosmalen, 1979). The conflict management of chimpanzees encompasses different mechanisms that act in conflicts of interest at different times. Avoidance of dominant conflict partners or tolerance of conflict partners by sharing are actions that are carried out before the conflict of interest escalates into aggression. This will be referred to as pre-conflict management. In order to understand pre-conflict management, I investigated whether or not chimpanzees dominance relationships are linear and based on contest competition (chapter 4; Wittig & Boesch, 2003a,c), and if the decision to initiate aggression is influenced by the resource benefit or the dominance relationship between opponents (chapter 5; Wittig & Boesch, 2003b). Using ritualised aggression instead of contact aggression or fighting non-friends instead of friends are strategies that can be used

2

see 2.4 6


during the escalation to aggression – referred to here as peri-conflict management. Thus, in terms of peri-conflict management, I next investigated if opponents use strategies of cost reduction by varying the aggression and type of opponent (chapter 5; Wittig & Boesch, 2003b). Finally, many interactions of conflict management are reactions to costs that have been already created. These interactions are carried out after the conflict and are part of post-conflict management. For the post-conflict management, I examined the economic rules for choosing a post-conflict interaction (chapter 6; Wittig & Boesch, 2003d) and the way to repair the disturbance of usually relaxed relationships after the conflict (chapter 7; Wittig & Boesch, in review). Conflict management is naturally a system of decision-making. Although triadic relations are also important (de Waal & van Hooff, 1981; Judge, 1991; Das et al., 1997), conflict management mostly contains dyadic interactions. Thus my analyses were based on dyadic interactions. Multivariate analyses were used to detect co-variances and interactions between influencing factors (see chapter 33). This study is the first to my knowledge that has concurrently investigated all aspects of conflict-management in a species. Moreover this study places the patterns of conflict management in an economic framework of costs and benefits. As Pan and Homo are sister-groups, knowing about the conflict management of chimpanzees may help us to understand our own conflict management and aspects of our own evolution (chapter 84).

3 4

see 3.4.1 see 8.5

7

Chapter 25: Biology of common chimpanzees (Pan

troglodytes)

Figure 2.0 Members of the North community in Taï are feeding of and resting around a fallen Palm-tree (Photo by R. Wittig).

5

This chapter provides the reader with information on the biology of chimpanzees that is relevant to this study.

Chapter 2: Biology of chimpanzees

2.1 Distribution of chimpanzee

Chimpanzees (Pan troglodytes) are found in the forests and woodlands of tropical Africa (Figure 2.1a). They group in four sub-species (in order from west to east, Figure 2.1b): (a) the West African Chimpanzee (Pan troglodytes verus), ranging in the west of the Niger River, from Senegal to Ghana; (b) the Nigerian Chimpanzee (P.t. vellorosus), found between the rivers Niger and Sanaga in Nigeria and the northern part of Cameroon; (c) the Central African Chimpanzee (P.t. troglodytes), existing in the west of the Congo River between the rivers Sanaga and Ubangi in Gabon, Central African Republic (C.A.R.), Congo and northern part of Democratic Republic of Congo; and (d) the East African Chimpanzee (P.t. schweinfurthii), ranging between the rivers Congo and Ubangi and spread out east beyond the Great Rift Valley, into the triangle of Uganda, the eastern part of Democratic Republic of Congo and Tanzania (Gagneux et al., 2001; Gagneux, 2002). Chimpanzees and Bonobos (Pan paniscus) represent together the genus Pan, which is the phylogenetic sister-group to humans (Homo sapiens) (Kaessmann et al., 2001; Ebersberger et al., 2002). Our knowledge about the biology of wild chimpanzees originates mainly from seven long-term studies (see Figure 2.1b). The two oldest long-term studies on chimpanzees are located in east Africa with Pan troglodytes schweinfurthii. One is in the Gombe Stream National Park, Tanzania (since 1960: Goodall, 1986) and the other is in the Mahale Mountains National Park, Tanzania (since 1965: Nishida, 1990), both on the shores of Lake Tanganyika. Two further long-term studies were established in the second half of the 1970s in west Africa on Pan troglodytes verus. One is located in Bossou, Guinea (since 1976: Sugiyama, 1984) and the other one in the Taï National Park, Côte d’Ivoire (Figure 2.1c,d; since 1979: Boesch & Boesch-Achermann, 2000; Wittig, 2003). Another three studies also investigate Pan troglodytes schweinfurthii, of which two are placed in the Kibale National Park, Uganda (since 1987: Kanyawara, Wrangham et al., 1991; since 1995: Ngogo, Watts, 1998) and one in the Budongo Forest Reserve, Uganda (since 1994: Newton-Fisher, 1999a). All of these long-term studies are located in eastern or western Africa and represent only two of the four subspecies (P.t. schweinfurthii and P.t. verus). However, recently another two study sites were established with the potential to become long-term studies. One study site is in the Gashaka Forest Reserve, Nigeria (Sommer et al., 2003) on Pan troglodytes vellorosus and the other one in the Goualougo Triangle, Nouabalé-Ndoki National Park, Republic of Congo (Morgan & Sanz, 2003) on Pan 9


troglodytes troglodytes (Figure 2.1b). The present study was conducted in the Taï National Park, Côte d’Ivoire.

P.t. verus P.t. vellerosus P.t. troglodytes P.t. schweinfurthii

Senegal Guinea Bossou Côte Liberia d’Ivoire Ghana Taï

Nigeria Gahshaka

C.A.R.

Cameroon Goualougo Gabon

Budongo Uganda

Congo

Democratic Republic of Congo

Kibale Gombe Tanzania Mahale

Africa

(b) (a)

(5°52’ N, 7°22’ W) Taï

Study Area of the Taï Chimpanzee Project

Côte d’Ivoire

Taï National Park

Yamoussoukro

Liberia

Taï National Park

Abidjan

(c)

(d)

Figure 2.1 Approximate distribution of the four subspecies of chimpanzees (Pan

troglodytes) over Africa (a+b), following the suggested distribution by Gagneux (2002), locations of the long-term studies (

) and the precise location of the Taï Chimpanzee

Project (c+d).

10


2.2 Social structure and mating system of chimpanzees

Chimpanzees live in multimale – multifemale communities (=unit-groups: Nishida, 1968; Goodall, 1983). These communities have a fission–fusion grouping pattern, as they travel around in temporary grouping units (= parties) with constantly fluctuating compositions (Sugiyama, 1984; Goodall, 1986; Nishida, 1990; Boesch, 1991; Wrangham et al., 1992). All party members seen together in various parties belong to the community. Communities consist of between 10 (middle community in Taï: Herbinger et al., 2001) and 150 individuals (Ngogo community: Mitani & Watts, 2001) of all age classes. Males are the philopatric sex in chimpanzees. They stay in their natal community, while females transfer permanently to another community usually at the end of adolescence (Nishida & Hiraiwa-Hasegawa, 1985; Watts & Pusey, 1993). Chimpanzees inhabit a home range of between 9 km2 (Bossou: Sakura, 1994) and 35 km2 (Ngogo: Watts, 2002), with home range size increasing with number of males (Lehmann & Boesch, in press). As only about 10% of the home range area is exclusively used by the resident community, whilst the remaining area is also used by neighbouring communities, chimpanzees can face up to a mean of four inter-community encounters per month (Lehmann & Boesch, in press). The males of the resident community, sometimes supported by the females, defend the their home range by means of drumming displays accompanied by pant-hoot and long bark vocalisations (Crockford & Boesch, 2003). In case of direct contact between the communities, serious fighting can emerge, which can be lethal (Nishida et al., 1985; Fawcett & Muhumuza, 2000). Chimpanzees have a promiscuous mating system, where females mate with several males and males with several females (Tutin, 1979; Hasegawa & Hiraiwa-Hasegawa, 1983; Matsumoto-Oda, 1999). Female chimpanzees show a sexual swelling of their perineum (Dixson, 1998). This sexual swelling (=oestrus swelling; Figure 2.2) lasts for about 10 days (the time period which I refer to as being in oestrus) during each fertile cycle in female chimpanzees (about 36 days: Wallis, 1982) and indicates for the probability of ovulation (Deschner et al., 2003). After conception female chimpanzees have a gestation period of about 8 months, following a lactation time of about 2 years and a total inter-birth interval of almost 6 years (Furuichi & Hashimoto, 2002; Wrangham, 2002). Only when a younger sibling is born do children stop travelling on their mother’s

11


back, however they still stay together with her mother until they are adolescent (another five years).

Figure 2.3 Male Macho and female Venus are feeding on Figure 2.2 Female Mystère with a full sexual swelling a carcass of a red colobus (Procolobus badius), while (Photo by T. Deschner).

females Goma and Belle are watching the carcass (Photo by R. Wittig).

Social relationships play an important role in the social life of chimpanzees. In most communities chimpanzee are probably male bonded, as males are philopatric and seem to have stronger bonds than females (Mitani et al., 2002). Males, for example, build long lasting alliances, that can be sometimes strong enough to perform a coup, overturning the alpha male (Nishida, 1983). In Taï chimpanzees, however, relationships in females can become so strong that a female may adopt the children of another female who dies (Boesch & Boesch-Achermann, 2000). Chimpanzees of both sexes have many benefits to give to or to receive from their partners. Partners can support each other in agonistic situations (e.g. de Waal, 1984; Hemelrijk & Ek, 1991), they can share special foods (e.g. de Waal, 1989a; Mitani & Watts, 2001), or they can groom each other (e.g. Huffmann, 1990; Wrangham et al., 1992; Watts, 2000). Chimpanzees can trade benefits in order to built up or to maintain good relationships. Since chimpanzees can become 40 years and older (Hill et al., 2001), many of their relationships can be long lasting and may influence the structure of the community (e.g. grooming and association: Boesch & BoeschAchermann, 2000; Watts, 2000).

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2.3 Diet and aspects of feeding ecology of chimpanzees

Chimpanzees are omnivorous, although most of their diet consists of ripe fruits (Goodall, 1986; Yamakoshi, 1998; Newton-Fisher, 1999b; Boesch & Boesch-Achermann, 2000). Most fruits are eaten in large plentiful patches under and in the trees and therefore are not usually a limited food source. However apart from fruits chimpanzees feed on many other food items. They hunt other mammals, mostly smaller primates, but in some communities also small forest antelopes and bush pigs (Boesch, 1994a; Stanford et al., 1994; Mitani & Watts, 1999; Watts & Mitani, 2002). In Taï, group hunting is common and hunters collaborate by taking different roles6 (Boesch, 1994b). A carcass of a hunted monkey can be characterised as nutritious but monopolisibale (Figure 2.3). Moreover chimpanzees in Taï extract nut kernels from their shells using pieces of wood or stones as hammers and anvils (Boesch & Boesch, 1983; Boesch & Boesch, 1984a,b). Some nut species possess very hard shells and chimpanzees are obliged to use stone hammers to crack them (Figure 2.4). Since large stones are rare in the Taï forest, stone hammers are also considered to be a monopolisable resource, needed to extract nutritious food. Chimpanzees also feed on insects. For some types of insects they use tools, for example when extracting maggots out of dead wood (Figure 2.5) or dipping for ants (Figure 2.6). When dipping for driver ants (Dorylus spec.), the right position at the nest is important to avoid painful ant bites – also a situation of possible monopolisation. Despite fruits, meats, nuts and insects, chimpanzees feed also on mushrooms, leaves, terrestrial herbivorous vegetation and insect products, such as honey. Some of these food items can be rare and monopolisable, but most of them are widely spread and usually common.

Figure 2.4 Female Goma is cracking Panda-nuts (Panda

Figure 2.5 Female Belle is extracting maggots from a

oleosa) with a stone hammer, while her daughter Giselle

dead tree with a stick that she is using like a fork (Photo

is feeding on the left-over (Photo by R. Wittig).

by R. Wittig).

6

coordination in time, space and function of behaviour 13


2.4 Reconciliation (and conflict management) in chimpanzees – a brief history

With such a social structure of long-term cooperative relationships and competition over food and mating partners, it is not surprising that reconciliation was discovered first in chimpanzees. In 1979 de Waal & van Roosmalen published the first detailed study on unexpected friendly interactions among former opponents after aggression in a group of chimpanzees in Arnhem Zoo, The Netherlands. They found that former opponents were more likely to be partners in the first friendly interaction after a conflict and that the time interval between the aggressive and the first interaction was shorter than between the first and the second interaction. De Waal and van Roosmalen called those first friendly interactions reconciliation and intuitively used the right functional label for these interactions (chapter 77; Cords & Thurnheer, 1993; Wittig & Boesch, in review). Although reconciliation has been studied in many other species after de Waal’s and van Roosmalen’s discovery (Aureli & de Waal, 2000a; Appendix A), it needed more than 10 years before the first comparison was conducted with another chimpanzee group in a zoo (Baker & Smuts, 1994). Since then several other studies have partly re-evaluated the results of de Waal’s and van Roosmalen’s pioneering work with data on other captive chimpanzees (Fuentes et al., 2002; Preuschoft et al., 2002).

Figure 2.6 Male Marius is ant dipping from a nest of driver ants (Dorylus spec.) on the ground (Photo by R. Wittig).

7

see 7.3.2 14


However in wild chimpanzees reconciliation was forgotten for many years. In 1986 (Goodall, 1986) mentioned reconciliation in a short paragraph and stated: “The gestures (note: that is reconciliation) used by the Arnhem chimpanzees were similar to those observed at Gombe (note: her study site), but it was obviously still more important in the captive situation (where the chimpanzees could not escape from one another) that quarrels be speedily settled.” It needed more than 20 years before the first study on reconciliation was published for wild chimpanzees (Arnold & Whiten, 2001). However, only with the present study were we are able to analyse all aspects of conflict management in wild chimpanzees.

15

Chapter 3: General methods of the study

Figure 3.0 The author, Roman M. Wittig, fully equipped (hand-held computer, watch, binoculars, pencil and paper) in the forest while observing chimpanzees in a tree (Photo by C. Crockford).

Chapter 3: General methods

3.1 Data collection

3.1.1 Study Site Data was collected in the study area of the Taï Chimpanzee Project, Taï National Park, Côte d’Ivoire (West Africa, 5°52 N, 7°22 W). The study was conducted between October 1996 and April 1999 on the North-community, which has been observed continuously since 1979 (further information: Boesch & Boesch-Achermann, 2000). In October 1996 the community consisted of four males (three adults, one adolescent), 14 females (11 adults, three adolescents) and 13 juveniles and infants. Almost all individuals, except for the female Ricci and her adolescent son Nino, were fully habituated to humans and were followed from nest to nest. The end of 1997 Nino became fully habituated and was followed for full days. During the observation period five chimpanzees disappeared or died (one adult male, two adolescent females, two juveniles) and six infants were born. 3.1.2 Observation Procedure I collected the following four types of data during all-day follows of the four males and 10 adult females. (1) I did all-day focal animal sampling (Altmann, 1974) of a target chimpanzee, recording activities, social interactions and vocalisations. (2) I recorded the target’s party composition by scanning the presence of individuals in visibility of the target every 10 minutes. (3) I documented the identity and number of females with genital swellings per day. (4) I noted specific information at each feeding site regarding the foods consumed by the target, food monopolisability and number of competitors present. All these data I entered directly into a Psion Organiser® hand-held computer via The Observer® (Noldus, 1989). Additionally I recorded quickly changing behaviours on a tape recorder and entered this information the same evening into the daily computer file (example for a daily computer file: Appendix B). I aimed to change the target chimpanzee each day, observing females once and males twice per month. There was however, some variability in individual observation frequency due to the fission-fusion character of chimpanzee societies (chapter 28), death and the habituation level. The result was 80 all-day follows of males (Macho/Marius: 31 days each, Brutus/Nino: 9 days each) and 123 all-day follows of females (between 10 and 15 days per female). 1071 conflicts with complete information were collected. Of these,

17


876 conflicts were analysed in chapter 59 (Wittig & Boesch, 2003b) and 610 (Wittig & Boesch, 2003d), as I excluded conflicts involving juveniles and infants. However in chapter 711 (Wittig & Boesch, in review), I analysed only conflicts with the complete information required for testing for reconciliation, which basically left 791 conflicts to analyse. 3.1.3 Ethogram The ethogram, I used for my study, was a subset of the ethogram of the Taï chimpanzee Project (C. Boesch, unpublished). It included 11 activities of the focal animal, 6 behaviours related to the proximity between partners, 11 social behaviours related to food, 6 submissive social behaviours, 21 aggressive social behaviours, 12 affiliative social behaviours and 16 vocalisations and gestures (Table 3.1). Some of the behaviours had additional modifiers to separate subtypes of the same behaviour (e.g. Table 3.1, No. 62, french kiss vs. directed kiss). Names of food plants were taken from a list including all species consumed by the Taï chimpanzee (D. Anderson, Z. Gone Bi and C. Boesch, unpublished). Table 3.1 Ethogram of the study on conflict management in Taï chimpanzees. Behaviours have a serial number and are ordered in the 7 categories: activity, proximity, food related social behaviour, submissive social behaviour, aggressive social behaviour, affiliative social behaviour and gestures/vocalisations. A represents the actor (or initiator) and B the reactor (or receiver) in the description of the behaviour. Occasionally either the French or the German name for the behaviour refers to the code, which is presented then in brackets. No. Name activity 1 move (déplacement) 2 eat (manger) incl. what A is eating incl. how many competitors incl. monopolisability 3 play (jeux)

JEU

4

hunt (jagen)

JAG

5

wait

WAI

6 7

rest (repos) and sleep groom incl. direction incl. duration of grooming nest (nid) contact handhold have

REP GRO

8 9 10 11

Code

Description

DEP MAN

A is moving on the ground or in the trees A is harvesting food and/or ingesting food, nut-cracking, and/or travelling within feeding site fruits, meat, nuts, insects, leaves, mushrooms, THV (terrestrial herbal vegetation), others absolute number of competitors relative amount and distribution of food source in relation to the number of competitors A plays with social partner or object. Play contains either behavioural elements out of the aggressive range or object manipulation, but is always accompanied by play face. A is at the height of monkeys and/or is moving towards monkeys with the “intention” (trying) to catch one A interrupts travelling movement when others lag behind or do not follow, and continues to travel once others have appeared or start following A is lying, sitting, standing or sleeping, not involved in other activities A grooms B indicating whether A grooms B or grooming is reciprocal indicating the end of grooming session A constructs a nest to lie in A rests in physical contact with B A rests holding B’s hand A possesses X, with X being a food item, a tool, or other object

NID CON HAN HAV

8

see 2.2 see 5.2.1 10 see 6.2.1 11 see 7.2.1 9

18


proximity 12 approach 13 hesitant approach 14 fast approach 15 leave 16 retreat 17 pass

APP HAP FAP LEA RET PAS

A A A A A A

food related social behaviour 18 stare at (fixer) 19 beg (mendier)

FIX MEN

20 21

PRE RE* PPA PAP PAC CAD REF TBA SUP

A approaches within 1m of B and stares at food B is possessing A, who is within 1m of B, lightly touches the food B is holding, or touches B with his hand (when B is eating), or puts his hand, palm up, toward food, or puts cupped hand under the food, or attempts to take food from B A takes food belonging to B A takes over food or object that B left on the ground either after B left the spot, or when B is still around fruits, meat, nuts, insects, leaves, mushrooms, THV (terrestrial herbal vegetation), others, tools B takes bits of food or object which A controls. A neither defends food nor encourages B B takes bits of food from A and A makes a gesture to encourage B A actively gives a piece of food to B A gives B all or more than half of food source A moves food or himself away from B A takes back some or all the food taken by B A approaches B who leaves, A takes over feeding/resting site or grooming partner

submissive social behaviour 29 present 30 crouch present 31 crouch 32 bobbing 33 intense greeting 34 hand reach

PRS CPR CRO BOB IGR HRE

A turns rump to B exposing genital area A presents to B with flexed legs A faces B with flexed legs A performs up and down movement by flexing and stretching arms and legs A sequence of fast approach, eye contact seeking, bobbing and intense pant-grunting A stretches arm out to B without actually touching B

aggressive social behaviour 35 piloerrection 36 head tipping 37 push away 38 arm wave

PIL HTI PAW AWA

39 40 41 42

throw shaking branch rocking hunch

THR SBR ROC HUN

43 44 45 46 47 48

shuffling bipedal swagger slapping or stamping bite dragging display

SHU BSW HIT BIT DRA DIS

49 50 51 52 53 54 55

short chase chase charge attack fight support flight

SCH CHA CHR ATT FIG SPP FLI

A has erected hair A makes a slight upward and backward jerk of the head towards B A stretches arm out and shoves B in direction away from himself A is flinging-out his arm towards B, a hitting down, punching-down or sweeping upwards movement A picks up an object and throws it unaimed (no target) or aimed (hitting a target) A takes hold of a branch, twig or sapling and sways it from side to side or forward and backward A rocks from side to side or forward and backward A raises shoulders accompanied by hair erection, can be bipedal, quadrupedal or in sitting position A walks dragging feet on the ground making the movement noisy A stands upright, sways from foot to foot, shoulders slightly hunched, arms out A hits substrate or B with hands or feet A puts his mouth to B’s body and bites him (presses his teeth) A pulls object or B over the ground A shows a variety of conspicuous and exaggerated motor patterns (e.g. bipedal running, stamping ground, waving arms in air) including piloerection A makes an movement of the whole body in the direction of B A runs after B who runs away from A A rushes towards B without B fleeing A suddenly, forcefully and deliberately contacts B with a series of aggressive contacts A attacks B while B attacks A C supports A or B via aggressive behaviours in an aggressive interaction A runs away from B

22 23 24 25 26 27 28

take (prendre) take over (récupération) incl. type of take over incl. type of food or object food share (partage) passive food share active-passive food share active present (cadeau) refusal take back replacement (suppléant)

affiliative social behaviour 56 bluff over 57 reach touch 58 finger in mouth 59 touch genitals 60 rump rub 61 mount 62 kiss incl. modifier 63 embrace incl. modifier 64 under arm 65 pat 66 copulation 67 genital investigation

BOV TOU FIM TGE RRU MOU KIS EMB UAR PAT COP GIN

enters at normal speed within 1 m of B enters at normal speed within 1 m of B, interrupted by pauses runs or walks fast directly over to B and enters within 1m of B casually walks out of 1m range of B makes sudden movement away from the vicinity (1m range) of B, but remains in visibility enters B’s 1m range, continues walking and leaves B’s 1m range without changing direction

A walks behind B putting his arms over him A reaches and touches any party of B’s body A puts his finger in B’s mouth A touches B’s oestrous or scrotum A rubs his rump against B’s rump A mounts B’s body, A makes pelvic thrusts, but does not ejaculate A presses his lips to B’s body french: kiss mouth to mouth; directed: kiss mouth to body part A stands/squats in front/behind B and puts his arms around B’s body reciprocal, directional A stands beside B and puts one arm around B’s body, while A and B look in the same direction A strokes or quickly caresses B’s body A approaches B, mounts, achieves penetration, performs pelvic thrusts A touches, sniffs or makes a few quick grooming movements on B’s oestrous

19


vocalisations, gestures & facial expressions 68 solicited grooming SGR A approaches B, stops in front and turns his side to B, presents an arm or scratches vigorously 69 solicited copulation SCO A presents penis erection and/or pelvic thrusts, bipedal swagger, knuckle knock to B 70 grin (fear) GRI A shows his teeth while maxilla and mandible are closed or almost closed 71 pant-grunt PGR A pants while inhaling and grunts with an open mouth while exhaling 72 bark BAR A produces a short and sharp vocalisation, that sounds like ‘waa’, ‘waaoo’ or ‘aaoo’ to the human ear 73 pant PAN A produces a panting sound whilst in- and exhaling with an open mouth 74 grunt GRU A produces a grunting sound with closed or open mouth 75 greeting-hooh GHO A produces a repeated and intense serious of ‘hoos’ 76 pant-hoot PHO A produces a series of alternating pants and hoots with increasing volume that usually ends in a climax scream 77 scream SCR A produces a loud and shrill vocalisation with an open mouth 78 crying (pleur) PLE A produces a series of short tonal hoos 79 drum (tambourinage) TAM A hits buttress of a tree with hands and/or feet 80 drumming sequence DSE A performs a sequence of behaviours starting with a warm up (rocking and/or leaf clipping in front of a tree), pant-hooting, drumming and climax scream 81 alarm call (cris d’alarme) CDA A produces a series of very loud bark-like vocalisations that sounds like ‘waaoou’ to the human ear 82 temper tantrum TTA A screams or cries and tumbles on the ground 83 smack SMA A makes a smacking sound with the lips while grooming B

3.2 Operational definitions

3.2.1 Behaviours Aggressive behaviours are threats (e.g. barks, arm wave), non-contact aggression (e.g. displays) and contact aggression (e.g. bites, hits). Affiliative behaviours are friendly behaviours with body contact (e.g. kiss, genital touch, embrace, grooming). Submissive behaviours are submission (e.g. greeting, crouching) and flights (e.g. fleeing, retreating). 3.2.2 Conflict variables A conflict was defined as a dyadic aggressive interaction that started with the first aggressive behaviour exchanged and ended with either submission, flight, reciprocal screaming, or a non-aggressive behaviour which was not immediately followed by further aggression. The conflict duration was measured from the first exchange of aggressive behaviour until the defined end of the conflict. Aggressive behaviours were classified into five ordinal intensities according to the likelihood of the aggression injuring the partner (Table 3.2). For each conflict the most intense aggressive behaviour used determined the intensity score for that conflict. The cross product of sex of both the initiator and the receiver of aggression are referred to as the sex combination of conflict partners in four categories (initiator-receiver: male-male, m-m; male-female, m-f; female-male, f-m; female-female, f-f).

20


3.2.3 Context variables I observed conflicts within three different contexts. In sex contexts, conflicts are with or over oestrus females either to copulate with them or to prevent others’ copulation attempts. Food conflicts are over food resources. In the social contexts, conflicts show or challenge dominance (e.g. subordinates approaching without greeting followed by aggression) or conflicts arise from competition for social partners. However, it is partly a default option (as conflicts can only occur with other social partners present), which other studies have summarised in an unknown category (e.g. de Waal & Hoekstra, 1980). Two further variables are the possibility to monopolise the resource as well as how many competitors are present. The resource monopolisation is a measure of the relative distribution and aquantity of the resource that is fought over. This is a relative scoring of the number of competitors needed to monopolise the resource. The competitor proportion is the proportion of competitors present in the party compared to all potential competitors for the specific resource. 3.2.4 Dominance variables Hierarchies in chimpanzees are classically determined by greeting vocalisations (de Waal, 1978). I used pant-grunts among males and additionally greeting-hoo and greeting-

pant among females to determine the hierarchy (chapter 412; Wittig & Boesch, 2003a). One rank change occurred in the males during the data collection period due to the death of Brutus, the b-male. Since the two lower ranking males rose up in rank accordingly, conflicts between them and the a-male had two rank differences each in the data set, depending on whether the conflict occurred before or after the rank change. The relative dominance of the initiator towards the recipient of aggression was scored in the initiator’s

rank variable. The winner of the conflict was defined as the one able to access the resource. In food contexts the winner was the one possessing the food after the conflict. In sex contexts the winner was able to assert his or her choice (e.g. a female refused copulation, a male interrupted a copulation of another male). In social contexts the winner was the one who neither showed submission, screams nor flight, or, in cases of competition over the access to a social partners, the one who affiliated with this social partner afterwards. I defined conflicts as a draw when neither conflict partner won. Whether or not the initiator won was reflected in the initiator victory outcome, while

12

see 4.2.1 21


whether or not the dominant won the conflict was marked by the variable of the winner’s

rank. 3.2.5 Social variables Association is defined as being present in the same party (being within visibility). I used the dyadic association index (DAI) to measure how frequently two individuals were associated: DAIAB =

å ( A + B) , where A is the time individual A was seen, B å A + å B - å ( A + B)

is the time individual B was seen and A+B is the time A and B were seen together (Nishida, 1968). I gave the relationship benefit three categories according to the presence of food-sharing and support events within the dyads (Table 3.2). Dyads of high

relationship benefit are called high value partners or friends.

Table 3.2 Main variables with definitions used in the present study on conflict management to describe the conflict situation. Parameters names of the variables are underlined for a better contrast. Some variables are adapted for comparisons (see 3.2.5) and thus same parameters have different ranges for e.g. different sex combinations. Name Conflict Duration Conflict Intensity

Type continuous ordinal multinomial

Initiator’s Sex Recipient’s Sex Conflict Context

binomial binomial multinomial

Resource Monopolisation

ordinal multinomial

Competitor Proportion

ordinal multinomial

Rank Difference (r.d.)

ordinal multinomial

Initiator’s Rank

binomial

Initiator Victory Outcome

multinomial

Winner’s Rank

binomial

Association Index

ordinal binomial ordinal multinomial

Relationship Benefit

Variable definitions and parameter scoring duration from start of first aggressive behaviour to end of the conflict (in seconds) intensity of the conflict scored by the most intensive single aggressive behaviour: 1 = aggressive vocalisation or gesture; 2 = non-contact aggression, no movement; 3 = noncontact aggression, including movement; 4 = physical contact aggression, one action; 5 = physical contact aggression, many actions sex of the initiator of aggression: m = male aggressor; f = female aggressor sex of the recipient of aggression: m = male aggressed; f = female aggressed context in which the conflict occurred: sex = conflict about oestrus females; food = conflict about food or possession of tool; social = conflict about hierarchy or social partners and default category relative scoring of the monopolisability of the resource that is fought over: monopolisable by one = resource can be monopolised by one competitor; monopolisable by few = resource can be monopolised by some of the competitors; not monopolisable = resource is not monopolisable proportion (prop.) of competitors present in the party in relation to all potential competitors (in social context: adults in party / all adults; in sex context: males in party / all males; in food context: adults feeding / all adults in party): few for prop.£0.33; some for 0.330.66 difference of ranks in the linear dominance hierarchy (Wittig & Boesch 2003a; chapter 4) between conflict partners (rank subordinate – rank dominant); different category ranges, because of different numbers in sex classes; categories of r.d. in m-m dyads (4 males): small for rank neighbours, middle for r.d.=2, large for r.d.=3; categories of r.d. in f-f dyads (12 females): small for r.d.£3, middle for 36; categories of r.d. in m-f dyads (16 individuals) small for r.d.£5, middle for 510 dominance relationship of the conflict partners from the perspective of the initiator: dom = aggressor dominant over aggressed; sub = aggressor subordinate to aggressed outcome of the conflict from the perspective of the initiator: w = initiator wins aggressive interaction; l = initiator loses; n = no winner dominance relationship of the conflict partners from the perspective of the winner: d = winner is dominant over loser; s = winner is subordinate to loser; n = no winner relative scoring of the dyadic association index (Nishida 1968): rare = m-m: £50% of observation time, others: £25%; frequent = m-m: >50%, others: >25% scoring of the cooperative character of the relationship of dyads: low = no food sharing, no support within dyad; medium = either food sharing or support within the dyad; high = both food sharing and support within the dyad

22


3.2.6 Variable adaptations for statistical comparison Some variables are affected by the sex of chimpanzees (e.g. DAI: higher for males than females, Boesch & Boesch-Achermann, 2000; dominance-rank: males dominant over females, Bygott, 1979). For the multivariate analysis, I corrected for sex differences by calculating indices. The indices relate data to average values of sex combinations in order to enable a comparison between the sexes (detailed description: Table 3.2).

3.3 Multivariate dyadic approach

I employed a multivariate approach to detect the factors that affected the variation within conflict management. Table 3.2 displays variables (with definitions and scoring of parameters) that I considered either as dependent variables or as independent variables for variation within conflict management. In addition to the variables of Table 3.2, I tested some chapter specific variables and interactions of variables, which are explained in detail within the methods part of each chapter. Conducting a multivariate analysis I considered all independent variables simultaneously. As many of the variables (e.g. rank

difference, association index, relationship benefit) were different for the same individual with different partners, I conducted the analyses on a dyadic level. However, since repeated measurements of individuals can inflate the Type I (a) error, I controlled for this and ruled out the influence of repeated measurements on variables (see statistical process). The strength of the results was therefore similar to an individual based analysis, but included the advantage that one is able to detect dyadic variability.

3.4 Statistics

Despite the usual parametric and non-parametric statistic, I conducted two less common types of statistical tests throughout almost all four chapters. One type of tests was multivariate analysis in two different versions (for categorical and continuous dependent variables) and the other type included dyadic permutation tests. When a

23


question required another unusual statistical approach (e.g. chapter 4: linearity tests13), then I have explained it in the methods specific to that chapter. 3.4.1 Multivariate analysis To detect the variables that affect the variation found within conflict management, I executed multivariate analyses as follows: 1. In order to eliminate repeated measurements of the same conflict type per dyad, I summarised conflicts in one data point for identical cases. Identical conflicts had the same initiator and receiver of aggression, the same value of some chapter specific variables and were identical in all other independent variables, apart from conflict

duration and conflict intensity, which were scored with mean and median values respectively. However each specific question of my thesis (each of the chapters 4, 5, 6 and 7) had a different set of independent variables. Thus the previously described balancing process lead for each question to a different final data set for the multivariate analysis, which is referred to in each chapter again. 2. I used a generalisation of the Logistic Regression called General Linear Models (GLM: McCullagh & Nelder, 1989; Agresti, 1996) to examine the functional relationship between the occurrence of one dependent variable and several independent variables. GLMs have been successfully applied in studies on reconciliation and dominance relationships (Call et al., 1999; Côté & Festa-Bianchet, 2001). (a) Investigating the variation of categorical dependent variables I conducted a Generalised Linear Model (GLZ). For GLZs I chose an ordinal multinomial, or binomial respectively, error distribution and a logit link function (McCullagh & Nelder, 1989). The best model was selected by the best subset method, which is an iterative method based on maximum likelihood estimation (LR: likelihood statistics), and the Akaike’s Information Criterion (AIC), which penalises for the number of independent variables in the model (Akaike, 1973). The significance of the independent variables and their parameters was assessed using Wald statistics for the best model (Dobson, 1990). The estimate-coefficient b is an indicator for the strength of the effect that an independent variable-parameter

13

see 4.2.3 24


has on the occurrence of the dependent variable. The probability that the tested dependent variable occurred was eb more likely with one unit increase of the independent variable, after adjusting for all other variables. This means that, for example, a positive b indicates an effect in favour for the first parameter of the dependent variable that enters the model, while a negative b indicates the opposite direction. The further away b was from 0 the bigger the influence of the independent variable-parameter. (b) Investigating the variation of continuous dependent variables I conducted a General Linear Model (GLM). The GLM is based on the least square method and can estimate the effect of several variables on one dependent variable. After calculating the all effect model, I calculated the effect size for the significant variables by excluding trivial effects (effect size £ 0.1) from the model (Cohen, 1988). For variable interactions I used Fisher LSD post-hoc test (equivalent to ttest) with Bonferroni correction. 3. Since I was analysing on a dyadic level, I controlled for a possible inflation of the Type I (a) error due to multiple measurements of the same actor. Therefore I included the identity of the conflict initiator as an additional independent variable and tested the best model again (similar procedure was used by Côté & Festa-Bianchet (2001)). When the significant explanatory independent variables remained significant, one is able to assume that the effect was not due to the replicated observations of the same individual (Bland & Altman, 1995). For the sake of simplicity I have presented only the remaining significant variables of the best models in the results, where p