why small groups of tamarins do not reconcile conflicts

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Colleen M. Schaffner, PhD, Department of Psychology Centre for Stress Research, University College Chester Parkgate Road, Chester CH1 4BJ (UK) Tel. +44 1244 392 732, ext. 3358, Fax +44 1244 392 823 E-Mail [email protected]

Reviewed Article

Folia Primatol 2005;76:67–76 Received: February 6, 2004 DOI: 10.1159/000083614 Accepted after revision: August 5, 2004

Following the Rules: Why Small Groups of Tamarins Do Not Reconcile Conflicts

Colleen M. Schaffnera Filippo Aurelib Nancy G. Cainec a Department of Psychology, Centre for Stress Research, University College Chester, Chester, b School of Biological & Earth Sciences, Liverpool John Moores University, Liverpool, United Kingdom; c Department of Psychology, California State University San Marcos, San Marcos, Calif., USA

Key Words Reconciliation � Tamarins � Post-conflict behaviour � Relationship repair

Abstract Reconciliation is the post-conflict friendly reunion between opponents. A

series of conditions and rules in order for reconciliation to take place has been recently proposed. One critical condition is that the relationship between oppo-nents must be disrupted. We tested this condition using post-conflict and matched-control observations on 4 small groups of tamarins (Saguinus labiatus). Our previous lack of evidence for reconciliation was confirmed. No post-conflict relationship damage was therefore expected. We found evidence that relation-ships were disturbed following conflicts over food but, as in other primates, no evidence for reconciliation following such conflicts was found. For non-food-related conflicts there was no evidence that relationships were disturbed, as opponents were in close proximity to each other and resumed the exact same activity as frequently in the post-conflict observations as they did in the matched-control observations. We conclude that ‘everyday’ aggression may not disrupt the relationships among individuals from the same family group and therefore reconciliation is not needed.

Introduction

There is a variety of possible negative consequences associated with aggres-sion in group-living animals. Recipients of aggression are at risk of renewed attack [e.g., Aureli & van Schaik, 1991; Kutsukake & Castles, 2001], may spend less time

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http://dx.doi.org/10.1159%2F000083614

68 Folia Primatol 2005;76:67–76 Schaffner/Aureli/Caine

foraging [Aureli, 1992], and other individuals may be less likely to cooperate with them [de Waal, 1986]. In recent years research has focused on how animals deal with these negative consequences, especially on the post-conflict phenomenon that was originally termed ‘reconciliation’ [Aureli & de Waal, 2000]. ‘Reconciliation’ was defined as a friendly reunion between former opponents soon after a conflict has ceased [de Waal & van Roosmalen, 1979], and many studies have supported a relationship-repair function of reconciliation [Aureli et al., 2002].

A predictive framework with conditions and rules pertaining to the occurrence of reconciliation was recently presented [Aureli et al., 2002]. Conditions for recon-ciliation include that species have individually recognised relationships within their social groups; that aggressive conflicts occur between individuals in socially living groups (as individuals that do not resolve disputes with aggression – e.g., avoidance of the opponent – would not need reconciliation to repair their relationships), and finally that in the aftermath of aggression the opponents’ relationship is disturbed in the form of reduced tolerance around resources, increased risk of renewed attack or loss of cooperation. Of course, even if species fulfill all the conditions they will not necessarily reconcile unless there is a loss of benefits. Therefore rules have been proposed to predict when reconciliation should occur [Aureli et al., 2002]. Rule 1 states that reconciliation can take place when the benefits of signalling the end of hostility outweigh the costs of renewed attacks. Rule 2 states that reconcilia-tion is necessary for species in which aggression compromises mutually valuable relationships, and that it should occur more often between individuals that have more valuable relationships.

Reconciliation has been studied in a variety of species, predominantly pri-mates, and in nearly all cases reconciliation has been found [Aureli et al., 2002]. However, it should be noted that in a few cases reconciliation was reported only for certain age-sex class dyads [Watts, 1995; Leca et al., 2002; Sommer et al., 2002]. Only two studies reported that no evidence of reconciliation could be found. Kap-peler [1993] failed to find evidence of reconciliation in ring-tailed lemurs (Lemur catta), but Rolland and Roeder [2000] suggested that reconciliation could take place. Schaffner and Caine [2000] found no evidence of reconciliation in red-bellied tamarins (Saguinus labiatus). Ring-tailed lemurs fail to reconcile probably because conflicts are usually limited to individuals that do not have valuable rela-tionships (i.e., no post-conflict loss of benefits), whereas valuable partners almost never, if ever, fight [Pereira & Kappeler, 2000]. However, the situation is not as clear-cut in the case of red-bellied tamarins as all group members, given their high degree of relatedness and shared breeding interest, are expected to have valuable relationships [for review see Schaffner & Caine, 2000].

Red-bellied tamarins are therefore expected to reconcile according to rule 2 (see above). The reason why this is not the case could be because tamarins’ rela-tionships may not be disturbed by aggressive interactions. Therefore the aim of this study was to test whether aggression disturbed tamarins’ relationships using the same data set as Schaffner and Caine [2000]. We made the following predictions: first, post-conflict tolerance between former opponents should decrease if their relationship is disturbed by the aggressive interaction; second, in the event that post-conflict renewal of aggression occurs, the decrease in tolerance between for-mer opponents should be even stronger, and finally, if aggression does disturb tamarins’ relationships, then aggression should disrupt the mutual social activities

Why Tamarins Do Not Reconcile 69 Folia Primatol 2005;76:67–76

of the opponents. Since most previous studies have shown that conflicts over food have a different aftermath [reviewed in Aureli et al,. 2002; Judge, 2003] than non-food-related aggression, we also carried out separate analyses for food-related and non-food-related conflicts.

Method

Subjects and Husbandry Ten captive red-bellied tamarins (S. labiatus) living in 4 social groups were observed

for the study. All the groups were small (table 1) though not incongruent with group sizes that occur in the wild [Sussman & Garber, 1986]. The groups were housed inside the primate laboratory at Bucknell University (Lewisburg, Penn., USA) within 4 different enclosures that varied in size. The largest enclosure measured 2.5 � 1.9 � 1.7 m, 2 of the enclosures measured 1.3 � 1.6 � 1.9 m and the smallest enclosure measured 0.76 � 0.76 � 1.42 m. (The smallest enclosure was a temporary housing arrangement due to pending refurbishment of all the tamarin enclosures at Bucknell University.) Even the smallest enclosure allowed the animals to separate themselves much further than the proximity measure used in this study. For further details on housing and husbandry routines see Caine et al. [1995].

Data Collection Caine [1990] found that red-bellied tamarins might alter their behaviour in the presence

of familiar and unfamiliar observers. Therefore C.M.S. observed the tamarins from behind a blind or one-way mirrored glass planes. In all cases the animals were clearly visible in all parts of their enclosure and all vocalisations could be readily heard. The data were collected using an Observational Systems OS-3 event recorder programmed to encode the identity of the actor, the behaviour and the recipient as well as the time the behaviour occurred. Each group was observed for a minimum of 30 min on every observation day, approximately 5 days/week, spanning a period of 5 months (November 1990 to April 1991). Data collection consisted of two main components: aggressive behaviour and its aftermath.

The onset of aggression was recognised by headshake-teethchatter [Coates & Poole, 1983] and confirmed by pilot observations. The agonistic behaviours that were recorded during an aggressive interaction were based upon pilot observations and the findings of Ep-ple and Alveario [1985] on aggression in saddle-back tamarins (table 2). In addition to not-

Table 1. Characteristics of study subjects

Subject Sex Age class1 Group membership

Hourly rate of aggression

1C female adult 1 0.11 1B male adult 1 0.30 1W male adult 1 0.16 2Jt female adult 2 0.60 2J female sub-adult 2 0.10 2D female sub-adult 2 0.23 3N male adult 3 0.28 3M female adult 3 0.38 4S male adult 4 0.05 4M female adult 4 0.03

1Adults were >2 years of age, sub-adults 1–2 years of age.


ing the actor, behaviour and recipient, the activity (if any) opponents were involved in with each other just before the aggressive conflict was also noted (table 2). Aggressive interac-tions were recorded on an all-occurrences basis [Altmann, 1974].

The second component of the study examined the effects of aggressive behaviour, fol-lowing de Waal and Yoshihara’s [1983] procedure. Immediately after an aggressive interac-tion, when the opponents ceased headshaking and teethchattering, a post-conflict obser-vation (PC observation) began, using continuous focal animal sampling [Altmann, 1974]. The time of each aggressive and/or friendly behaviour with the former opponent was re-

Table 2. Description of aggressive and affiliative behaviours and the activity at the time of the aggression recorded during the study

Behaviour category Behaviour Description

Aggressive Headshake-teethchatter

The rapid movement of the animal’s head from side to side accompanied by raspy vocalisation that is emitted in short consecutive bursts.

Facial threat Stare at another individual, can occur in con-junction with a frown or with ear flicking and tongue protrusions.

Piloerect Portions or all of the animal’s fur is erect. Lunge Head and shoulders are thrust toward another

individual. Chase One animal pursues another. Bite Animal puts mouth to any other part of another

individual. Attack Individual grabs or scratches the skin or fur of

another animal. Fight Prolonged kicking, scratching and biting, often

characterised by animals falling to the cage floor and remaining locked together in combat.

Proximity Animal is within arm’s reach of another indi-vidual.

Affiliative Allogroom Animal uses hands or teeth to manipulate the skin or fur of another.

Huddle Animal lies across another or sits or lies with limbs intertwined.

Play Two individuals wrestle or chase after one another while emitting the play vocalisation.

Sexual Male attempts to mount or mounts a female with or without thrusting, often accompanied by tongue flicking by one or both individuals.

Activity the opponents were engaged in with each other at time of aggression

Feeding At least one individual was eating while in proximity with the other or an individual at-tempted to snatch the food away from the other.

Allogroom One individual was grooming the other. Play Individuals were involved in a play bout. Sex Individuals were engaged in copulation.


corded for 20 min. If aggression re-occurred during the PC observation, the observation was aborted and a new PC observation was started following the end of the aggressive interac-tion. Then, on the next possible observation day and at precisely the same time of day, the focal animals involved in the conflict were observed for 20 min and the same type of data as during the PC observations was collected to provide matched-control observations (MC observations). In the event that aggression erupted 10 min before the scheduled MC observa-tion or during an MC observation, data collection was postponed until the following obser-vation day. Social behaviour categories recorded during the PC observations and MC obser-vations (table 2) were based on pilot observations and another study of captive red-bellied tamarin behaviour [Coates & Poole, 1983].

Statistical Analyses We collected data on 77 conflicts; the majority of the conflicts had adequate detail to

be included in this study, resulting in 74 PC observation-MC observation pairs. To confirm our previous finding (based on the time rule method [Schaffner & Caine, 2000]) that recon-ciliation does not take place, we analyzed the PC observation-MC observation data set to ascertain whether former opponents were attracted to each other or dispersed following con-flicts. ‘Attracted pairs’ were former opponents who affiliated with each other sooner during the PC observation than during the MC observation (table 2), whereas ‘dispersed pairs’ were opponents who made friendly contact sooner during the MC observation than the PC obser-vation [de Waal & Yoshihara, 1983].

To test whether aggression disturbed the tamarins’ degree of tolerance we compared the mean proportion of PC observations or MC observations in which former opponents were in proximity (see table 2 for definition) to each other during the first 5 min of the PC observations (the same was done for the MC observations). We also distinguished whether renewal of aggression between opponents occurred within the first 2 min of the PC observa-tion or not, and then performed the same comparison for proximity levels in PC observations where there was a renewal of aggression, in PC observations without a renewal of aggres-sion, and in MC observations. Finally, to test whether aggression disrupted the former oppo-nents’ activity (see ‘activity at time of aggression’ in table 2), we examined whether indi-viduals resumed the same activity that they had been engaged in with each other prior to the aggressive conflict less often than what could be expected (calculated from activity rates in periods without previous conflict). To do so, we compared the proportion of PC observa-tions in which the former opponents resumed the activity within 5 min following the conflict with the proportion of MC observations in which the same activity occurred within the first 5 min.

Analyses were carried out at the opponent dyad level; however, not all dyads had PC observation-MC observation pairs for every analysis and thus the degrees of freedom vary across statistical tests. Typically in studies on reconciliation, nonparametric statistical tests are used. We used two-tailed paired t tests and one-way repeated measures ANOVA tests, which are more powerful parametric tests and more likely to detect significant differences with small samples sizes. We present only the results from these tests; however, we obtained the same pattern of findings when we performed the analyses with non-parametric alterna-tives. An α-level of 0.05 was adopted for all tests.

Results

We found no evidence that tamarins reconcile following conflicts. There was no significant difference in the mean proportion (± SE) of attracted versus dis-persed pairs [t(7) = 0.58, p = 0.58]. The mean proportion of attracted pairs was 0.39 (± 0.08), whereas the mean proportion of dispersed pairs was 0.48 (± 0.08). When we examined only conflicts over food we found no support for reconciliation. Actu-ally, there was a trend in the opposite direction [t(7) = 2.28, p = 0.06]. The mean


proportion of attracted pairs was 0.28 (± 0.07), whereas the mean proportion of dispersed pairs was 0.57 (± 0.07).

Next, we examined whether aggression led to a decrease in tolerance between former opponents following aggression. We found no evidence that former oppo-nents decrease their tolerance to each other within 5 min following non-food-related conflicts: there was no statistical difference in proximity levels between the non-food-related PC observations and the corresponding MC observations [t(6) = 0.18, p = 0.86; fig. 1]. By contrast, we found a significant difference in the propor-tion of observations in which former opponents were in proximity to each other following food-related conflicts compared to the analogous measure in the MC observations [t(7) = 3.27, p = 0.014; fig. 1]. Thus they avoided each other following food-related conflicts.

Although aggression was renewed during 23 of the PC observations (31%), post-conflict tolerance between former opponents was not altered. The proportion

Fig. 1. Mean (± SE) proportion of observations in which individuals were in proximitywithin the first 5 min after the start of the observation.

Fig. 2. Mean (± SE) proportion of observations in which individuals were in proximitywithin the first 5 min after the start of the observation.


of observations in which former opponents were in proximity to each other within 5 min of the beginning of the observation was not different between the PC observa-tions with renewed aggression, PC observations without renewed aggression and MC observations [F(2,10) = 0.27, p = 0.77; fig. 2]. In addition, when we excluded conflicts over food from the analysis we again failed to find a significant difference [F(2,10) = 0.29, p = 0.75].

Finally, we tested whether aggression affected the resumption of activities former opponents were engaged in when aggression started. The proportion of ob-servations in which former opponents resumed the same activity within 5 min of ceasing a conflict did not differ statistically from the proportion of MC observa-tions during which they were engaged in the same activity within the first 5 min [t(7) = 0.72, p = 0.50]. The finding did not change significantly when we excluded conflicts over food [t(4) = 0.98, p = 0.38; fig. 3].

Discussion

Our findings are in line with the condition that reconciliation will take place only when relationships are disturbed by aggressive interactions [Aureli et al., 2002]. We confirmed that the study tamarins did not reconcile [cf. Schaffner & Caine, 2000]. We did not find support for the three predictions we made if aggres-sive conflict disrupts former opponents’ relationships. We found no statistical evi-dence that there was a decrease in tolerance between former opponents in the after-math of non-food-related conflicts as measured by close proximity in PC observa-tions and MC observations. We also failed to find evidence for such a decrease when post-conflict renewal of aggression did take place. Furthermore, no evidence was found for former opponents’ on-going activities to be altered by the previous aggressive conflict as they resumed the activity they were engaged in with each other prior to the conflict as often as they were involved in such an activity in MC observations.

Fig. 3. Mean (± SE) proportion of PC observations in which former opponents resumedthe same activity they were engaged in with each other at the time of aggression within thefirst 5 min after the conflict and the proportion of MC observations in which they wereengaged in the same activity within the first 5 min of the observation.


We found that former opponents were less likely to be in close proximity to each other in the aftermath of food-related conflicts than during MC observations. This finding could be interpreted as evidence for disturbance between former oppo-nents and thus reconciliation would be expected to occur. However, we found no evidence for reconciliation following food-related conflicts. This finding is in full agreement with the results of several other studies [see review by Judge, 2003] which, unlike ours, found evidence for reconciliation after non-food-related con-flicts and suggested that former opponents’ relationships may not be disturbed by food-related conflicts.

Although the three predictions were not supported by our data on non-food-related conflicts, we have to be cautious in our conclusions because the sample sizes for the analyses were small. One hundred and forty-nine hours were spent collecting data, but only 77 PC observation-MC observation pairs could be re-corded (74 of which met the criteria for the present study) because of the low rate of aggressive interactions in the study groups (see below). However, two lines of evidence suggest that our failure to reject the null hypothesis is probably not simply due to the small sample size. First, we found a significant difference in the prox-imity patterns of former opponents between PC observations after food-related con-flicts and MC observations. Our result was obtained using only a subset of the data (the 37 PC observations after food-related conflicts), indicating that we could have identified other statistically significant differences in spite of the small sample sizes. The second line of evidence is that the PC observation and MC observation mean values (± SE) were very similar to each other (fig. 1, 2) or in stark contrast to that expected if a relationship disruption had occurred (fig. 3). It is especially re-markable that the PC observation mean value for proximity was similar to that for MC observations, even for PC observations with renewal of aggression (fig. 2). The examination of these values therefore corroborates our interpretation of the failure to reject the null hypothesis for the three predictions.

It is interesting to compare our results with the findings of the only other study on post-conflict behaviour of a callitrichid species. Westlund et al. [2000] reported the occurrence of reconciliation in common marmosets (Callithrix jacchus), but a close inspection of their results reveals striking similarities with our study. Marmo-set opponents remained together after approximately 20% of their aggressive con-flicts, and they were in close proximity within 3 min in a further 30% of PC obser-vations. Post-conflict overt affiliative behaviour, such as grooming or playing, which are expected to be used for post-conflict resolution in highly co-operative and affiliative species, was rare. In addition, there was evidence for rapid post-conflict resumption of the same activity that the former opponents were engaged in prior to the conflict (although this issue was not explicitly examined in the study). Furthermore, the authors reported that the marmosets hardly showed any behav-ioural sign of distress in the aftermath of aggressive conflicts [Westlund et al., 2000]. A re-assessment of Westlund et al.’s findings in light of the framework of rules and conditions for reconciliation put forward by Aureli et al. [2002] would likely support the results of our study.

The anomalous lack of evidence for reconciliation in a species with highly valuable relationships may not be unique to our study of red-bellied tamarins, and further investigation of other callitrichid species is necessary to determine if it is indicative of a general pattern. Our own results provide support for one of the criti-


cal conditions for reconciliation: post-conflict friendly reunions are expected to occur when former opponents’ relationships need to be repaired [Aureli et al., 2002]. If aggressive conflicts do not disturb such relationships, reconciliation is not expected to occur. Thus we have found a plausible answer to the question: ‘Why do tamarins not reconcile?’ that actually fits our knowledge of post-conflict patterns in other primates well. The tamarins we studied follow the rules of the predictive framework [Aureli et al., 2002]. The next challenge is then to answer the critical question why their relationships are not disturbed by aggressive conflicts. It is pos-sible that the nature of their relationships, which serve the overriding needs to co-operate for avoiding predation and caring for infants, reduces the extent to which they are vulnerable to disruption from rare, mild forms of aggression, which are the norm in these species [reviewed in Schaffner & Caine, 2000]. The high tolerance and dependency may also generate sufficient social security [Cords & Aureli, 2000] that relationships do not require reaffirmation following mild altercations.

Group size may affect the occurrence of reconciliation as relationship value and the likelihood of post-conflict relationship disturbance are expected to vary depending on the availability of alternative partners [Aureli et al., 2002]. This ‘market effect’ [cf. Noë et al., 2001] could well apply to callitrichid social systems. When groups are small, like our study groups, the high degree of co-dependency between group members reduces the extent to which social relationships are vulner-able to disruption from mild aggression. However, when groups are larger, more partners are available to carry out co-operative tasks. The possibility to replace co-operative partners with other individuals may result in a lower degree of co-dependency from specific group members. When groups are very large, such ten-dency could lead to the actual eviction of group members under particular circum-stances. Thus one possible scenario for the occurrence of reconciliation in callitri-chids is when mild aggression in middle-sized groups results in a loss of benefits because individuals can ‘shop around’ for alternative cooperative partners [cf. Lazaro-Perea, 2001]. Under these circumstances, former opponents are likely to be motivated to reconcile their conflicts to reaffirm the co-dependency to each other and thus maintain the benefits associated with their relationships [i.e., condition 4 in Aureli et al., 2002].

Of the many studies on post-conflict behaviour, only a handful have provided evidence for the actual need for and effectiveness of reconciliation by showing data on post-conflict disturbance of the opponents’ relationship and its restoration follow-ing friendly exchanges [Cords, 1993; Schino, 2000]. Our study shows that verifying the presence of the critical conditions expected for reconciliation to take place is fundamental for meaningful interpretation of any finding of post-conflict studies. After failing to demonstrate reconciliation at group level, few studies selected poten-tially valuable relationships (e.g., male-female, kin) and managed to find evidence for reconciliation between such valuable partners [e.g., Watts, 1995; Leca et al., 2002]. This is certainly a method for the identification of reconciliation that fits the overall idea behind the predictive framework put forward by Aureli et al. [2002]. We believe however that a more sensitive method would be to test explicitly whether the conditions for reconciliation are actually present in the study group, a subset of rela-tionships or a particular situation. Our study is the first example of the use of the predictive framework to test specifically whether such conditions are satisfied or not. We encourage others to do the same in future post-conflict studies.


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