Many patterns of affiliative behaviour have been described for primates, for instance: reciprocation and exchange of grooming, grooming others of similar rank, reconciliation of fights, and preferential reconciliation with more valuable partners. For these patterns several functions and underlying cognitive processes have been suggested. It is, however, difficult to imagine how animals may combine these diverse considerations in their mind. Although the co-variation hypothesis, by limiting the social possibilities an individual has, constrains the number of cognitive considerations an individual has to take, it does not present an integrated theory of affiliative patterns either. In the present paper, after surveying patterns of affiliation in egalitarian and despotic macaques, we use an individual-based model with a high potential for self-organisation as a starting point for such an integrative approach. In our model, called GrooFiWorld, individuals group and, upon meeting each other, may perform a dominance interaction of which the outcomes of winning and losing are self-reinforcing. Besides, if individuals think they will be defeated, they consider grooming others. Here, the greater their anxiety is, the greater their “motivation” to groom others. Our model generates patterns similar to many affiliative patterns of empirical data. By merely increasing the intensity of aggression, affiliative patterns in the model change from those resembling egalitarian macaques to those resembling despotic ones. Our model produces such patterns without assuming in the mind of the individual the specific cognitive processes that are usually thought to underlie these patterns (such as recordkeeping of the acts given and received, a tendency to exchange, memory of the former fight, selective attraction to the former opponent, and estimation of the value of a relationship). Our model can be used as a null model to increase our understanding of affiliative behaviour among primates, in particular macaques.

Individual primates distribute their affiliative behaviour (such as grooming) in complex patterns among their group members. For instance, they reciprocate grooming, direct it more to partners the higher the partner's rank, use it to reconcile fights and do so in particular with partners that are more valuable. For several types of patterns (such as reconciliation and exchange), a separate theory based on specific cognitive processes has been developed (such as individual recordkeeping, a tendency to exchange, selective attraction to the former opponent, and estimation of the value of a relationship). It is difficult to imagine how these separate theories can all be integrated scientifically and how these processes can be combined in the animal's mind. To solve this problem, we first surveyed the empirical patterns and then we developed an individual-based model (called GrooFiWorld) in which individuals group, compete and groom. The grooming rule is based on grooming out of fear of defeat and on the anxiety reducing effects of grooming. We show that in this context this rule alone can explain many of the patterns of affiliation as well as the differences between egalitarian and despotic species. Our model can be used as a null model to increase our understanding of affiliative patterns of primates.

Introduction

Patterns of affiliative behaviour have long puzzled primatologists. One of the most frequent behavioural acts is grooming. It has been explained as serving several functions, such as cleaning the fur [1], reducing anxiety, tension and stress [2], social bonding [3], repairing relationships [4] and social reciprocation and exchange [5]. As regards the mechanisms of exchange, individuals have been supposed to direct grooming up the hierarchy in order to receive more effective support in return, and due to competition for partners of high rank they may end up grooming others of similar rank [6]. Besides, they were also supposed to groom others of similar rank, because individuals of similar rank have similar needs [7]. Grooming between two former opponents immediately after a fight has been interpreted to function as a means to repair the relationship or ‘reconcile’, because it occurred significantly earlier after a fight than otherwise in matching control periods the next day. Besides, individuals appeared to reconcile in particular with those partners that appeared more valuable to them, the so-called ‘valuable-relationship hypothesis’ [8].

To complicate matters, the degree of exchange and reciprocation [9] appeared to differ between egalitarian and despotic species. Applying market theory [10],[11], this was explained by assuming that the exchange rate of services differed between the two competitive regimes [9]. Further, the co-variation hypothesis explained the lower conciliatory tendency in despotic societies by the greater danger involved in reconciliation in these species [12].

Many specific cognitive considerations have been suggested to underlie these affiliative patterns. For instance, as regards reciprocity and exchange, the individuals are supposed to keep records of the acts of grooming and tune them to frequencies of receipt of being groomed or another act, such as support [13], and to use their knowledge of the ranks of others to obtain more effective support [6],[14]. Besides, individuals have been supposed to be attracted to others of higher rank [6] and to others of similar rank [7]. The supposed cognition underlying reconciliation consists of the ability to remember the former opponent and of selective attraction to the former opponent or a conciliatory disposition [15],[16]. As to their inclination particularly to reconcile fights with opponents that are of greater value to them, the so-called ‘valuable-relationship hypothesis’ [8], [17]–[23], three key components are supposed to influence the quality of a relationship, namely its security, its value, and the compatibility of both partners [8],[24]. According to Silk [25] this implies that assessing the value of a relationship over the long-term requires cognitive sophistication, because it asks for a precise memory of what happened in the past and for a correct evaluation of the relationship in the long run.

These theories of affiliation pose several problems. First, evidence for each of these theories is not conclusive [5], [26]–[30]. Second, from a scientific perspective, these numerous different theories for specific patterns of affiliation (such as exchange and reconciliation) must be integrated in some way. Third, the use of grooming as a ‘currency of exchange’ is dangerously anthropomorphic according to us and others [25],[31],[32]. As a more parsimonious alternative, we suggest to follow a more distributed approach based on local interactions and rules of thumb [31], [33]–[36]. Fourth, even though primates are obviously intelligent [37],[38] it seems much to ask of primates to combine intentionally all these rational considerations in the distribution of their affiliative behaviour (e.g. to consider what incidence of grooming was used in exchange for something, and what for reconciliation or maintenance and development of social bonds). Fifth, often simple rules suffice to cause many of the observed patterns and herewith an integrative theory [39],[40]. Therefore fewer cognitive processes may suffice as shown for instance in a model for dominance style [32],[41]. A similar integrative approach based on fewer cognitive processes is also suggested by the co-variation hypothesis (or theory of social epigenesis). In this theory part of the behavioural acts is supposed to be forced by constraints due to the specifics of the social structure [12].

For these reasons, we use in the present paper a computer model to develop an integrative approach to patterns of social affiliation in primates. We first precede this by a survey of the precise patterns of dominance style and affiliation found in the literature. In the model, we assume very little cognitive deliberations by the individuals to groom others: Individuals merely groom others out of fear of being defeated and to reduce their own anxiety. Individuals do not intend to reconcile fights nor to exchange or reciprocate grooming. Our model is an extension of our earlier model of grouping and competition, called DomWorld [42],[43]. We choose DomWorld, because it has reproduced many of the patterns of aggression, dominance and spatial structure that have been observed in despotic and egalitarian societies of primates, in particular of macaques. These have arisen merely as a side effect of local rules for grouping and competition through the feedback between hierarchical development and spatial-social structure with dominants in the centre and subordinates at the periphery [35], [41], [44]–[46]. Note that the hierarchy develops via self-reinforcing effects of victory and defeat, which have been described for many species including primates [45], [47]–[50]. Through these self-reinforcing effects, occasional victories of low ranking individuals may lead to rank reversals. This is important, because dominance hierarchies in empirical data are not entirely stable [51]–[55].

Interactions in our new model, called GrooFiWorld (a contraction of groom and fight), are extended with the option to groom. When individuals meet each other at close proximity, they will consider whether to groom, to fight or to rest. As to the order of what to do first, we are led by four observations: first, those on baboons by Kummer [56] who inform us that upon their first encounter individuals first fight and later groom; second, by the empirical finding that an individual builds up anxiety (as indicated by the increased heart rate) when approaching an opponent by whom it may be defeated [rhesus monkeys, 57]; third, that anxiety increases after a fight as is indicated by the increase in frequency of scratching and heart rate in both opponents [25], [58]–[64]; fourth, that anxiety may subsequently be reduced (in many species) by the receipt of affiliative behaviour as indicated by the reduced heart rate and the rate of self-directed behaviour [57],[61],[62],[64] and to a lesser degree by active grooming [65]. Furthermore, our model is informed by empirical studies on grooming and opiate administration which indicate that not being groomed for some length of time reduces the concentration of endorphins and increases the motivation to be groomed, and that grooming increases the level of endorphins in the brain and reduces the motivation to groom [66]–[71].

In sum upon encountering someone else, an individual in our model first deliberates whether or not to attack. This decision depends on the risks involved (whereby risk concerns the chance of losing a fight), as is the case among primates [72], and as in our earlier model: a fight is only initiated when the individual expects to win [41],[73]. If defeat is expected, its fear of losing makes the individual consider grooming the other. Its decision whether or not to groom depends on its degree of anxiety: an individual that is more anxious is more inclined to groom (instead of resting close by). After being groomed by another and (a little less) after actively grooming another, its anxiety and therefore its tendency to groom diminishes. Its anxiety also increases after a fight and after a period of not having been involved in grooming. Note that we do not distinguish between anxiety, tension or stress.

In order to compare the patterns of affiliative behaviour in our model with those in real primates, we used the same statistical measures as applied in empirical data and we confined ourselves to macaques for two reasons. First, because their social behaviour has been studied extensively and shown to differ in interesting ways between the typical egalitarian and despotic societies [74],[75]. Second, because in our earlier model, DomWorld patterns of dominance and aggressive interaction were remarkably similar to those of macaques [41],[45]. Since GrooFiWorld is an extension of this model, we assume it to also be suitable for comparing to macaques.

The paper is organized as follows. First, we summarise the literature on the common patterns of affiliative behaviour in females of egalitarian and despotic species of macaques (Table 1). Second, we tune the percentage of grooming time and the unexpectedly emerging percentage of reconciliation to empirical data for despotic societies. Third, by varying the intensity of aggression we show the emergence of all these common patterns of affiliation and their differences between typical egalitarian and despotic macaque species in GrooFiWorld. Fourth, in order to understand how these patterns emerge, we remove different assumptions in turn, such as the self-reinforcing effects of victory and defeat and effects of spatial proximity. Fifth, the explanation of the causation of these patterns in the model leads to new hypotheses about the interconnection between other traits which we confirm in the model. Part of these predicted patterns appear also to be found in empirical data described by scientists in other contexts. Other patterns still need to be tested empirically. Since for all patterns empirical data are insufficient, we list them together in Table 2 so that the relevance of our model to empirical data may be tested in the future.