Fair play and its connection with social tolerance, reciprocity and the ethology of peace
Elisabetta Palagi 1,2 , Giada Cordoni 1, Elisa Demuru 1 & Marc Bekoff 3
1 Natural History Museum, University of Pisa, Pisa, Italy
2 Unit of Cognitive Primatology and Primate Center, Institute of Cognitive Sciences and Technologies, CNR, Roma, Italy
3 Ecology and Evolutionary Biology, University of Colorado, Boulder
Corresponding author: elisabetta.palagi@unipi.it
SHORT TITLE: Play and the ethology of peace
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Summary
The concept of peace, with its corollary of behaviours, strategies and social implications, is commonly believed as a uniquely human feature. Through a comparative approach, we show how social play in animals may have paved the way for the emergence of peace. By playing fairly, human and nonhuman animals learn to manage their social dynamics in a more relaxed and tolerant way that results in a more effective management of conflicts. We show that play promotes tolerance, cooperation, fairness and reciprocity, which are essential elements of the so-called positive peace. This kind of peace is reached through an evolving process in which individuals continually modify social relationships to attain peaceful coexistence. In conclusion, we assume that the concept of peace has deep biological roots that constitute the basis for more sophisticated cultural constructions.
Keywords: Egalitarian species - Despotic species - Pre-school Children - Positive peace - Playful expression 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
The behavioural, emotional and mental continuity among different animal species, proposed by Charles Darwin (1859), must be taken into account when investigating the extremely complex, and apparently unique, pinnacles of the human brain. More recently, some authors have argued that evolutionary continuity is universally acknowledged in anatomy, genetics, and neuroscience, but not in cognition and emotions where, nonetheless, continuity also is clearly present (Bekoff & Pierce, 2009; de Waal & Ferrari, 2010; Pierce & Bekoff, 2012). Indeed, comparative cognition is based on a top-down approach that focuses more on the expression of complex skills than on their basic building blocks. The human rational concept of peace, with its corollary of behaviours, strategies and socio-cultural implications, is undoubtedly one of the pinnacles of human cognition. Can therefore, the concept of peace be considered an exclusive feature of our own species without any connection to other species? Here, we argue against human exceptionalism in the arena of peace, and show that individuals of other species also have evolved behavioural strategies to live in harmony with another.
The concept of peace is not straightforward and many efforts have been made to unify the numerous definitions coming from the different disciplines dealing with this topic. Peace is generally defined as the “absence of conflict”, even if the issue appears to be much more complicated than this reductive analysis. Indeed, in some cases an almost total absence of conflict does not necessarily translate into a peaceful and tolerant society as well as the presence of conflict does not necessarily translate into a violent and despotic society. A key contribution in expanding the concept of peace stems from the pioneering ideas of one of the founders of peace research, Johan Galtung. In his editorial essay for the founding edition of the Journal of Peace Research (1964), Galtung philosophically distinguishes between negative and positive peace in humans, two opposite routes that both lead to the absence of conflict. Negative peace implies “the absence of violence” between individuals, groups and communities and it can be reached through an oppressive regime that suppresses individual freedom and social harmony (Verbeek, 2008). On the contrary, positive peace is more than the absence of violence and is linked to the concepts of tolerance, fairness and social 3 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 8
justice (Verbeek, 2008) which are essential components for play to occur. Tolerance, fairness and social justice can work at all levels and at different scales involving individuals, groups, communities and populations. Hence, the separation of intra- and inter-group peace appears unnatural.
Can we expand this philosophical concept to non-human animals by scientifically exploring those behaviours that could represent the biological bases of human negative and positive peace both at intra- and intergroup level? In other words, what are the evolutionary foundations (or the building blocks, sensu Flack & de Waal, 2000) that paved the way for the emergence of negative and positive peace in humans? The demonstration that chimpanzees are capable of elaborate strategies to restore a relationship damaged by aggression gave rise to a great body of research focusing on post-conflict mechanisms in non-human animals (de Waal & van Roosmalen, 1979). Further studies on post-conflict dynamics showed that many animals are not only able to reconcile (Schino, 1998; Aureli et al., 2002; Cordoni & Palagi, 2008; Cozzi et al., 2010; Baan et al., 2014; Cordoni & Norscia, 2014), but also that they can console the victims (Palagi et al., 2004, 2006, 2014; Cordoni et al., 2006; Fraser et al., 2008; Romero et al., 2009; Clay & de Waal, 2013; Palagi & Norscia, 2013) or appease the aggressors (Judge, 1991; Das, 2000; Palagi et al., 2008; Romero et al., 2011). Therefore, the importance of restoring harmony in a social group not only involves the two opponents, but it also expands to other members of the group itself. Beside the “post-conflict machinery”, many other (and, maybe, less evident) behaviours are constantly employed to reach and maintain social cohesion between group members (Aureli & de Waal, 2000). Play, in its social form, represents one of these hidden tactics used by animals to gain social harmony. In humans, free play helps facilitate more ritualized means of dealing with disputes and helps subjects acquire the tools to do so. Free play differs from structured games because the latter are based on the observance of pre-existing and externally imposed rules. In this sense, structured games represent a form of ritualized play, which is a powerful tool to enhance peace in humans (Lorenz, 1966). However, from an evolutionary perspective, only free social play is tightly linked to fairness, 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82
tolerance, reciprocity, and peaceful relationships among group-living animals and humans (Bekoff & Pierce, 2009; Pierce & Bekoff, 2012).
Play is an activity that has no obvious immediate benefits for the performer and that includes many motor patterns typical of other serious functional contexts, such as agonistic, antipredatory, and mating behaviour (Martin & Caro, 1985; Pellis & Pellis, 1996; Bekoff, 2001, 2014). Playful patterns do not structurally differ from those included in the serious contexts except for the way in which they are performed (Bekoff & Allen, 1998). Fragmentation, disorder, absence of inhibition and incompleteness mainly characterize sequences of playful patterns (Bekoff & Byers, 1981; Fagen, 1981; Burghardt, 2005; Bekoff, 2014). Separate from its formal definition, play represents a challenge for a vast array of scholars. Ethologists and psychologists are interested in discovering its consequences on behavioural ontogenetic trajectories, anthropologists focus their attention on defining its role in the evolution of social and cognitive abilities, and evolutionary biologists aim at understanding the functions of an apparently "useless" behaviour (which it is not; Burghardt, 2005; Bekoff, 2014). Although many questions remain unsolved, many efforts have been recently made to expand knowledge regarding some potential functions of play, both in human and non-human animals. The difficulty of identifying precise roles/functions of free playful activities is rooted in its multifaceted nature. Indeed, the different benefits of play can vary and overlap within a single play bout depending on the individuals who are playing (e.g. sex, age and rank), context (e.g. tense situations, mating period, mother-offspring interactions) and species (e.g. social versus less social and tolerant versus despotic).
This emergent scenario informs us about the importance of a comparative approach in the study of animal play in order to assess the relative meaning of its different functions (Bekoff & Byers, 1981; Fagen, 1981; Pellis & Pellis, 2009; Bekoff, 2014). Different play components (e.g. more or less unbalanced or complex) employed in different periods of life can be predictive of different functions of play (Pellegrini 1988, 1995; Cordoni & Palagi, 2011). Yet, a constant and widespread feature of animal play is the endless search of symmetry or equality between playmates. The 5 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 14
intrinsic nature of social play requires an implicit agreement and cooperation and negotiation between players, who have to trust one another. They do this by fine-tuning their own movements and by avoiding the performance of behaviours that might be interpreted as cheating. In this view, the term fair play assumes a biological connotation and it cannot be considered as the pure outcome of some cultural artefacts. By playing fairly, animals (including humans) may acquire the social skills and social rules that are at the basis of a peaceful coexistence (Bekoff, 2014).
Comparative research on social play in diverse mammalian species, including humans, shows that playing fairly is the rule, rather than the exception, and that a number of different strategies have evolved to make sure fairness and cooperation prevail (Bekoff & Pierce 2009; Bekoff, 2014). By a comparative approach, and adopting a bottom-up perspective (Darwin, 1859; de Waal & Ferrari, 2010), we aim to add empirical evidence to Carr's pioneering idea (1902) that social play, thanks to its multiple effects, may represent a building block of peaceful homeostasis by catalyzing the biological and cultural prevention and resolution of social conflicts.
Play, tolerance and reciprocity
The terms "despotic" and "egalitarian" derive from sociological studies and represent the two poles of the gradient describing the way in which power (i.e. the capacity to control a resource) is distributed within a social community (Flack & de Waal, 2004). The power distribution within a social group determines the rigidity of its hierarchy and, consequently, shapes the social relationships among its members (Norscia & Palagi, 2015). In despotic species, when the resources are monopolizable, the power is concentrated in the hands of few individuals; whereas, in egalitarian species the power is shared among many individuals so the power asymmetry is much weaker, especially when resources are abundant and scattered. As a consequence, in despotic species, social relationships are predominantly managed through dominant and submissive displays and this causes a reduction in the communicative complexity (Maestripieri, 2005). On the other 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133
hand, egalitarian species characterized by more relaxed, fluid and negotiable hierarchical relationships, must have evolved a more complex communicative system to manage constantly changing social dynamics (Maestripieri, 2005). Social play is one of the most complex forms of communication due to its improvisation, strategic timing, and creative nature, and thus is a cognitively and socially demanding activity (Palagi et al., 2015). The connection between social play and tolerance has been highlighted by comparative research demonstrating that egalitarian species show higher levels of social play than do despotic ones (Palagi, 2006; Antonacci et al., 2010; Ciani et al., 2012; Palagi & Cordoni, 2012; Hare et al., 2012).
As for the modality of play, the tactics used during play fighting can be predictive of the long-term function of play itself. In egalitarian cooperative species, defensive and offensive playful patterns are generally slowly and strategically performed so that the playmates have the chance to counterattack and reverse roles thus making the session more balanced and symmetric (50:50 rule; Altmann, 1962; Aldis, 1975). In these species, play represents a tool to promote a greater sense of trust, build social relations and manage them in successful ways. All these capacities are extremely adaptive because it is essential to integrate individuals within a cooperative community where cheaters are kept away from the group without benefiting from the advantages typical of social living and facing higher survival costs (Dugatkin & Bekoff, 2003).
In contrast, in despotic competitive species, offensive and defensive patterns are very rapidly performed thus making the counterattack difficult (Pellis & Pellis, 2009). Moreover, offensive and defensive patterns occur simultaneously because a player must attack the playmate and protect itself at the same time. In some canid species, including the domestic dog, players often violate the 50:50 rule as their relative hierarchical distance increases so that rank rules dictate play rules (Bekoff, 2014; Smuts, 2014). What differentiates competitive play from real aggression is the absence of attempts to take advantage of the situation once an attack has been launched. The higher level of play asymmetry typical of despotic species leads to a predominance of one of the players insomuch as it is possible to clearly recognize a "winner". As a whole, the level of symmetry/asymmetry 7 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 20
strongly influences the outcome of the playful interaction with asymmetric sessions showing a higher probability of escalating into a “real aggression”. This symmetry/asymmetry model has been theorized by comparing two rodent species that show opposite trends in their expressions of play (Rattus norvegicus and Octodon degus, Pellis et al., 2010), but comparative research has shown that it is applicable to several mammal species such as bonobos and chimpanzees (Palagi & Cordoni, 2012), Tonkean and Japanese macaques (Reinhart et al., 2010; Ciani et al., 2012), and wolves and coyotes (Bekoff & Pierce, 2009). Young coyotes, for example, are much more aggressive than young wolves and show higher levels of rank-related fights early in life (Bekoff, 1974). In coyotes, play is much more competitive and, from an ontogenetic point of view, emerges only after dominance relationships have been established at a dyadic level. To avoid the risk of being misinterpreted, coyotes need to communicate their playful intentions in a clearer way both for initiating a play bout and maintaining a playful mood once play has started (Bekoff, 1974; 1975; 1977; 1995). For example, when a coyote stands over a playmate (standing-over is usually used by dominants to establish or to reaffirm their rank) the behaviour pattern is followed by a high proportion of play bows (a specific playful signal used to punctuate the session). This signal redundancy is less necessary in wolves who play less competitively and in which the standing-over used in the playful context seems not to serve to assert dominance (Bekoff, 1975) as it occurs also in other carnivore species (e.g. bears, Burghardt & Burghardt, 1972). Similarly, in dogs, it has been recently demonstrated that the roll-over posture (an animal rolls over onto its back by exposing its ventrum) during play is not predictive of the real dominance relationship characterizing the dyad (Norman et al., 2015). One of the potential factors explaining why wolves do not use as sophisticated a set of signals (e.g. the play bow) to maintain a playful mood could be due to the fact that play activity is less frequent between individuals who greatly differ in social rank (Cordoni, 2009).
The degree of tolerance among individuals of a given species can affect many other features of play such as the involvement of adult subjects (adult/immature and adult/adult sessions) (Propithecus 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185
verreauxi, Antonacci et al., 2010; Callithrix jacchus, Norscia & Palagi, 2011; Pan paniscus, Palagi et al., 2006; Loxodonta africana, Lee & Moss, 2014), the number of players contributing to the same session (> 2 players) (Pan paniscus, Demuru et al., 2015), the rate of success of playful interactions (low level of aggressive escalation or the duration of the interactions) (Rattus norvegicus, Pellis & Pellis, 2009; Pan paniscus and Pan troglodytes, Palagi & Cordoni, 2012) and the amplitude of the play clique size characterizing each player (technically defined as the number of individuals with whom a certain individual plays divided by the number of subjects who played at least once during the observation period) (Macaca tonkeana, Scopa & Palagi, submitted). Even though the subject is profoundly neglected it seems that in tolerant and cooperative species, play is a pervasive activity that can be used to recruit all members of the group independently from sex, age and rank (Palagi, 2006; Hare et al., 2007; Pellis & Pellis, 2009; Antonacci et al., 2010; Ciani et al., 2012).
In despotic species immature playful behaviour has a competitive nature and can be viewed as sort of a substitute for real aggression. In these species play serves to test the agonistic capacities of juvenile playmates and to develop and establish hierarchical relationships among them (Pan troglodytes, Paquette, 1994; Palagi & Cordoni, 2012; Lemur catta, Pereira, 1993; Crocuta crocuta, Drea et al., 1996). This strict linkage between playful and agonistic behaviour explains the severe decline of the playful motivation in despotic species during adulthood when play becomes a potentially dangerous activity (Ciani et al., 2012).
The reasons why we observe a steep decrease in play from juvenility to adulthood in despotic species must be searched in the early phases of life. The social inhibition characterizing despotic species also reflects on maternal style and particularly on the degree of permissiveness of mothers that is a fundamental factor influencing the distribution of play in infants (Thierry, 1985; Maestripieri, 2004). Indeed, mothers of Macaca fuscata, a despotic macaque species, are extremely protective towards their infants thus limiting their social contacts with other members of the group. This inhibitory control limits the infant's relational network and creates the conditions for a strong 9 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 26
canalization that has implications for social and emotional competence later in life (Berman, 1982). The process of social canalization is not present in more tolerant species such as Tonkean macaques in which mothers allow their infants more freedom to interact with other group members. As a result, despite their phylogenetic closeness, the pattern of play distribution differs deeply between these two macaque species with the tolerant Tonkean macaque showing a larger play network involving also adult subjects (Ciani et al., 2012). Social inhibition also acts in shaping the playful attitude in the Pan genus with bonobos (Pan paniscus) showing a lower degree of social inhibitory control and an apparent developmental delay in acquiring such control compared to chimpanzees (Hare et al., 2007; Wobber et al., 2010). This delay permits bonobos to expand the age window in which play can be expressed. The ontogenetic pathways of play in immature bonobos seem to show similarities with those of children in more egalitarian human cultures. Indeed, even though human cross-cultural ethological data are limited, there is evidence that playful attitudes are linked to the tolerance levels of the different socio-economic structures and the rigidity in social relationships of a given population (Fouts et al., 2013). For example, playful activity strongly differs between the children of Bofi farmers (an African population subsisting on slash-and-burn horticulture and trading) and those of Bofi foragers (a semi-nomadic African population subsisting on net hunting and gathering). The Bofi farmers typically show more bridled social roles and base their power on status, age and gender. In these groups, gender segregation is evident also during children playful sessions where boys tend to play with same-sex subjects. On the contrary, Bofi foragers support personal autonomy and respect value sharing by daily engaging in cooperative behaviours within and between households. By rough joking and storytelling children and adults of Bofi foragers encourage and support the sharing of every item (toys or objects) and the cooperation between group members (Fouts et al. 2013). In these communities, the play of children does not show any gender segregation thus reflecting the egalitarian social relationships typical of adults.
A further example of how the play of children reflects adult social style comes from two Zapotec communities in Oaxaca (Mexico) (Fry, 1992). The author found inter-community differences in 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237
children’s serious and non-serious aggression (play fighting) that corresponded to differences in adult behaviour. Children of the La Paz community, which is characterized by high tolerance and peacefulness, showed lower levels of play fighting and serious aggression than children of the San Andrés community, which is characterized by higher levels of verbal and physical aggression. The analogy between adult and children's behaviour within each community indicates that the social environment shapes and maintains across generations divergent social dynamics related to violence or peacefulness.
Proximate mechanisms for play
The role of play in promoting social affiliation and cooperation is related to complex neuro-endocrine mechanisms (Burghardt, 2001; Pellis & Iwaniuk, 2002; Lewis & Barton, 2006; Naber et al., 2010; Mancini et al., 2013a,b; Pellis et al., 2014; Romero et al., 2014; Martin et al., 2015). The use of play by animals to navigate social possibilities also implies that to a certain degree these activities are self-rewarding (Panksepp, 2005). The proximate causes increasing the propensity to play are linked to the instinctual action apparatus of the mammalian brain (Burghardt, 2001; Panksepp, 2005). Social, but not solitary play, leads to an increased development of particular brain areas such as amygdala (the subcortical area implied in recognizing and generating simple and more complex emotions, Adolphs, 1999; Bartels & Zeki, 2000) and hypothalamus (implied in facilitating the experience and understanding of pleasure - von Borell, 2000 - such as that deriving from playful activity - Lewis & Barton, 2006). Some authors have argued that social play improves social competence, behavioural flexibility and the ability to appropriately respond to facial expressions (Lewis & Barton, 2006; Montgomery, 2014). Moreover, experimental studies using rats have demonstrated a linkage between play, well-being and affiliation (Panksepp, 2005). Panksepp and Panksepp (2013, p. 492) highlighted the importance of rough-and-tumble play in "providing fundamental learning experience for higher forms of empathy." Play behaviour in rats has great importance in the maturation of the medial prefrontal cortex, a brain area that receives input from 11 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 32
all other cortical regions and functions to plan and direct motor, cognitive, affective, and social behaviour across time (Bell et al., 2010). Panksepp and Panksepp (2013) indicated that the medial prefrontal cortex is one of the most important "empathic" parts of the brain, which is also sensitive to social play during ontogeny.
Perceiving and sharing others’ emotions, the so called "affective empathy", is a phylogenetically old capacity and is linked to a mechanism in which mirror neurons seem to be implicated (de Waal, 2008; Iacoboni, 2009). While observing a facial expression (the observable outcome of emotions, Darwin, 1872), the observer involuntary re-enacts the same motor pattern by recruiting neural mechanisms that concurrently activate the same affective state associated with that specific facial expression (Perception-Action Model, Preston & de Waal, 2002; Gallese, 2003; Gallese et al., 2004). This mechanism is essential for successful social interactions (Preston & de Waal 2002), including play (Mancini et al., 2013b). Motor mimicry, not requiring any conscious awareness, generates an emotional connection that can synchronize the affective state of subjects (Hatfield et al., 2009). The connection, in turn, fosters mimicry and sustains a positive feedback which is at the basis of social bonding. In short, the more mimicry, the greater the affiliation. This emotional circuitry has been found not only in humans (Singer, 2006; Pfeifer et al., 2008) but also in dogs (Palagi et al., in press), monkeys (Ferrari et al., 2009; Palagi et al., 2009; Paukner et al., 2009; Mancini et al., 2013a,b) and apes (Anderson et al., 2004; Davila-Ross et al., 2008; Campbell & de Waal, 2011, 2014). During playful interactions, animals frequently express their positive emotions through the use of a specific facial expression named play face or open mouth display, the homologue of the visual component of human laughter (van Hooff, 1972). As with the other emotionally driven facial expressions, play faces can be mirrored by the observer in a very rapid and automatic way (Provine, 1992; Davila-Ross et al., 2008; Dimberg & Thumberg, 2012; Mancini et al., 2013a). There is growing evidence across primates that a high level of affiliation and emotional proximity strongly increase the frequency of facial mirroring, with more strongly bonded subjects showing greater sensitivity, and increased likelihood of responding, to one another's facial 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289
expressions. Due to the abundance of positive facial expressions during play, this context is particularly fruitful in providing opportunities to investigate the phenomenon of rapid facial mimicry (RFM). In geladas, for example, the mimicry of play face differs both quantitatively and qualitatively according to the emotional closeness between playmates: mother-infant dyads show the highest level and the fastest facial responses compared to mother-nonkin infant dyads (Mancini et al., 2013a). The temporal coordination of face-to-face interaction during play occurring between mothers and infants has been extensively documented in humans (Feldman, 2007) and it serves to build an attachment of the infant with the caregiver (Feldman, 2010). The findings on gelada indicate that also in non-human primates, RFM reflects one of the core elements of the mother-infant relationship thus representing an important indicator of the quality of interpersonal bonding. Moreover, the effectiveness of facial expressions can be amplified when it is matched by the observer. Facial mimicry appears to convey more important information to the playmate than is the case with the mere perception of the signal, because the observer's mimicry indicates that the intrinsic message of the play face has been correctly interpreted. Facial mimicry, therefore, can facilitate communicative exchanges and behavioural coordination. Accordingly, the mimicry of play faces significantly prolongs the duration of playful interaction in geladas (Mancini et al., 2013b).
It has been recently demonstrated that stress is an important factor limiting the expression of empathic sensitivity (e.g. emotional contagion) in mice and humans and that the stressor preventing the emergence of emotional contagion is related to the forced social interaction between strangers in a novel environment (Martin et al., 2015). The pharmacological blockage of the synthesis of glucocorticoids (stress hormones which reduce the empathic response in both mice and humans) significantly improves the empathic response in the two species (Martin et al., 2015). In a novel environment, a similar effect can be obtained for humans by playing a cooperative game that enhances the expression of emotional contagion in dyads in which the partners were strangers.
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Therefore, social play in all its forms, is a fertile ground for the evolution of emotional sharing and cooperation, two phenomena strongly linked to empathic proclivity (Palagi et al., 2015).
Play and the ethology of peace
Each new encounter, or social situation, is a new challenge with which animals have to cope. Play can be considered as a useful tool kit to ‘break the ice’ and make animals, from invertebrates (Pruitt et al., 2012) to vertebrates (Antonacci et al. 2010), capable of responding to such challenges. Prosimians represent a useful taxon with which to explore this phenomenon because, being arboreal and primarily nocturnal, there are barriers to their use of visual and acoustic signals (Fleagle, 2013). For these reasons, the onset of new social relationships is based on tactile forms of communication. Play fighting is one of them. The function of play in prosimians can vary according to the ecological habits and social structure of each species, however, some generalizations can be made (Pellis & Pellis, 2010).
Many nocturnal species belonging to the family Lorisidae are solitary with individuals actively defend their own territories (Charles-Dominique, 1977; Charles-Dominique et al., 1980). Males meet females only during the annual mating season lasting only a few days, therefore animals are relatively unfamiliar with one another. During the mating season, males visit females' territories and engage in complex courtship sequences involving grappling which is a form of play fighting typical of the arboreal prosimian species (Pellis & Pellis, 2009; 2010). Such playful interactions favour the familiarization process which increases the chance for mating. A similar situation can be also found in the family Cheirogaleidae. Males and females of Coquerel's mouse lemur share part of their territories where they regularly meet to exchange social interactions, including arboreal play fighting (Pagés, 1978). These playful exchanges are maintained all year round but peak just before copulations. Some species belonging to the family of Galagidae live in family groups including one adult male, several adult females and immature individuals (Charles-Dominique et al., 1980). 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339
During the day, the individuals of different families share communal sleeping sites where they engage in inter-family grooming and play fighting interactions (Doyle, 1974). These positive interactions do not necessarily anticipate sexual behaviour but they confer in increasing tolerance between the different families. Once more, this is an example of the role of play fighting in overcoming the reticence to interact with unfamiliar subjects.
In the Indridae family, and specifically in Propithecus verreauxi, a diurnal species characterised by relaxed relationships and mild levels of aggressiveness (Norscia et al., 2009; Norscia & Palagi, 2015), play is a passport to enter new groups. In this species, females are the choosey sex and subjects of both sexes can mate with multiple partners in their own and neighbouring groups (Norscia et al., 2009). During the pre-mating period, males began to visit other groups to search for receptive females, who experience a very short oestrus period per year (up to 72 h). After the first chases to keep intruders away, resident adult males began to engage in play sessions with them, and aggressive behaviour decreases after the first playful interactions. The baseline levels of play among resident males were much lower than those between between in-group and out-group males, thus indicating that play is a strategic tool kit to overcome their reticence to contact unfamiliar subjects and so to reduce xenophobia (Antonacci et al., 2010).
The ice breaking effect of play is also present in monkeys and apes. The social system of geladas (Theropithecus gelada) is organized in a multilevel structure (Dunbar & Dunbar, 1975). The two most basic components of this social system are the individual reproductive units (one-male unit) and a cluster of units (bands or herds) that share a common home range (Dunbar & Dunbar, 1975). A one-male unit consists of a single adult male, his reproductive females and their offspring.
The most frequent interactions between members of different units consist of playful contacts between immature subjects, who form temporary playful aggregations named ‘play units’ (Dunbar & Dunbar, 1975). Juvenile geladas do not show any preferences in choosing playmates as a function of the membership of the one-male unit (Mancini & Palagi, 2009). By playing with peers, independently of their unit membership, immature geladas may create and reinforce social 15 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 44
relationships with those individuals who could interact with them in the future. In this case, inter-unit play can be interpreted as a social launching pad for youngsters (Mancini & Palagi, 2009). An even more striking example comes from wild bonobos. It has been recently observed that adult males of different communities can meet each other and play together, thus demonstrating social tolerance through play (Behncke, 2015). This finding strongly contrasts with what has been discovered in wild chimpanzees belonging to different communities that are extremely aggressive towards each other (Wrangham et al., 2006). Behncke (2015) suggests that play can be viewed as "a biological wild card" whose value and form can shift according to the different environmental and social contexts in which it occurs. This “biological wild card” is profoundly rooted in the primate domain, as it is evident from what we know about prosimians. Probably, along with the encephalization process, this “wild card” becomes more and more sophisticated and tactically efficient in overcoming the difficulty linked to the phases of socialization between unfamiliar individuals. In this sense, play behaviour can represent the face of the kaleidoscope that we call the ethology of peace.
Conclusions
The complex picture emerging from studies of social play in humans and other animals strongly indicates that this activity promotes the maintenance of social harmony and cohesion. The pervasiveness and incidence of play within a society are predictive of the tolerant nature of that society. This pattern of distribution of play begins at early stages of development (e.g. social canalization), has strong repercussions later in life and mirrors the social organization typical of a given species (for example, wolves are usually more social than coyotes, but lone wolves and packs of coyotes can occur depending on social and also nonsocial factors such as the distribution of food in their home range). Although it is impossible to establish a precise cause-effect relationship, it is evident that play and tolerance are tightly linked by positive feedback in which the expression of 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390
one supports the expression of the other. The positive feedback includes all those social skills that are learned and fine-tuned through play, such as cooperation, fairness, reciprocity, trust and empathy. Moreover, during play animals (including humans) can learn how to better manage their competitive and aggressive drives. These skills are fundamental as competition and aggression are intrinsic components of social life, and tolerant species are no exception. Indeed, as is illustrated by comparative studies of macaque societies (Thierry et al., 2004), tolerant societies are not distinguished by the absence of aggression, but rather, by the capacity to manage, solve and overcome it to restore a peaceful equilibrium within the group. It is therefore feasible to theoretically apply the concepts of positive and negative peace to both human and non-human animal societies and, in the end, to consider these categories as evolutionary adaptations aimed at avoiding conflicts. A clear social hierarchy and a nepotistic social system help to keep individuals in their particular social rank and there is no room for social negotiation. These fixed social roles lead to a reduction in overt aggression, thus creating the condition for forced peace with very few, but severe conflicts. On the other hand, in tolerant societies, social relationships have to be continuously renegotiated to prevent and reconcile possible conflicts, that can be recurrent but milder and frequently bidirectional. Indeed, we should consider as positive peace that employed by tolerant societies and negative peace that characterizing despotic species. Play, with all its correlates, enables individuals to manage their social dynamics in a more relaxed way that is reflected in a more effective management of conflicts. Therefore, play represents a building block for positive peace - a foundation of peace and fairness (Bekoff, 2014) - that achieves its goals through an evolving process in which individuals continually modify social relationships to reach peaceful coexistence. Following this line of reasoning, play can be a costly behaviour as well as functioning to maintain positive peace. Because this kind of investment is widespread across tolerant species, we assume that positive peace has deep biological roots that constitute the basis for more sophisticated cultural constructions.
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Acknowledgments
We wish to thank all the participants to the workshop "Obstacles and catalysts of peaceful behavior" held in Leiden (Lorentz Center) organized by Peter Verbeek and Douglas Fry.
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