Feelings of Excitement and Brotherhood
A coalition is defined as an aggressive attack of two or more individuals against one or more targets. Most frequently coalitions involve two individuals aggressing against a common target. Coalitions are prevalent throughout the animal kingdom and their frequency between males is surprising as males are most frequently the dispersing sex and thus there should be no kin-related benefits of cooperation. The formation of a coalition can be opportunistic whereby one individual joins one of two individuals during a dyadic aggression. Coalitions can be very basic and involve no communication between the coalition partners and an individual purely intervenes in an on-going aggression. Alternatively, coalitions can be coordinated attacks with individuals aggressing against a common target with similar forms of aggression and movement. These aggressions can involve recruitment of specific individuals from a population or social group. Individuals can use specific behaviors and vocalizations to recruit their partner and the aggression often appears to occur spontaneously between the coalition partners.
In the animal kingdom coalitions have been observed among many taxa from birds to cetaceans to social carnivores and ancestral human species. Coalitions are most frequently observed in nonhuman primate species and occur under a variety of circumstances and constellations described below (reviewed in; Smith et al. 2010).
Many different functions can be provided by coalitions including defense of or access to resources, rise in rank position for one or both partners, defense against aggression from a more dominant individual or defense against group take-overs from rivals (reviewed in Harcourt and de Waal 1992; Olson and Blumstein 2009; Smith et al. 2010). Nepotism and kin-related benefits of cooperating and supporting relatives are considered to be the main drivers behind coalitions between group-living females (Isbell 1991; Sterck et al. 1997; van Schaik 1989; Wrangham 1980). In the majority of primate species, males are the dispersing sex, and therefore move from their natal group at maturity. They are without kin or may not recognize kin in their new group but coalition formation is still often seen. Coalitions can either be between individuals within the same group (within-group coalitions) or individuals of one social group join together to fight against another group of individuals of another social group (between-group coalitions).
Gregarious male primates compete for access to mates within their social groups, and in many primate species males can use within-group coalitions to gain access to females. This form of cooperation is intriguing as under such circumstances males are competing for access to a non-divisible resource (access to females) and so cooperation should be unlikely. Recently a mathematical model has been developed to explain the evolution of different male-male coalition types within groups (Pandit and van Schaik 2003; van Schaik et al. 2004, 2006; also see Young et al. 2014b; for a review of alternative coalitionary models see Bissonnette et al. 2015). These models predict when certain coalition constellations and types are likely to occur within social groups. The model assesses the profitability (the coalition must provide fitness benefits for the partners) and feasibility (the coalition partners must be able to defeat the target) of certain coalitions occurring at different levels of contest competition within groups. Contest competition is based on the level with which the alpha male can monopolize mating access to females. Coalitions can be classified into three constellations based on the ranks of the allies (coalition partners) and target in the coalition: (1) all-up, where both allies are lower ranking than the target, (2) all-down, where both allies are higher ranking than the target, and (3) bridging, where the target is ranked between the allies. These coalition constellations were further categorized into types by their function, being either “rank-changing” when they affect participants’ rank position, or “levelling,” when they reduce the inequality in the distribution of matings/paternities across ranks. Rank-changing coalitions are also known as “political” or “revolutionary” coalitions. One of the main assumptions of the model is that payoffs for the coalition partners are mediated by dominance rank rather than male fighting abilities.
Contest potential within groups’ ranges from very high (where the alpha male has complete monopoly of mating access) to very low (where all males can mate with females and scramble competition occurs). In species with very high contest potential, coalitions are not expected between males as mating access is highly skewed to the alpha male. There are often few subordinate males and the high contest leads to large power differentials between males, hence, coalitions become unfeasible. Conversely, if contest potential is extremely low, then scramble competition is likely to develop and all males in the group can gain access to females and thus cooperation provides costs but no benefits to the allies. The mathematical models, therefore, predict that male-male coalition formation should occur from high to medium levels of contest potential but not at the extremes. The models further predict the coalition constellations which will occur at differing contest potential, these are outlined below.
Levelling coalitions become feasible as contest competition begins to decrease to a mid-high level as males can gain benefits from cooperation, which were not possible at extremely high contest. Here males can cooperate opportunistically to gain access to receptive females, by breaking up a consortship between a male and female dyad: as has been observed in savannah baboons, Papio cynicephalus (Bercovitch 1988; Noë and Sluijter 1990, 1995; Packer 1979) and Barbary macaques, Macaca sylvanus (Bissonnette et al. 2011). By combining their extrinsic fighting abilities, males are able to reduce mating opportunities of higher ranked rivals, effectively creating a level mating skew for subordinate males. When recruiting a partner, males must weigh-up the available allies and the target to recruit a partner who will provide enough combined intrinsic fighting ability to defeat the target. The undecided nature of the payoff outcome can make the opportunistic coalitions attractive for both partners (Noë 1990) as the highest ranked partner does not always gain access to the female. Both benefit by breaking up the consortship and simultaneously reducing the future mating opportunities of the consorting male and increasing the probability of future mating opportunities for the coalition partners. Levelling coalitions provide a short-term, opportunistic alternative mating strategy to gain direct benefits in the form of access to consorted females.
Coalition formation is not only beneficial in terms of gaining temporal access to females, and the payoffs to individuals are not only related to immediate benefits of access to the female. Male dominance rank position is highly correlated to reproductive success, thus, attaining high rank position can increase a male’s reproductive success. As contest potential decreases further, males can cooperate through rank-changing coalitions to increase the rank position of one or both partners. Rank changes through coalition formation can be achieved with one interaction but may require repeated coalitions afterwards to defend the new rank position and thus partners may need to interact over longer periods than with levelling coalitions. Rank-changing coalitions carry high immediate costs to the participants as higher ranked targets can show counter-aggression, and sexual selection has equipped males with extensive weaponry (e.g., large canines and body size). Recruitment of a coalitionary partner should therefore consider the reliability of the partner previously because if they were to defect (i.e., leave the fight as it is on-going) then their partner would be left in a vulnerable position possibly leading to serious injury during the fight (Ostner and Schülke 2014; Young et al. 2014b). As the rank change occurs with the cooperation of another individual, the intrinsic fighting abilities of those involved remain the same and as a result the partners should continue to form coalitions to defend and cement the newly acquired higher rank position (Ostner and Schülke 2014; van Schaik et al. 2006; Young et al. 2014b).
Regular coalitionary partners can provide further benefits as they can act to increase a males extrinsic power even in the absence of the partner and function to intimidate opponents and reduce the probability of aggression from higher rank individuals (Berghänel et al. 2011). Under such circumstances, male-male social relationships might be the key as the formation of social bonds between group members can help to facilitate this cooperation between individuals. Repeated affiliation between individuals through social behaviors such as grooming, being in close proximity without aggression or triadic male-infant-male buffering may show a willingness to cooperate and develop strong social bonds between individuals (Ostner and Schülke 2014). A correlation between grooming given and support received has been shown in both male and female primates (Schino and Aureli 2008). This is highlighted in Assamese macaques (Macaca assamensis) where male future rank position could be predicted by the frequency with which they formed coalitions and those forming coalitions more often rising in rank in the future (Schülke et al. 2010). Additionally, those males without strong bonds, formed coalitions less frequently and fell in rank. Furthermore, male coalitionary partner choice is related to social bond strength. In Barbary macaques, males recruited the partner with the greatest social bond strength more frequently from the pool of available bystanders when a dyadic aggression occurred. Males were also more likely to fail to recruit a partner if their opponent had a stronger social relationship or was of higher rank to the bystander than between the bystander and the recruiter (Young et al. 2014a). Only a small number of primate species show male-male affiliation, but where affiliation is frequent and social bonds are formed and differentiated, these factors are expected to play a major role in coalitionary partner choice. Although strong social bonds are important for the selection of a reliable partner, rank-changing coalitions have also been observed where male-male affiliation is infrequent or absent. In general, rank-changing coalitions can be considered a more long-term strategy to increase an individuals’ position in the dominance hierarchy and lead to long-term reproductive benefits of higher status.
Besides rank-changing and levelling coalitions, the third constellation is bridging coalitions. These coalitions involve a high ranked individual and a subordinate against a target ranked between them. These coalitions are predicted to occur between kins as only by assisting a relative can they remain profitable for the high ranked partner. But they are also observed in species where individuals show strong male-male social bonding and where individuals have a regular socially bonded partner. For example, if a rank-changing coalition has occurred involving two bonded partners, it may be one partner remains low ranked and the two continue to form coalition’s post-rank change to cement the higher individuals’ new rank position. As a result these coalitions would then be bridging. A subordinate may support a high-ranked partner against a mid-ranked challenger in order to gain future favor and support. Thus, where males form social bonds and recruit their bonded partner, bridging coalitions are more likely to occur.
Besides within-group coalitions, males can form coalitions with members of their own social group against members of another social group (known as between-group coalitions). These usually occur in defense of resources such as feeding sites or females against single males, multiple males, or other groups. These between-group coalitions are common as males cooperate against a common enemy and success leads to a common benefit for those involved defending the resource in question. Currently, a mathematical model predicting their feasibility, constellation, and occurrence is lacking. Additionally, future costs of cooperation can be lower as the target(s) of the aggression will not be in their social group at a later date when possible retaliation could occur. These coalitions are often very large in size compared to within-group coalitions. Males can also use coalitions to takeover a group; for example, an influx of bachelor males can usurp the current alpha male and even other resident males to assume control of the group (Borries 2000). In some instances, male coalitions against neighboring groups and within their own social group can be lethal and either directly or due to the injuries sustained in the aggression lead to the death of the target. In chimpanzees (Pan troglodytes), in particular, this behavior has been witnessed many times and has been likened to human warfare. Lethal coalitionary aggression is seen more frequently in species with fission-fusion dynamics where one party can have a large numerical advantage over their rival. These lethal aggressions can benefit the victors by increasing their territory, hence, food resources and reduce the number of future competitors or rivals who can form coalitions against them (Watts et al. 2006; Wrangham and Peterson 1996). The exact motivation behind lethal aggressions is still debated as death can merely be the unintentional result of fierce aggression and large injuries sustained to one of the competitors. Lethal coalitionary aggression has also been observed in ancestral human societies. For example, the Yanomamö tribal society of Amazonia where individuals who show stronger alliances are more likely to fight together and this can often be in the form of lethal aggression (Macfarlan et al. 2014). Although it is worth noting these coalitions often differ from those of chimpanzees and other nonhuman primates as the coalitionary partners can be from neighboring communities against a common enemy (Macfarlan et al. 2014). Hunter-gatherer societies also used levelling coalitions mainly involving males, and these were of larger size than seen in nonhuman primate species (Bowles 2006; Pandit and van Schaik 2003). It is thought that these coalitions carry lower costs due to the use of language allowing greater coordination and the greater number of participants reduces the costs for the aggressors (Pandit and van Schaik 2003). Historically, in human societies males are generally more closely related than females and as a result coalitions are thought to have evolved between related individuals within groups against a common out-group enemy similar to that of nonhuman primates.
Coalitions can occur both within and between groups in nonhuman primates. The level of contest within a group can determine the probability and feasibility of the occurrence of within-group coalitions. At too high or too low contest coalitions, within-groups are not expected. At mid-high contest levelling, coalitions are feasible as males fight over access to females and either male could gain access to the female once a coalition has broken-up a consorting pair. At lower contest, rank-changing coalitions are more likely and may require several aggressive bouts to allow one or both partners to rise in rank. Thus these coalitions should require a reliable partner. In species where male affiliation is common, those males with stronger social bonds may be more likely to form coalitions. Individuals in the same group may also form a coalition against members of another social group. For these, the individuals involved share a common goal of defeating a common enemy, making such coalitions more feasible. In some species, these contests can be intense and lead to physical injury or even death. Such lethal coalitionary aggression has been described as similar to that of human warfare.
- Borries, C. (2000). Male dispersal and mating season influxes in Hanuman langurs living in multi-male groups. In P. M. Kappeler (Ed.), Primate males: Causes and consequences of variation in group composition (pp. 146–158). Cambridge: Cambridge University Press.Google Scholar
- Harcourt, A., & de Waal, F. (1992). Coalitions and alliances in humans and other animals. Oxford: Oxford University Press.Google Scholar
- van Schaik, C. P. (1989). The ecology of social relationships amongst female primates. In V. Standen & R. A. Foley (Eds.), Comparative socioecology. The behavioural ecology of humans and other mammals (pp. 195–218). Oxford: Blackwell Scientific Publications.Google Scholar
- Wrangham, R. W., & Peterson, D. (1996). Demonic males: Apes and the origins of human violence. New York: Houghton Mifflin Harcourt.Google Scholar
- Young, C., et al. (2014a). Male social bonds predict partner recruitment in cooperative aggression in wild Barbary macaques. Animal Behaviour, 80, 675–682.Google Scholar