Intergroup Contest Research Articles

Contests within and between groups: Theory and experiment

We examine behavior in a two-stage group contest where intra-group contests are followed by an inter-group contest. Rewards accrue to the winning group, with winners of the intra-group contest within that group receiving a greater reward. The model generates a discouragement effect, where losers from the first stage exert less effort in the second stage than winners. In contrast to the related literature, we show that a prior win may be disadvantageous, generating lower profits for first stage winners as compared to losers. We consider exogenous asymmetry between groups arising from a biased group contest success function. Although the asymmetry occurs in the second stage, its effect plays out in the first stage, with higher intra-group conflict in the advantaged group. Experimental results support the qualitative predictions of the model. However, losers from the first stage bear a higher burden of the group contribution than the theoretical prediction.

The Election Effect: Democratic Leaders in Inter-Group Conflict

Abstract How does the experience of being elected alter subsequent leader behavior at the international level? We argue for the existence of an election effect, through which a democratic election intensifies in-group identification and generates a sense of obligation to voters, while simultaneously increasing out-group hostility. These combined effects cause leaders to overexert costly efforts in competitive situations against other groups. Using an online laboratory experiment, we show that democratic leader selection increases effort in inter-group contest games, which share key features with interstate conflicts. We use a carefully specified decomposition strategy to distinguish the election effect from better-known selection effects, wherein eventual leaders are nonrandomly chosen. Our results show that a democratic election has negative implications in inter-group games, despite the near-universally positive effects of democracy found in intra-group ones.

Fighting force and experience combine to determine contest success in a warlike mammal

Conflicts between social groups or "intergroup contests" are proposed to play a major role in the evolution of cooperation and social organization in humans and some nonhuman animal societies. In humans, success in warfare and other collective conflicts depends on both fighting group size and the presence and actions of key individuals, such as leaders or talismanic warriors. Understanding the determinants of intergroup contest success in other warlike animals may help to reveal the role of these contests in social evolution. Using 19 y of data on intergroup encounters in a particularly violent social mammal, the banded mongoose (Mungos mungo), we show that two factors, the number of adult males and the age of the oldest male (the "senior" male), have the strongest impacts on the probability of group victory. The advantage conferred by senior males appears to stem from their fighting experience. However, the galvanizing effect of senior males declines as they grow old until, at very advanced ages, senior males become a liability rather than an asset and can be evicted. As in human conflict, strength in numbers and the experience of key individuals combine to determine intergroup contest success in this animal society. We discuss how selection arising from intergroup contests may explain a suite of features of individual life history and social organization, including male eviction, sex-assortative alloparental care, and adult sex ratio.

Environmental stress increases out-group aggression and intergroup conflict in humans.

Peaceful coexistence and trade among human groups can be fragile and intergroup relations frequently transition to violent exchange and conflict. Here we specify how exogenous changes in groups' environment and ensuing carrying-capacity stress can increase individual participation in intergroup conflict, and out-group aggression in particular. In two intergroup contest experiments, individuals could contribute private resources to out-group aggression (versus in-group defense). Environmental unpredictability, induced by making non-invested resources subject to risk of destruction (versus not), created psychological stress and increased participation in and coordination of out-group attacks. Archival analyses of interstate conflicts showed, likewise, that sovereign states engage in revisionist warfare more when their pre-conflict economic and climatic environment were more volatile and unpredictable. Given that participation in conflict is wasteful, environmental unpredictability not only made groups more often victorious but also less wealthy. Macro-level changes in the natural and economic environment can be a root cause of out-group aggression and turn benign intergroup relations violent.This article is part of the theme issue ‘Intergroup conflict across taxa’.

Words and deeds – Experimental evidence on leading-by-example

The paper studies how leadership recommendations and leading-by-example shape group behavior. The experimental results from an intergroup contest show that the leaders are the key drivers of contest expenditure. They increase their expenditure substantially once they make recommendations to the followers. The leaders' expenditure recommendations have a highly significant impact on the investments of followers. The additional impact of leading by example confirms the relevance of the alignment of words and deeds. A second experiment replicates the treatments in a non-competitive voluntary contribution game. In this setting, the opportunity for making recommendations has an even stronger impact on the leader's expenditure. For a given level of leader expenditure, the behavior of followers remains remarkably consistent across the two experiments but the additional impact of leading by example is not significant anymore.

Quadratic relationships between group size and foraging efficiency in a herbivorous primate

The effect of feeding competition on foraging efficiency is an important link between ecological factors and the social organization of gregarious species. We examined the effects of group size on daily travel distances, activity budgets, and energy intake of mountain gorillas in Rwanda. We measured daily travel distances of five groups, activity budgets of 79 gorillas in nine groups, and energy intake data for 23 adult females in three groups over a 16-month period. Travel distances and the proportion of time spent traveling increased with size for most groups, which would be expected if their foraging efficiency is limited by intragroup feeding competition. However, travel distances and times decreased for the largest group, which also had higher energy intake rates than intermediate sized groups. The improved foraging efficiency of the largest group may be explained by advantages in intergroup contest competition. The largest group had much lower home range overlap than the other study groups which may be due to groups avoiding one another as a result of male mating competition. Collectively, our results indicate that intermediate sized groups had the lowest foraging efficiency and provide a new twist on the growing evidence of non-linear relationships between group size and foraging efficiency in primates.

The effect of intergroup competition on intragroup affiliation in primates

Researchers from various disciplines have hypothesized a positive correlation between the level of intergroup contest competition (IGCC) and the evolution of behavioural traits, such as cooperation, altruism and friendship, which promote intragroup affiliation. Empirical support for this hypothesis is, however, scarce and mainly available from humans. We tested whether the level of IGCC affects intragroup affiliation (i.e. intragroup grooming exchange) among male and female nonhuman primates. To quantify intragroup affiliation, we used social network measures and a grooming index. Our measure of IGCC combined frequency of intergroup encounters and proportion of aggressive encounters and was calculated separately for males and females. We ran our analyses on 27 wild groups of primates belonging to 15 species (13 Cercopithecinae, one Colobinae and one Cebinae). Our analyses reveal a clear pattern of correlated evolution between grooming network density and interindividual variation in the number of grooming partners on the one hand and the intensity of IGCC on the other in females, but not males. Thus, our results suggest that the exact nature of the relationship between IGCC and intragroup affiliation is sex specific. These results may be explained by the differential costs and benefits males and females experience during aggressive intergroup confrontations and by sex-specific differences in intragroup affiliation.

Comparison of feeding behavior between two different-sized groups of Japanese macaques (Macaca fuscata yakui).

Group-living animals face intragroup scramble and intergroup contest competitions. Many studies have shown that larger groups bear the costs of intragroup scramble competition, which negatively affects the reproductive success of females. Unlike most primate species, Japanese macaques in the Yakushima coastal forest show increased reproductive success with group size. However, it remains unclear how group size affects the behavior of macaques. The present study examined the effects of group size on the feeding behavior of Japanese macaques in the Yakushima coastal forest. We investigated 9-13 adult females from two different-sized groups via focal animal sampling during October 2012-August 2013. We compared the feeding behavior, including patch use, between the two groups. The larger group had a larger home range and spent more time feeding, especially on mature leaves. This suggests that intragroup feeding competition should be more intense in the larger group than in the smaller group. The feeding of mature leaves might enable the larger group to increase the number of co-feeding individuals. Contrary to the predictions that the larger group travels longer distances and spends more time moving, the smaller group traveled longer distances, and spent more time moving, although the number of visited patches did not differ between the two groups. The immediate consequences of the loss of inter-group encounters could accumulate as daily travel costs, considering that group size is associated with inter-group dominance and that intergroup aggressive encounters occur frequently in the Yakushima coastal forest. This suggests that the smaller group has increased travel costs as a result of intergroup contest competition, which leads to decline in reproductive success. Am. J. Primatol. 77:986-1000, 2015. © 2015 Wiley Periodicals, Inc.

Communication in intergroup conflicts

Numerous studies suggest that communication may be a universal means to mitigate collective action problems. In this study, we challenge this view and show that the communication structure crucially determines whether communication mitigates or intensifies rent-seeking for pure public goods. We observe the effect of different communication structures in the context of a finitely repeated intergroup contest and demonstrate that conflict expenditures are significantly higher if communication is restricted to one's own group as compared to a situation with no communication. However, expenditures are significantly lower if open communication within one's own group and between rivaling groups is allowed. We show that under open communication intergroup conflicts are avoided by groups taking turns in winning the contest. Our results do not only qualify the role of communication for collective action but may also provide insights on how to mitigate the destructive nature of intergroup conflict and group rent-seeking.

Prize Allocation in Contests with Size Effect through Prizes

We examine prize allocation in contests in which the number of contenders affects prizes. The government allocates a fixed prize between two groups of contenders. The prize share of each group is determined endogenously through the inter-group share contest. Then, the contenders in each group respectively compete for the prize. Examining prize allocation in such contests, we obtain the following results. The contenders expend more resources when they cooperate than when they contribute non-cooperatively in the inter-group share contest. The size effect through prizes increases the prize share of the larger group. The larger group can even obtain the larger share if the size effect through prizes is sufficiently strong.

Location and group size influence decisions in simulated intergroup encounters in banded mongooses

In social species that cooperatively defend territories the decision to retreat or attack in contests between groups is likely to depend on ecological and social factors. Previous studies have emphasized the importance of the encounter location or the size of competing groups on the outcome. In addition, the identity of the intruder, whether familiar or stranger, may also play a role. To test whether the same factors affect the resident group’s decisions already at the beginning of contests, we simulated intergroup encounters in banded mongooses (Mungos mungo). When spotting rival groups banded mongooses emit ‘‘screeching calls’’ which lead group members to bunch up. With playbacks of these calls, we tested how the groups’ response was affected by the following factors: 1) the location of the playback in relation to their territory (exclusive use vs. overlap); 2) the number of resident individuals; and 3) the origin of calls (neighbor vs. stranger) used. Subjects were more likely to approach the loudspeakers and arrive within 1 m of the speakers in the exclusive use zone than in the overlap zone. Moreover, larger groups tended to be more likely to move toward the loudspeakers and were also more likely to arrive there. The origin of calls used in the playbacks did not affect the groups’ responses. These findings exemplify the importance of the combined effect of location and group size on group decisions during impending intergroup contest. Key words: banded mongooses, group decision, group size, intergroup encounter, Mungos mungo, territory. [Behav Ecol]

Food conditions, competitive regime, and female social relationships in Japanese macaques: within-population variation on Yakushima

Feeding conditions, competitive regime, and female social relationships of Japanese macaques (Macaca fuscata) on Yakushima were compared between the two habitats at two different altitudes (coniferous forest, 1,000-1,200 m and coastal forest, 0-200 m). Fruit availability was higher in the coastal forest. There was no consistent difference in the frequency of agonistic interactions within a group during feeding between the two habitats. The coastal forest evoked stronger inter-group contest competition compared to the coniferous forest as evidenced by a higher inter-group encounter rate and a higher proportion of aggressive encounters to non-aggressive ones. Birth rate was higher in larger groups compared to smaller ones in the coastal forest, but did not differ in the coniferous forest. In spite of these differences in competitive regime, no variation in female social relationships was observed, such as direction and concentration on particular individuals in grooming, linearity in dominance rank, counter-attack, and support of juvenile kin during agonistic interactions. The present results indicate that the female social relationships of Japanese macaques are robust and do not change according to changes in the current environment.

Simultaneous inter- and intra-group conflicts

This paper models the trade-off between production and appropriation in the presence of simultaneous inter- and intra-group conflicts. The model exhibits a ‘group cohesion effect’: if the contest between the groups becomes more decisive, the players devote fewer resources to the intra-group conflict. Moreover, there is also a ‘reversed group cohesion effect’: if the intra-group contests become less decisive, the players devote more resources to the inter-group contest. The model also sheds new light on normative questions. I derive exact conditions for when dividing individuals into more groups leads to more productive and less appropriative activities. Moreover, I show that there is an optimal size of the organization which is determined by a trade-off between increasing returns to scale in production and increasing costs of appropriative activities.

Simultaneous Inter- and Intra-Group Conflicts

This paper models the trade-off between production and appropriation in the presence of simultaneous inter- and intra-group conflicts. The model exhibits a 'group cohesion effect': if the contest between the groups becomes more decisive, or contractual incompleteness between groups becomes more serious, the players devote fewer resources to the intra-group conflict. Moreover, there is also a 'reversed group cohesion effect': if the intra-group contests become less decisive, or contractual incompleteness within groups becomes less serious, the players devote more resources to the inter-group contest. The model also sheds new light on normative questions. I derive exact conditions for when dividing individuals in more groups leads to more productive and less appropriative activities. Furthermore, I show that there is an optimal size of the organization which is determined by a trade-off between increasing returns to scale in production and increasing costs of appropriative activities.

The evolution of bipedalism in hominids and reduced group size in chimpanzees: alternative responses to decreasing resource availability

One hypothesis for the evolution of hominid bipedalism is that bipedalism was more efficient than quadrupedalism for long-distance terrestrial locomotion, and was favored when resources became scarcer and more widely separated during the drying of African forests in the Miocene. Here we extend this scenario for the evolution of bipedalism based on principles of behavioral ecology of extant primates. Daily travel distance is not only an increasing function of resource scarcity, but also of group size (because of intragroup scramble competition). When faced with Miocene drying, hominoids were forced to evolve either energetically more efficient locomotory abilities or smaller group sizes. The cost of the latter strategy is that smaller groups are less successful than larger groups in intergroup contest competition (smaller groups are supplanted from limiting resources). Among the earliest hominids, bipedalism may have been favored over small group size as an alternative energetic response to decreased resource availability. The alternative was to maintain quadrupedal locomotion but evolve fission–fusion grouping to reduce daily travel distance for individuals and hence, their energetic costs of travel. We suggest that this strategy represents the evolutionary pathway taken by chimpanzees. This divergence of strategies may have been a result of inherent differences in feeding ecology, but could also have been enhanced by the pre-empting of niche space by the two closely related and presumably competing hominoid ancestors of humans and chimpanzees. If so, it could have been a potential factor in the speciation process that led to modern humans and chimpanzees.