We examine how group and individual factors combine to determine success in intergroup conflicts in a model wild mammal species, the banded mongoose. However, few studies have examined whether key individuals enhance collective fighting ability in nonhuman animals, that is, whether certain individuals have disproportionate effects on intergroup contest success ( 15, 20). Key individuals contribute to group success in contexts like foraging, patrolling, and defense ( 8, 18, 19). Similarly, in nonhuman animals, group size, or sometimes the number of members of a certain subgroup (e.g., males), are important factors in intergroup contest success ( 13– 17), and key individuals, such as leaders, may also improve group fighting ability. Effective leadership or the presence of “key individuals” is also crucial in human intergroup contests ranging from small raiding parties to battles between vast armies ( 8, 9), as charismatic warriors or gifted leaders can help smaller fighting units to prevail against larger ones ( 9– 12). However, what determines success in collective fights? In historical human battles, strength in numbers was often a major determinant of conflict success ( 6, 7). These intergroup contests have been proposed as a major force favoring the evolution of within-group altruism and between-group hostility in humans and other animals ( 3– 5). 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.įrom raiding termites ( 1) to warring humans ( 2), many cooperative animal societies engage in aggressive intergroup fights, or contests, over resources. As in human conflict, strength in numbers and the experience of key individuals combine to determine intergroup contest success in this animal society. 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. The advantage conferred by senior males appears to stem from their fighting experience. 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. Understanding the determinants of intergroup contest success in other warlike animals may help to reveal the role of these contests in social evolution. 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. 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.
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