Warfare and reproductive success in a tribal population

Abstract

Intergroup conflict is a persistent feature of many human societies yet little is known about why individuals participate when doing so imposes a mortality risk. To evaluate whether participation in warfare is associated with reproductive benefits, we present data on participation in small-scale livestock raids among the Nyangatom, a group of nomadic pastoralists in East Africa. Nyangatom marriages require the exchange of a significant amount of bridewealth in the form of livestock. Raids are usually intended to capture livestock, which raises the question of whether and how these livestock are converted into reproductive opportunities. Over the short term, raiders do not have a greater number of wives or children than nonraiders. However, elders who were identified as prolific raiders in their youth have more wives and children than other elders. Raiders were not more likely to come from families with fewer older maternal sisters or a greater number of older maternal brothers. Our results suggest that in this cultural context raiding provides opportunities for increased reproductive success over the lifetime.warfare | human behavioral ecology | reproductive success | pastoralists | status

Formation of raiding parties for intergroup violence is mediated by social network structure

“…To address how social networks influence the emergence of violence, we mapped the social ties in a nonstate society in which groups engaging in violent intergroup raiding formed organically, analyzing the role of social networks in instigating and sustaining intergroup conflict. Using observations derived from long-term ethnographic fieldwork, coupled with detailed mapping of the social network of raiding-aged men, we present data from a complete set of 39 discrete intergroup conflict events among the Nyangatom, a society of nomadic agro-pastoralists inhabiting a remote region along the border of South Sudan and Ethiopia largely outside the reach of state institutions (38,39).…”

“…To marry, a male is required to provide the family of the bride with bride wealth, often 30-60 cattle but sometimes as many as 100 cattle. Therefore, livestock are highly sought after, and violent conflict with other groups to obtain them is common (38)(39)(40). The Nyangatom also have a distinctive social organization involving sequential generation sets and age sets (38); most males engage in activities such as herding, socializing, and raiding with members of their age group, creating strong social bonds between members (38).…”

“…Casualties among members of raiding parties are unusual as they seek to minimize personal risk, but injuries and deaths of members of enemy groups are common, and fatalities among raiders are not unheard of. Successful raiders receive captured livestock (18,39) and sometimes other social benefits, such as status, honorific names and scars, and public praise (9,41). Raids generally begin with one or two individuals recruiting other participants, a process that typically takes several days.…”

“…Raiding parties can also emerge when large groups of young men are congregated, such as during a ceremony. Individuals are not compelled to join a raiding party, and many young men elect not to join; and there are no formal sanctions for cowardice, desertion, or failure to participate (39).…”

The Conditions Favoring Between-Community Raiding in Chimpanzees, Bonobos, and Human Foragers

“…It is frequently claimed that in human wars, cultural factors, including ideology, are major determinants (e.g., Robarchek 1989), and that any comparison with nonhuman species is therefore moot. Here, we do not deny the role of ideology or other cultural influences, but we do assume that in simple wars their effect does not systematically skew individual decisions away from their fitness interests (see also Glowacki and Wrangham 2015).…”