Defensive networks provide basis for late pre-Hispanic Andean polities

Regional polities may emerge from the development of a centralized hierarchy of a specific political regime, or from bottom–up patterns of interaction and collective action, especially mutual defense. In this study, we assess the proposition that the development of late pre-Hispanic polities of the south-central Andes was significantly shaped by defensive networks of pukaras (hillforts), which were in turn shaped by variable affordances for fortification in the mountainous Andean landscape. Pukaras are very common in the Andean highlands and date overwhelmingly to 1250 to 1500 CE, reflecting a period of widespread concern with violent conflict. We apply a combination of formal spatial and network analyses to a dataset of 1,287 hillforts identified in a satellite imagery survey of over 150,000 km 2 in the south-central Andean highlands. Starting from the premise that interaction and mutual aid were key to defensive cooperation, we define and weight network edges between hillfort nodes based on intervisibility and walking cost. Groupings of pukaras are identified using formal network analysis and community detection. Analysis reveals substantial regional diversity in the structure of these defensive networks. Larger but less cohesive networks developed in open terrain that facilitated movement, interaction, and visual connection, while smaller, more cohesive networks emerged in more rugged terrain. These patterns correspond broadly to the size and characteristics of sociopolitical units evidenced in independent archaeological and ethnohistorical sources. This study highlights how interactions between local landscapes, levels of conflict, and social patterns of defensive cooperation participated in the formation of late pre-Hispanic polities.