Signaling Shifts and Economic Defensibility at Boomplaas Cave, South Africa

This study tests predictions of the Economic Defensibility Model (EDM) regarding the relationship between resource density and distribution, as well as signaling behavior among Late Pleistocene foragers. The EDM proposes that territorial signaling intensifies when resources are dense and predictable, as the benefits of broadcasting group membership and maintaining alliances outweigh signaling costs. We evaluate this model using data from Boomplaas Cave, South Africa, with deposits spanning the Last Glacial Maximum (29–19 ka) to the early Last Glacial–Interglacial Transition (19–14 ka). We examine proxies of signaling—ochre procurement and ostrich eggshell beads—relative to measures of resource density and predictability inferred from the site's faunal data. Results reveal a strong positive correlation between non-local ochre use and ostrich eggshell bead production, while ostrich eggshell bead densities are negatively correlated with gregarious grazer abundance. Contrary to EDM predictions, explicit signaling and cultural material social mediation behaviors expand when dense, predictable prey decline. These findings suggest that signaling technologies at Boomplaas were not mechanisms of territorial defense over defensible resources but strategies for maintaining social networks and mitigating subsistence risk under reduced ecological productivity. The intensification of bead production and the use of non-local ochre reflect social boundary defense and alliance-building strategies during periods of resource unpredictability. By integrating behavioral ecology with costly signaling theory, this study highlights the adaptive role of material signaling in buffering risk and sustaining social cohesion during climatic and environmental transitions in Late Pleistocene southern Africa.