Need-based transfers on a network: a model of risk-pooling in ecologically volatile environments

Need-based transfers are a widespread form of human cooperation across cultures that enhance survival in marginal environments. Examples include central place food sharing among foragers and stock friendships among pastoralists. Previous models have demonstrated that such systems lead to higher rates of herd survival under volatile ecological conditions, such as those experienced by the Maasai of East Africa. The Maasai use a need-based transfer system called osotua that leads to risk pooling. Here we implement osotua-style asking and giving rules on a network in order to understand which network features promote herd survival. We find that (1) greater network size increases herd survival when individuals selectively ask their wealthiest partner for livestock but not when they ask a partner at random, (2) greater network connectedness improves herd survival regardless of whether individuals ask their wealthiest partner or ask a partner at random, (3) greater network heterogeneity leads to higher herd survival with selective asking of wealthy partners and decreases herd survival for random asking. In general, selective asking of wealthy partners is associated with higher rates of herd survival. We also examined the features of survival networks in order to understand the characteristics of the networks that result from 50 simulated years of osotua-style sharing under ecologically volatile conditions and the elimination of individuals who do not stay above sustainability threshold. These results will help inform further fieldwork on the need-based transfer systems and increase our understanding of features of sharing networks that enable risk pooling. Simple decentralized sharing rules can be highly effective for pooling risk, suggesting that complex cultural institutions may not be necessary for expansion into ecologically marginal environments.