A Simple Model of the Tsembaga Pig Population

A simple model of the pig population is shown in Figure 1. The stock keeps track of the pig population. The net pig births is based on a net birth rate of 14%/yr. The pig kill fraction is set at 85%, and the pig deaths in festival lowers the pig population whenever there is a festival. A binary variable takes on the value of 1 when there is a festival. Otherwise, it is zero. Shantzis and Behrens (1973, 273) describe the pressures for a festival in great detail, but the introductory model uses the simple assumption that a festival is declared whenever the ratio of pigs to humans exceeds 1.0. The human population is fixed at 160, and the pig population is initialized at 40.

Figure 1. Introductory model of the pig population.

Figure 2 shows the simulated behavior over a 40 year time interval if the step size (DT) is set at 1 year. The pig population grows from 40 to 160 during the first ten years of the simulation thereby triggering the first festival. The simulation shows three festivals with a time interval of around 11 years between festivals. This interval corresponds to the time interval reported by Rappaport.

Figure 2. Simulated cycles in the pig population.