Figure 1. Sustainability of a colony with/without inactive workers.

(a) The number of processed tasks in a time step is always larger in a colony with an invariable response threshold among workers irrespective of parameter values (degree of fatigue and task appearance rate), indicating that variable threshold systems are generally more productive in short-term. (b) Variable threshold systems persisted longer than systems with equivalent thresholds. The advantageous parameter space of the variable threshold system differs depending on the task appearance rate. When there is no fatigue (black line) there is no difference in the time of persistence between the two systems. (c) Change in sustained rate of colonies against task appearance rate. In both the variable and invariable systems, the sustained rates decreased with the task appearance rate, but the decline began earlier in the system with invariable thresholds. This difference generates convex shapes of advantageous areas of parameter space in which variable threshold systems persist for longer.