Optimality theory predicts the maximization of individual fitness1,2,3 (also 7–9 pp. in ref. 2; 5,6 pp. in ref. 3). This theory also predicts an increase in the average short-term productivity of a functional unit because such a change will usually increase the fitness of all members4 (see also 119–129 pp. in ref. 5). A colony of social insects is a functional unit subjected to natural selection f