Shorter-lived workers start foraging earlier

In social insects it is often observed that young workers perform tasks inside the nest and later switch to tasks outside the nest. By doing this the workers maximize their expected longevity, because tasks inside the nest are safe and tasks outside the nest are risky. The optimal strategy of workers should be expected to depend not only on their age but also on their health status if it is associated with reduction of longevity. Here a mathematical model is used to calculate the optimal time of switching between safe and risky tasks in a colony consisting of both healthy and unhealthy workers. The model predicts that unhealthy workers, with shorter longevity, should perform more risky tasks at an earlier age than their healthy nest mates should. The optimal time to switch between safe and risky tasks depends on the proportion of healthy and unhealthy workers in the colony, but the workers need not perceive the health status of their nest mates in order to adopt the optimal strategy. The workers need only perceive their own life expectancy, because the life expectancy of healthy and unhealthy workers should be the same at the time of switching from safe to risky tasks. The model predictions agree with a wide range of empirical data presented in this paper. Workers that are infected, poisoned, injured or affected by other harmful factors start to forage and perform other risky tasks at an earlier age than their healthy nest mates.