Dynamic model of optimal age polyethism in social insects under stable and fluctuating environments

Social insect workers from many species perform division of labor based on age (temporal division of labor). The age polyethism hypothesis postulates that the kind of labor is determined by age, independent of the environmental condition. Previous theoretical studies on this subject have used only static models which do not describe time developmental factors such as population growth or age structure dynamics. However, age structure dynamics is an essential factor for temporal division of labor. In this paper, we develop an age-structured population model with division of labor between tasks inside the nest and a foraging task outside the nest and analyse the adaptive form of age polyethism by computer simulation. We show that in a constant environment age polyethism is adaptive when specialization for a single job is efficient or when the mortality during foraging task is higher than that during inside task. And computer simulations suggest that when the excess foraging mortality exists and life expectancy of workers decreases monotonously with age, an age polyethism where workers drastically switch from inside to outside labor at a certain age is adaptive, which is consistent with field observations. In a fluctuating environment, the balance of labor can not stay optimal because age structure fluctuates due to temporal variation of newly produced workers and because the necessary balance itself may fluctuate. Computer simulations reveal that when environmental fluctuation affects the efficiencies of both inside and outside labors, a "soft age polyethism" form tin which each worker of an age class performs both labors and the ratio of the labors gradually changes with age) becomes more adaptive than any "hard age polyethism" form tin which all workers of an age class perform the same single labor and the kind of labor abruptly changes at a certain age class) as the magnitude of fluctuation increases. However, when environmental fluctuation affects only outside labor, hard age polyethism forms tend to stay adaptive even as the fluctuation increases. (C) 1998 Academic Press.