REPRODUCTIVE COMPETITION AND THE EVOLUTION OF WORK LOAD IN SOCIAL INSECTS

Work load is analyzed as a heritable summary measure of individual activity, such that high work loads benefit the colony but increase the mortality rate of individuals. Thus, worker genotypes that adopt low work loads are expected to accumulate during colony growth at the expense of hard-working types if both occur in the same colony. When mature colony size is reached, this accumulation could, through nepotism, bias the production of sexuals toward the accumulated genotypes. A formal model shows that a gene coding for low levels of work load could indeed spread in a population of hard-working colonies, if colony workers are not always full sibs, that is, with polygyny or polyandry. The work load that is evolutionarily insensitive to slightly deviating mutants is predicted to decrease with the degree of polygyny (number of egg-laying queens), with polyandry (number of males contributing sperm), and with an increase in the degree of nepotism. Work load should also vary with ecological variables, such as the length of the colony's growth period before reproduction. To evaluate this hypothesis, several measures for work load (inactivity, self-grooming, brood care, risky tasks) are compared across species of ants. Average work load increases with reported colony size, but activity levels in colonies of monogynous and polygynous species are remarkably similar. -from Authors