Modeling long lifespans in eusocial insect populations

Along with division of labour, and life-history complexities, a characteristic of eusocial insect societies is the greatly extended lifespan for queens. The colony structure reduces the extrinsic mortality of the queen, and according to classical evolutionary theories of ageing, this greatly increases the lifespan. We explore the relationship between the evolution of longevity and the evolution of eusociality by introducing age-structure into a previously proposed evolutionary model and also define an associated agent-based model. A set of three population structures are defined: (i) solitary with all reproductive individuals, (ii) monogynous eusocial with a single queen, and (iii) polygynous eusocial, with multiple queens. In order to compare the relative fitnesses we compete all possible pairs of these strategies as well as all three together, analysing the effects of parameters such as the probability of progeny migration, group benefits, and extrinsic mortality on the evolution of long lifespans. Simulations suggest that long lifespans appear to evolve only in eusocial populations, and further, that long lifespans enlarge the region of parameter space where eusociality evolves. When all three population strategies compete, the agent-based simulations indicate that solitary strategies are largely confined to shorter lifespans. For long lifespan strategies the solitary behaviour results only for extreme (very low or very high) migration probability. For median and small values of migration probability, the polygynous eusocial and monogynous eusocial strategies give advantage to the population respectively. For a given migration probability, with an increase in lifespan, the dominant strategy changes from solitary to polygynous to monogynous eusociality. The evolution of a long lifespan is thus closely linked to the evolution of eusociality, and our results are in accord with the observation that the breeding female in monogynous eusocial species has a longer lifespan than those in solitary or polygynous eusocial species.