Evolution of Life History Strategies in Ants: Variation in Queen Number and Mode of Colony Founding

Abstract

Most theoretical models for the evolution of polygyny in ants assume a stable population. Here, we discuss the evolution of life history strategies with perennial life cycles, overlapping generations, and fluctuating populations. We assume two alternative strategies, i.e., monogyny = independent founding strategy (in which the survival rate of foundress queens is low but fecundity of individual queens is high), and polygyny = dependent founding strategy (in which the initial survival rate of queens is high, but queens have low fecundity and short longevity). Adopting the intrinsic rate of natural increase (r) as the fitness measurement, we compare the two strategies under various ranges of life history parameters. Our model suggests that a higher survival rate of queens may compensate for a low fecundity of queens adopting the dependent founding strategy. Since dependent foundresses can skip over the non-sexual producing founding and ergonomic stages, they can quickly initiate sexual production and gain a higher r. This implies that dependent founding, which may result in secondary polygyny, tends to be an r strategist. This early reproduction effect has not been explicitly dealt with in previous discussions of polygyny, because they have omitted population demography. We predict that dependent founding (or polygyny) would be more common in open areas where ant populations might suffer from random density independent population fluctuation than in forests where density dependent regulations could stabilize ant populations. A recent empirical data-set from Okinawa Island actually suggested that polygyny is more common in open areas than in forests. Some other published data that enabled intraspecific comparison also supported our prediction.