A replaceable soma enables negative senescence in harvester ants

Most vertebrates and insects undergo senescence: as they age, their rate of mortality increases while reproduction declines. We present data from a 34-y study on 3,285 colonies of known age in the western harvester ant, Pogonomyrmex occidentalis, that show the reverse pattern, negative senescence, in which mortality decreases and reproduction increases with age. Theoretical models have identified indeterminate growth and a lack of reproductive cost to growth as the key traits favoring the evolution of negative senescence. Ant colonies continue to grow throughout their lifetime, leading to older colonies being larger on average. We found no evidence of a tradeoff between colony growth and reproduction: growth and reproduction were positively correlated, a synergism that may favor continued investment in growth. Harvester ants occupy a temporally variable environment that shapes investment in reproduction. At the population level, greater reproduction in 1 y does not lead to higher numbers of new colonies in the following year. Environmental unpredictability decouples reproduction from recruitment and selects for an extended reproductive lifespan, a trait that parallels changes in aging. We discuss the features that harvester ant colonies share with other organisms to identify functionally homologous mechanisms for avoiding senescence, as well as those traits that are specific to highly eusocial species.