Summary Recent studies have suggested that an extended postreproductive life span, such as life after menopause in human females, will evolve when the indirect (kin-selected) fitness benefits from altruistic behavior are greater than the direct fitness benefits from continuing reproduction [1–4]. Under some conditions in which postreproductive altruism is more beneficial and/or continuing reproduction is more costly, the postreproductive life span can be shaped by natural selection [5, 6]. However, indirect fitness benefits during postreproductive survival have been documented mainly in intelligent mammals such as humans and cetaceans, in which elder females possess enhanced social knowledge through learning [7–10]. Here we show that postreproductive females of the gall-forming aphid Quadrartus yoshinomiyai (Nipponaphidini) can gain indirect fitness benefits through their altruistic colony defense. These females cease reproduction around the time of gall opening and defend the colony by sticking themselves to intruding predators with a waxy secretion that is accumulated in their body with aging. Our results suggest that the presence of an age-related trait for altruistic behavior promotes the evolution of postreproductive altruism in this social insect via kin selection under natural selection imposed by predators.
