Short-lived ants take greater risks during food collection.

Life-history theory predicts that organisms should alter their behavior if life expectancy declines. Recent theoretical work has focused on worker life expectancy as an ultimate factor in allocating risk-related tasks among the workforce in social insects. A key prediction of this evolutionary model is that workers with shorter life expectancy should perform riskier tasks. We tested this hypothesis, using laboratory colonies of the ant Myrmica scabrinodis. We modified foraging so that it differed in level of risk by manipulating distances, temperatures, and the presence of competitors on foraging patches. The life expectancies of foragers were shortened by poisoning with carbon dioxide or by injury through removal of their propodeal spines. Both treatments significantly shortened worker life expectancy in comparison with untreated ants. We show, for the first time, that foragers with a shorter life expectancy foraged under risk more often than foragers in the control group. Thus, a worker's strategy of foraging under risky circumstances appears to be fine-tuned to its life expectancy.