Caste-specific lifespans emerge from the evolution of resource allocation in eusocial insects

Eusocial insects have extreme intraspecific lifespan variation, where queens are long-lived (up to 30 years in some species) whereas workers only live for a few months or years at most. Several studies have invoked the disposable soma theory to explain the evolution of caste-specific lifespans in eusocial insects, which proposes that senescence results from a resource allocation trade-off between maintenance and reproduction. An extension of this theory to eusocial insects is that caste-specific lifespans could emerge from a resource allocation trade-off between castes. However, to date this idea has not been formalised in a theoretical model. Here, we present an individual-based simulation model for the evolution of caste-specific lifespans in social insects. In our model, queens and workers die when their nutritional state becomes too low. The evolving trait in our model is the age-specific resource allocation of the queen and the workers, who can allocate resources to themselves, other workers, the brood, or the queen. We find that lifespan differences between queens and workers emerge from the evolved resource allocation within colonies. Caste-specific lifespans emerge in our model because queens obtain sufficient resources to be long-lived and highly fertile, whereas workers evolve to give resources away to enhance the queen's reproduction and thereby their own indirect fitness. In our model, we assume that the transition from nursing the brood to foraging is triggered by a decrease in the nutritional state of workers. If workers emerge with a high nutritional state, we observe that age polyethism emerges from the evolved resource allocation by the workers, where young workers nurse the brood and older workers forage. Overall, our model demonstrates that both caste-specific lifespans and age-related worker division of labour emerge as a consequence of evolved within-colony resource allocation.