Caste-specific ageing emerges 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) 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 ageing in eusocial insects, which proposes that senescence results from a resource allocation trade-off between maintenance vs. reproduction. An extension of this theory to eusocial insects is that caste-specific ageing 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 model for the evolution of ageing 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 individual workers, who can allocate resources between themselves, other workers, and the queen. We find that lifespan differences between queens and workers emerge from the evolved resource allocation within colonies, which are within the range of empirically observed lifespans of queens and workers in monogynous eusocial insects. Caste-specific ageing evolves in our model because queens obtain large amounts of resources, which allows them to be long-lived and highly fertile, whereas workers evolve to give resources away to enhance the queens reproduction and thereby their own indirect fitness. We also observe that age polyethism emerges, where young workers nurse the brood and older workers forage. Overall, our model demonstrates that both caste-specific ageing and age-related worker division of labour emerge as a consequence of evolved within-colony resource allocation.