Toward greater realism in inclusive fitness models: the case of caste fate conflict in insect societies

In the field of social evolution, inclusive fitness theory has been
successful in making a wide range of qualitative predictions on expected
patterns of cooperation and conflict. Nevertheless, outside of sex ratio
theory, inclusive fitness models that make accurate quantitative
predictions remain relatively rare. Past models dealing with caste fate
conflict in insect societies, for example, successfully predicted that if
female larvae can control their own caste fate, an excess should opt to
selfishly develop as queens. Available models, however, were unable to
accurately predict levels of queen production observed in Melipona bees –
a genus of stingless bees where caste is self-determined – as empirically
observed levels of queen production are ca. two times lower than the
theoretically predicted ones. Here, we show that this discrepancy can be
resolved by explicitly deriving the colony-level cost of queen
overproduction from a dynamic model of colony growth, requiring
incorporation of parameters of colony growth and demography, such as the
per-capita rate at which new brood cells are built and provisioned, the
percentage of the queen’s eggs that are female, costs linked with worker
reproduction and worker mortality. Our revised model predicts queen
overproduction to more severely impact colony productivity, resulting in
an evolutionarily stable strategy (ESS) that is ca. half that of the
original model, and is shown to accurately predict actual levels of queen
overproduction observed in different Melipona species. Altogether, this
shows how inclusive fitness models can provide accurate quantitative
predictions, provided that costs and benefits are modelled in sufficient
detail and are measured precisely.