Benefits of extended maternal care in a mass-provisioning bee at the cusp of sociality

Many invertebrates exhibit parental care, posited as a precursor to sociality. For example, solitary foundresses of the facultative social orchid bee <i>Euglossa viridissima</i> guard their brood for 6+ weeks before offspring emerge, when the nest may become social. Guarding comes at the fitness cost of foregoing the production of additional offspring. Yet it is unclear whether guarding (extended maternal care) can enhance offspring survival such that it outweighs those fitness costs, or if it is a consequence of the selective benefits of sociality, including extended female longevity. Experimental removal of solitary foundresses from nests of <i>E. viridissima</i> revealed an immediate fitness loss: decreased offspring survival. A mathematical model exploring the trade-off between extended maternal care versus non-guarding revealed that extended maternal care is immediately advantageous to a solitary mother if nest establishment takes longer than a threshold 1.7-12.5 days. Below this threshold, our model suggests that social fitness gains (acquiring helper daughters) need to be invoked to explain the evolution of extended maternal care. Enhanced survival of offspring through guarding and nest inheritance may nevertheless ease conditions for the evolution of sociality by favouring extended adult longevity and brood care in incipient social species like <i>E. viridissima</i>.