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 Euglossa viridissima 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: EMC) 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 E. viridissima revealed an immediate fitness loss: decreased offspring
survival. A mathematical model exploring the trade-off between EMC versus
non-guarding revealed that EMC is advantageous if the nest established is
greater than a threshold of 1.7-12.5 days. For much of the parameter
space, EMC may be a precursor to sociality. Below this threshold, our
model suggests that social fitness gains (acquiring helper daughters) need
to be invoked to explain the evolution of EMC. 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 E.
viridissima.