Reduced neural investment in post-reproductive females of the bee Ceratina calcarta

Abstract

Many insects show plasticity in the area of the brain called the mushroom bodies (MB) with foraging and social experience. MBs are paired neuropils associated with learning and memory. MB volume is typically greater in mature foragers relative to young and/or inexperienced individuals. Long-term studies show that extended experience may further increase MB volume, but long-term studies have only been performed on non-reproductive social insect workers. Here we use the subsocial bee Ceratina calcarata to test the effect of extended foraging experience on MB volume among reproductive females. Ceratina calcarata females forage to provision their immature offspring in the spring, and then again to provision their adult daughters in the late summer. We measured the volume of the MB calyces and peduncle, antennal lobes (AL), optic lobes (OL), central complex (CX), and whole brains of three groups of bees: newly emerged females, reproductive females in spring (foundresses), and post-reproductive mothers feeding their adult daughters in late summer. Post-reproductive late summer mothers had smaller MB calyces and ALs than foundresses. Moreover, among late mothers (but not other bees), wing wear, which is a measure of foraging experience, negatively correlated with both MB and OL volume. This is contrary to previously studied non-reproductive social insect workers in which foraging experience correlates postiviely with MB volume, and suggests that post-reproductive bees may reduce neural investment near the end of their lives.