A neural correlate of learning fails to predict foraging efficiency in the bumble bee Bombus terrestris

Mushroom bodies (MB) are integrative structures in the insect brain that, in social bees, contribute to both visual and olfactory learning. Changes in the density of presynaptic boutons (or microglomeruli) within the calyx region of the MB have been linked to various aspects of foraging, including forms of learning that are believed to be key in supporting foraging efficiency. Here, we directly tested the relationship between foraging efficiency and microglomerulus density in a bumble bee model, Bombus terrestris. We found no evidence for microglomerulus density predicting real-world foraging performance, nor any relationship with foraging experience. Instead, our data suggest a potential nonlinear relationship between an individual's age, which is independent of foraging experience, and microglomerulus density in the lip region of the calyx, which is associated with olfactory processing. Our findings suggest that in real-world scenarios there is no simple direct relationship between microglomerulus density, learning ability and foraging efficiency in bumble bees, highlighting the knowledge gap regarding the relationships between learning abilities, neuroanatomy and foraging efficiency. • We relate foraging efficiency to experience, age and brain structure in bumble bees. • Efficiency improved with more foraging trips, showing the importance of experience. • Synaptic density in the mushroom body calyx did not predict foraging efficiency. • Synaptic density in the calyx lip changed with age but not with foraging experience. • Our findings warrant further study of the functional correlates of synaptic density.