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

The 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 repeatedly linked to various aspects of foraging, including forms of learning that are thought to be key to support foraging efficiency. Here we set out to directly test the relationship between foraging efficiency and microglomerulus density in a bumblebee model, Bombus terrestris. We found no evidence that microglomerulus density predicted real-world foraging performance, nor any relationship with foraging experience. Instead, our data suggest a potential non-linear relationship between individual age, 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 microglomerular density, learning ability and foraging efficiency in bumblebees, highlighting the gap in knowledge regarding the relationships between learning abilities, neuroanatomy and foraging efficiency.