Bumblebee queens differ in brain morphology but not learning performance across life stages

Animal cognition and brain morphology can vary between individuals and across a lifetime as a function of social and ecological requirements. Bumblebees have distinct ecological pressures acting upon individuals at different times: young queens (gynes) and workers share similar challenges as they both forage as part of the colony, but only queens overwinter and start a colony the following season, switching to a reproductive stage. Here we compared bumblebee (Bombus impatiens) visual learning and brain morphology across age-matched workers and gynes and older reproductive queens. We expected foraging-stage gynes to be better at visual learning than reproductive queens and visual regions to be reduced in the reproductive queens, in line with work in other social insects. However, we found that gynes and queens performed similarly, while both performed better than workers. We also found that reproductive queens had larger olfactory regions (antennal lobes) than gynes, while visual regions (medulla and lobula) did not differ, indicating a greater investment in olfactory regions in this later reproductive stage. Brain regions also scaled differently with body size for gynes and workers. Overall, our results provide behavioral and morphological evidence that social and ecological roles in a colony shape cognition and neural investment.