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

Animal cognition and morphological investment across brain regions can vary between individuals and across a lifetime as a function of individual 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 once established in a (dark) colony. Past work shows that gynes are better at visual learning than workers, but we know little about this varies between a foraging (gyne) stage and reproductive (queen) stage. Here we compared bumblebee (Bombus impatiens) visual learning and brain morphology across age-matched naive 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 latter stage, in line with past 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 the reproductive stage. Brain regions scaled differently across gynes and workers with body size, suggesting that gynes are not simply large workers in terms of their neuromorphology. Overall, our results provide behavioral and morphological evidence that social and ecological roles in a colony shape cognition and neural investment.