Reproductive environment affects learning performance in bumble bees

Abstract

Despite a presumed fitness advantage for individuals with well-developed cognitive abilities, learning performance is usually found to be highly variable within a population. Although little is currently known about the mechanisms responsible for maintaining such variation, there is correlative evidence to suggest that learning performance may be linked to reproductive physiology in the social insects. Bumble bee colonies naturally undergo an initial co-operative phase, when only the queen reproduces, and a subsequent competition phase when all colony members compete to produce male offspring. We experimentally induced these distinct phases by manipulating the presence/absence of the queen and assessed changes in sucrose responsiveness and learning performance. We found that nest-based workers upregulated their reproductive potential in queenless colonies, and correspondingly, these bees were more responsive to sucrose than their queenright counterparts, performing better in an olfactory learning task as a result. These findings suggest that differences in ovarian development are responsible for at least some of the remarkable variation in learning performance that can be observed among very closely related members of social insect colonies. Cognitive abilities are often assumed to be inherently adaptive, so the question of why individuals vary in their learning ability has received relatively little attention. Here, we focus on reproductive status as a proximate cause of variation in learning ability in the social insects. We show that a significant proportion of the surprising variation that exists between genetically similar colony members can be explained by worker ovarian development; reproductively active workers are more sensitive to food rewards and thus learn more quickly. Learning ability may be one of a suite of correlated traits that are linked to reproductive physiology in social insects and therefore play an important role in the evolution of division of labour.