Caste-specific differences in risk sensitivity in honeybees, Apis mellifera

Honeybee workers (foragers) are risk averse to variability in volume of reward when measured by conditioning of the proboscis extension response, and the level of risk aversion depends on the coefficient of variation of the variable distribution. Since drones do not forage on flowers, they may not have been under selection for risk-sensitive choice behaviour. We compared risk sensitivity of workers and drones and their ability to discriminate between the reward volumes used in the risk sensitivity experiments. Both castes discriminated better between 0 and 0.4 μl than between 0.4 and 1.2 μl, consistent with Weber's law of relative discrimination. Workers discriminated between both volume pairs better than drones, and workers showed greater risk aversion than drones. This is the first demonstration of caste-specific differences in risk sensitivity. These differences do not appear to be the result of differences in energy budgets, since both castes were on positive energy budgets. Levels of risk aversion were consistent with the coefficient of variation model. We calculated the relative associative strengths of subjects to the reward volumes from their choice proportions in the discrimination tests. The relative associative strengths of workers were greater than those of drones, and in both castes the relative associative strength of 0.4 μl relative to 0 μl was greater than that of 1.2 μl relative to 0.4 μl. Owing to Jensen's inequality, the decreasing functions of differences in relative associative strengths could explain differences in degree of risk aversion between the castes. Our findings are consistent with both mechanistic and functional explanations.