Learning in honeybees as a function of amount of reward: Control of delay

Six experiments on learning in honeybees were prompted by the possibility that results previously attributed to a difference in amount of reward (20- versus 5-μl drops of sucrose solution presented on colored targets) might be due at least in part to a difference in delay of reward attendant on greater difficulty in locating the 5-μ1 drops. Substantial reduction in the diameter of the targets, which was designed to facilitate location of the drops, impaired discrimination of the colors, perhaps because their salience was reduced in the process (Experiment 3). White dots used to mark the location of the drops on larger targets also impaired discrimination of the colors, which presumably were overshadowed by the dots (Experiments 1, 2, and 4). That the dots did not serve merely to equate delay but were themselves discriminated was demonstrated in Experiment 5, which produced as well the first indication of an effect of amount of reward uncontaminated by the possibility of differential delay: Animals trained with a 5-μl drop on a dotted target of one color and a drop of the same size on an undotted target of a second color preferred the dotted target, but animals trained with a 5-μl drop on a dotted target of one color and a 20-μl drop on an undotted target of a second color preferred the undotted target. In Experiment 6, with odors substituted for the colors on the assumption that they were less likely to be overshadowed by the dots, what could be interpreted as a pure amount effect was found again. Aside from their relevance to questions about the role of amount of reward, the results have some interesting implications for the theory of discriminative learning in honeybees.