The Influence of Prior Learning Experience on Pollinator Choice: An Experiment Using Bumblebees on Two Wild Floral Types of Antirrhinum majus

Abstract

Understanding how pollinator behavior may influence pollen transmission across floral types is a major challenge, as pollinator decision depends on a complex range of environmental cues and prior experience. Here we report an experiment using the plant Antirrhinum majus and the bumblebee Bombus terrestris to investigate how prior learning experience may affect pollinator preferences between floral types when these are presented together. We trained naive bumblebees to forage freely on flowering individuals of either A. majus pseudomajus (magenta flowers) or A. majus striatum (yellow flowers) in a flight cage. We then used a Y-maze device to expose trained bumblebees to a dual choice between the floral types. We tested the influence of training on their choice, depending on the type of plant signals available (visual signals, olfactory signals, or both). Bumblebees had no innate preference for either subspecies. Bumblebees trained on the yellow-flowered subspecies later preferred the yellow type, even when only visual or only olfactory signals were available, and their preference was not reinforced when both signal types were available. In contrast, bumblebees trained on the magenta-flowered subspecies showed no further preference between floral types and took slightly more time to make their choice. Since pollinator constancy has been observed in wild populations of A. majus with mixed floral types, we suggest that such constancy likely relies on short-term memory rather than acquired preference through long-term memory induced by prior learning.

Fig 1. Y-maze apparatus used for bumblebee behavioral tests.

(a) Position of plexiglas plant chambers facing the arm ends. The chamber at the left contains no plant (control: empty pot with a wooden stick). The chamber at the left contains a A. majus pseudomajus plant with four open flowers. Connectors for plant scent collection are visible on the top of the chambers. (b) Fan causing an air flow from arm ends to the main channel. The hole above the fan was used to put the bees inside the Y-maze. The Y-maze neutral zone is red-shadowed.

Fig 2. Flight cage used for bumblebee training procedure.

Plants are of the subspecies A. majus striatum (yellow-flowered).

Fig 3. Color loci of plants used in bumblebee behavioral tests, in the B. terrestris color perception space.

Mean position ± SE (circle) per plant, on measures taken on the upper and lower petal on one to two flowers per plant. Magenta dots: pseudomajus plants; golden dots: striatum plants.

Fig 4. Partial least square-discriminant analysis (PLS-DA) on the 37 quantifiable VOCs in floral scent of A. majus subspecies (black: A. m. pseudomajus; gray: A. m. striatum).

(a) correlation circle of the most discriminant VOCs, and (b) samples projection, on principal components 1 and 2. (c) correlation circle of the most discriminant VOCs, and (d) samples projection, on principal components 1 and 3. The variation explained by each of the principal components is shown as percentages along axes. Hepta': heptanal; 'Bmy': beta-myrcene; 'Hexa': hexanal; 'Octa': octanal; 'Undeca': undecanal: 'Dodeca': dodecanal; 'Deca': decanal; 'Gt': gamma-terpinene; #62, #121 and #190: unknown compounds.

Fig 5. Proportion of bees choosing the striatum plant in the three groups of bees, and for the two plant positions in the Y-maze.

Error bars are standard errors on model parameters estimated from non linear mixed model. Plant positions are specified on the x-axis by a left / right couple of spots (light gray: plant of the subspecies striatum; dark gray: plant of the subspecies pseudomajus). Letters show significance in log-likelihood ratio tests between proportions for each position, in the three groups of trained bees separately (naive bees: normal font; pseudomajus-trained bees: italic font: striatum-trained bees: bold font). Stars emphasize a significant deviation from random choice (proportion 0.5). Samples sizes are provided for each group.

Fig 6. Time to first choice in the Y-maze experiment.

Mean ± SE, for naive bees, bees trained on pseudomajus plants, and bees trained on striatum plants. Samples sizes are indicated below error bars.