Bumblebees do not learn polarization cues in flower choice

Abstract

Until recently, sensitivity to the polarization of light had not been scrutinized in the context of flower choice in bees. In 2014, it was demonstrated that buff-tailed bumblebees (Bombus terrestris) could learn to distinguish between two artificial “flower” targets presenting patterns of different polarization angle orientations over 100 choices, but only when the polarization cue was in view of dorsal-most region of the eye, suggesting the presence of a previously undocumented dorsal rim area (DRA). The present study continues the investigation of the described phenomena behaviorally in a novel experimental setup and also provides the first images of a suggested dorsal rim area in B. terrestris. In contrast to the findings by Foster et al. (2014), we find no indication that differences in polarization patterns can be used by a B. terrestris worker for flower choice. This is put in to perspective through two additional experiments demonstrating the ability to learn to distinguish targets based on visual disparity; 1) an experiment for demonstrating the ability to distinguish targets of two different colors (blue and yellow) and 2) an experiment for demonstrating the ability to distinguish targets of different intensity, – both proving significant learning under the current experimental regime. Possible implications of the methods used and found conflicting results are discussed as it stands clear that more studies on the subject are needed.

Popular Abstract

Secret conversations of bees and plants

Bumblebees are very fond of flowers and they make no secret of it. The nectar and pollen of flowers provide the food for bees and other pollinating insects. The relationship is not one-sided; plants as in turn are equally fond of and dependent on bees for they are carrying pollen between their kin. Although we too find a flower beautiful, the flower is not talking to us, – it is advertising itself to the insect pollinators. While we can overhear parts of this conversation – vibrant colors, intricate shapes and wonderful scents, some parts we cannot perceive. Could signals shown by plants through polarized light be one of them?

What is polarized light and why is it that bumblebees can see it and we don’t? As light travels from a light source to our eye it moves like a wave, it oscillates, in all directions along its axis of motion. This is what we call normal, or unpolarized, light. As the light is reflected by molecules in the air, the surface of lake or a shiny leaf, something interesting happens: certain angles of oscillations become more common than others – the light becomes polarized (Fig. 1). While indistinguishable by us, in the insect world the ability to see and use this feature of light is common and its uses in navigation widespread. Honeybees can fly off into the unknown and find their way back to the hive by looking at the polarization patterns in the sky. Dung beetles, ants, crickets and Monarch butterflies - all use it to find their way. However, as far as we know, they do not use it to find their food - or in the context of bumblebees, the flowers. While being powerful tool for navigation, it can also problematic and interfere with the ability to see colors correctly and for this is one of the reasons humans do not perceive the polarization of light.

In 2014, a team of researchers in Bristol, UK, conducted a series of experiments involving bumblebees and flower choice and made very interesting discovery; it would seem that they can learn to pick flowers only based on their reflected polarization pattern. It was the first study to show the use of the polarization of light to choice between different flowers.

To investigate this phenomenon further we constructed a meadow of artificial flowers for bumblebees to choose from in our laboratory. In the experiments, the flowers were of two types, almost identical, but transmitting two different patterns of polarized light. To ask if the bees could learn to distinguish between the two, we made it so that one flowers with one of the patterns provided a reward of good-tasting sugar water, and the other provided only a very bitter tasting, though harmless, solution of quinine. Naturally, the bees would try to pick the good-tasting flower, if they could. But somehow, the bees just would not learn. Had we done something wrong? Instead of differences in polarization we now instead asked the bees to distinguish between colors – something we knew that they should be able to. And they did, easily. We then asked them about differences in brightness, and again, they learned. So was it we now, that had made a mistake, or was in the research group in UK?

We still do not know if they can or cannot learn polarization patterns when finding food. Maybe the truth lies somewhere in between - maybe it used as a part of another signal? It would not be the first of example of this, signals invisible to us that are discovered to be used in the communication between plants and pollinators. Bees have been shown to decide on which flower to visit based the electric field, or minute differences in the temperature, of flowers. These signals are part of a, to us, secret conversation between the insect pollinators and the plants. With more species of insects out there than there are of mammals, birds, fish, or reptiles combined, the question of what hidden signals they use to make decisions in their daily life remains a fascinating one, worth studying.

Supervisors: James Foster and Marie Dacke
Master´s Degree Project 60 credits 2015
Department of Biology, Lund University

• author: Ahlgren, Johan
• supervisor: James Foster; Marie Dacke
• organization: Degree Projects in Biology
• course: BIOP01 20151
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8567534
• date added to LUP: 2016-01-25 16:08:31
• date last changed: 2016-01-25 16:08:31