Foraging errors play a role in resource exploration by bumble bees (Bombus terrrestris)
Supplementary Information
Lisa J. Evans1*, Nigel E. Raine1,2
1 School of Biological Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK
2 School of Environmental Sciences, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
* email:
Innate colour preference test
METHODS
The aim of this test was to find a third flower colour that bees could distinguish from the blue and yellow artificial flowers used in previous learning studies (e.g. Raine et al. 2006, Raine & Chittka 2008), and also produced similar levels of innate attraction in colour naïve bees as these yellow artificial flowers. In addition to blue (Perspex 727) and yellow (Perspex 260) flowers, we tested the choice responses of bees to four more solidPerspex colours: cream (Perspex 128), pink (Perspex 4415), green (Perspex 6205) and red (Perspex 433). To test forager preference for these colours we individually trained colour naïve bees (n = 23) to forage for 50% (v/v) sucrose solution on colourless, transparent artificial flowers (25 x 25 mm) within a flight arena (120 x 100 x 35 cm). Bees that left the colony and fed on the colourless, transparent flowers for at least five consecutive foraging bouts were deemed to be motivated foragers and were selected for colour preference testing. These individuals were presented with an array of 30 unrewarded artificial flowers: five flowers from each of the six different colours. Flowers were unrewarded (empty) to avoid foragers having the opportunity to form associations between any colour and reward, and flowers were placedat random spatial positions in the flight arena. We recorded the choice sequence of eachindividual bee during the test, noting the colour of flowers it approached. The mean number of times each flower colour was chosen was then calculated for the first 30 choices made byeach tested forager (Figure S1). Each bee was then re-tested with two different pairs of colours from the original six colours (e.g., bluevs.green and yellow vs pink). Again the mean number of times each flower colour was chosen was then calculated(Figure S2).
The spectral reflectance of all six artificial flowers were measured in 1 nm increments over the wavelength range from 300 to 700 nm using a spectrophometer (Ocean Optics S2000) with a deuterium/ halogen light source (Chittka & Kevan 2005; Figure S3A). Colour loci for each of the six flowers were calculated from these reflectance data and plotted in the bee colour hexagon (Chittka 1992; Figure S3B).
RESULTS
There was significant variation in the number of flower visits received by each colour, with cream flowers being strongly preferred (Figure S1). Red flowers received the next highest number of visits, although not significantly more than blue flowers. Green, pink and yellow flowers all received a similarly low number of visits. Cream and red were not suitable to use as our novel flower colour in experiment 2 as they were both at least as innately attractive as blue flowers. The remaining two colours, pink and green, elicited similar responses to blue and yellow flowers in the innate colour preference tests suggesting they are similarly attractive to colour-naïve bees. Distances between loci for all colour pairs are larger than 0.045 hexagon units (Table S1) the limit for colour discrimination in B. terrestris (Dyer and Chittka 2004). Green was selected as our novel colour for experiment 2 because behavioural data from pairwise choice tests (blue vs green and yellow vs green) confirmed it was easily distinguishable from blue and yellow flowers (Figure S2), and its colour locus falls between these two colours.
Figure S1. Mean (± SE) number of visits to each flower colour made by23 colour-naïve foragers. Means were calculated from the first 30 choices made by each bee. Different letters indicate significant differences among flower colours (Tukey’s HSD, p <0.05).
Figure S2. Mean (± SE) number of visits to each flower colour made by 7 colour-naïve foragers. Different letters indicate significant differences between flower colours (Independent samples t test, p <0.05).
Figure S3. Spectral reflectance of the six artificial flowers colours tested and the arena background (floor colour of the flight arena) (a) and their colour loci within the bee colour hexagon (b).Spectral reflectance can vary from 0 (no reflectance) to 1 (all incident light is reflected). Reflectance functions for each flower type were measured in 1 nm increments from 300 to 700 nm using a spectrophometer (Ocean Optics S2000) with a deuterium/ halogen light source (Chittka & Kevan 2005). Differences in reflectance above 650 nm are not relevant for bees since their visual spectrum ends around that value (Chittka 1992; Peitsch et al. 1992). (b) The distance from the centre of the hexagon (zero) to each vertex is 1 hexagon unit. The locus generated by a coloured object within the hexagon informs us how bees will perceive the object through their ultraviolet, blue and green photoreceptors, and through further processing of receptor signals in the central nervous system. Each object, such as a flower, is categorised into one of the six bee-subjective colour categories defined by the colour hexagon (ultraviolet (U), UV-blue (UB), blue (B), blue-green (BG), green (G), and UV-green (UG)), depending on which of the three colour receptors of bees (UV, blue or green) they stimulated most strongly (Chittka 1992, Chittka & Kevan 2005). The spectral reflectance of all six artificial flower colours was quantified for the spectral properties of the fluorescent lighting used in laboratory colour tests (Figure S1), and converted into colour loci in bee colour space (Dyer & Chittka 2004, Chittka & Kevan 2005). These bee-subjective colour loci for the six artificial flower colours used in the laboratory preference tests are indicated by circles coloured as they would appear to humans.
Flower colour / Blue / Yellow / Green / Purple / RedYellow / 0.44
Green / 0.38 / 0.07
Purple / 0.15 / 0.52 / 0.46
Red / 0.24 / 0.38 / 0.32 / 0.21
Cream / 0.15 / 0.3 / 0.23 / 0.25 / 0.22
Table S1:Perceptual distances between pairs of colour loci within the hexagon for all six artificial flower colours tested (colour hexagon units).
REFERENCES
Chittka, L. 1992. The color hexagon - a chromaticity diagram based on photoreceptor excitations as a generalized representation of color opponency. Journal of Comparative Physiology A, 170, 533–543.
Chittka, L. & Kevan, P. G. 2005. Flower colour as advertisement. In: Dafni A, Kevan PG, Husband BC, eds. Practical Pollination Biology. CambridgeOntario: Enviroquest Ltd. pp 157–196.
Dyer, A.G. & Chittka, L. 2004. Biological significance of distinguishing between similar colours in spectrally variable illumination: bumblebees (Bombus terrestris) as a case study. Journal of Comparative Physiology A, 190, 105–114.
Peitsch, D., Feitz, A., Hertel. H., de Souza, J., Ventura, D. F. & Menzel, R. 1992. The spectral input systems of hymenopteran insects and their receptor-based colour vision. Journal of Comparative Physiology A, 170, 23–40.
Raine, N. E. & Chittka, L. 2008. The correlation of learning speed and natural foraging success in bumble-bees. Proceedings of the Royal Society B, 275, 803-808.
Raine, N. E., Ings, T. C., Ramos-Rodriguez, O. & Chittka, L. 2006. Intercolony variation in learning performance of a wild British bumblebee population (Hymenoptera: Apidae: Bombus terrestris audax). Entomologia Generalis, 28, 241-256.
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