Flower iridescence: what might a bee see? (#416)
The evolution of flowers to suit the visual systems of important pollinators is a classic system for understanding principles of visual ecology.1 Recent work shows that bees provided with extended appetitive- aversive differential condition can learn to discriminate between stimuli presenting different iridescence signals in controlled laboratory conditions. 2 This suggests that structural colours may act to enhance pigment colour to facilitate reliable pollination, and a variety of plant species contain structural colours.3 To date it has been unclear the extent to which iridescence might act as a robust signal in complex environments, where bees would approach multiple con-specific flowers from a variety of different viewpoints that potentially confound signal integrity.4 This visual problem is important because the bee’s compound eye has limited acuity, and for reliable signalling, iridescence should need to maintain signal integrity independent of approach angle. Therefore, we used linearised digital imaging 5 6 to capture different viewpoints (Figure a,e) in a comprehensive hemispherical grid to allow for precise mapping of both colour and iridescence components considering a model of bee colour perception.2 7 Images were filtered (Fig. b-d, f-h) with a ‘mechano-optical’ array to map bee acuity. 8 We show that whilst iridescence ‘signal’ integrity can be maintained considering the flight approach angle by a bee on the x-y plane of the hemispherical grid (linear-circular association P-value = 0.648), in the z-axis there is a significant change in the properties of image information that would be available to a free flying bee depending upon angle (linear-circular association P-value = 0.008) . We conclude that iridescence could be used as a cue considering certain restricted viewing conditions by a bee, but such visual information would not necessarily be a robust signal when considering the complex, three-dimensional space navigated by free flying pollinators.
Figure: Alyogyne huegelii from different viewpoints (upper/lower row). c, g threshold masks indicating perceivable iridescence. d,h overlay of mask images mapped onto pigment-based colour image.
- Dyer A.G., Boyd-Gerny S., McLoughlin S., Rosa M.G.P., Simonov V., Wong B.B.M. 2012 Parallel evolution of angiosperm colour signals: common evolutionary pressures linked to hymenopteran vision. P Roy Soc Lond B Bio 279(1742), 3606-3615. (doi:10.1098/rspb.2012.0827).
- Whitney H.M., Kolle M., Andrew P., Chittka L., Steiner U., Glover B.J. 2009 Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators. Science 323, 130-133.
- Glover B.J., Whitney H.M. 2010 Structural colour and iridescence in plants: the poorly studied relations of pigment colour. Ann Bot-London 105(4), 505-511. (doi:10.1093/aob/mcq007).
- Kooi C.J., Dyer A.G., Stavenga D.G. 2014 Is floral iridescence a biologically relevant cue in plant–pollinator signalling? New Phytol 205(1), 18-20.
- Garcia J.E., Dyer A.G., Greentree A.D., Spring G., Wilksch P.A. 2013 Linearisation of RGB Camera Responses for Quantitative Image Analysis of Visible and UV Photography: A Comparison of Two Techniques. PLOS ONE 8(11), e79534. (doi:10.1371/journal.pone.0079534).
- Stevens M., Párraga C.A., Cuthill I.C., Partridge J.C., Troscianko T.S. 2007 Using digital photography to study animal coloration. Biol J Linn Soc 90(2), 211-237. (doi:doi:10.1111/j.1095-8312.2007.00725.x).
- Chittka L. 1992 The colour hexagon: a chromaticity diagram based on photoreceptor excitations as a generalized representation of colour opponency. J Comp Physiol A 170(5), 533-543.
- Williams S., Dyer A. 2007 A photographic simulation of insect vision. J Ophthalmic Photogr 29(1), 10-14.