Flower Iridescence Increases Object Detection in the Insect Visual System without Compromising Object Identity.

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Whitney, Heather M 
Reed, Alison 
Rands, Sean A 
Chittka, Lars 
Glover, Beverley J 

Iridescence is a form of structural coloration, produced by a range of structures, in which hue is dependent on viewing angle [1-4]. One of these structures, the diffraction grating, is found both in animals (for example, beetles [2]) and in plants (on the petals of some animal pollinated flowers [5]). The behavioral impacts of floral iridescence and its potential ecological significance are unknown [6-9]. Animal-pollinated flowers are described as "sensory billboards" [10], with many floral features contributing to a conspicuous display that filters prospective pollinators. Yet floral iridescence is more subtle to the human eye than that of many animal displays because the floral diffraction grating is not perfectly regular [5-9]. This presents a puzzle: if the function of petals is to attract pollinators, then flowers might be expected to optimize iridescence to increase showiness. On the other hand, pollinators memorize floral colors as consistent advertisements of reward quality, and iridescence might corrupt flower color identity. Here we tested the trade-off between flower detectability and recognition, requiring bumblebees (Bombus terrestris) to identify artificial flowers that varied in pigmentation and degree of iridescence. We find that iridescence does increase target detectability but that "perfect" iridescence (produced by an artificial diffraction grating) corrupts target identity and bees make many mistakes. However, "imperfect" floral iridescence does not lead to mistaken target identity, while still benefitting flower detectability. We hypothesize that similar trade-offs might be found in the many naturally "imperfect" iridescence-producing structures found in animal-animal, as well as other plant-animal, interactions.

Animals, Bees, Color Perception, Flowers, Iridescence, Pattern Recognition, Visual, Time Factors
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Curr Biol
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Elsevier BV
We thank Divykriti Chopra, Matthew Dorling, Lucy Sandbach and James Philpott for assistance with experiments, and Edwige Moyroud for helpful discussions. We thank James Foster for assistance with measurement of flight arena light level measurements. HW is supported by ERC Starting Grant 260920. AR was supported by a BBSRC doctoral training grant studentship. LC is supported by a Royal Society Wolfson Research Merit Award and ERC Advanced Grant 339347.