Convergent evolution of sexual deception via chromatic and achromatic contrast rather than colour mimicry

Abstract

The Orchidaceae is characterised by the repeated evolution of sexual deception, one of the most specialised pollination strategies. In orchids, sexual deception involves long-range pollinator attraction via mimicry of female insect sex pheromones. At close range, visual signals involving colour mimicry, contrast to the background, and exploitation of pollinator sensory biases could attract pollinators, but remain largely untested. Here we focus on a remarkable system in which species from two only distantly related sexually deceptive orchid genera with strikingly different flowers (Drakaea livida and three species of Caladenia) share the same pollinator, males of the thynnine wasp Zaspilothynnus nigripes. We used spectral reflectance measurements and modelling to investigate pollinator perception of colour, including the first examination of overall colour patterns in flowers via colour pattern geometry analyses. Rather than closely matching the colours of female Z. nigripes, these orchids had strong chromatic and achromatic contrast against their backgrounds. For Caladenia, the sepals and petals show high contrast, while in D. livida, which has diminutive petals and sepals, it is the labellum that contrasts strongly against the background. Despite varying in colour, the Caladenia species all had strong withinflower contrast between a UV-bright central target (column and labellum) and a corolla of radiating stripes (petals and sepals). The colour pattern geometry analyses also indicated that the orchids' overall colour patterns are highly conspicuous against their backgrounds. Contrast, UV, and target patterns could all enhance detection, and exploit pollinators' innate preferences. Since colour contrast may function with a range of colours and floral forms, attracting pollinators via contrast rather than visual mimicry may be a critical but previously overlooked process facilitating the evolution of sexual deception.

Clothing color mediates lizard responses to humans in a tropical forest

“…The idea that animals better tolerate anthropogenic colors that are used in their species‐specific sexual displays might be explained through an existing sensory bias. Sensory biases for colors have been demonstrated in a variety of taxa from insects to birds and fish (Gaskett, Endler, & Phillips, ; Ninnes, Adrion, Edelaar, Tella, & Andersson, ; Spence & Smith, ), and many studies have examined color preferences in regard to foraging and mating (see Kemp & Grether, ). Yet, animals also show preferences for sexual colors outside of foraging and mating.…”

A Comprehensive Study of Orchid Seed Production Relative to Pollination Traits, Plant Density and Climate in an Urban Reserve in Western Australia

“…Australia is a global center of diversity for plants with deceptive pollination [15,16]. Orchids that attract insects by visual deception, exploit insect color perception biases and often resemble other flowers that contain nectar or pollen rewards and are scented [17][18][19][20]. In these orchids, some insect specificity can occur because the mimicked flowers are preferred by particular pollinators, but these usually include multiple categories of insects (Table A1).…”

The neural selection and the emergence of ‘beauty canons’ as signaling codes in co-evolving species

“…Indeed, the visual apparatus of insects has elementary functions similar to the simplest retinal functions. Pollinators have different types of photoreceptors with opponent visual receptive fields that differ in spectral sensitivities (Hempel de Ibarra et al 2014;Horridge 2015;Stach et al 2004) and display a preference for green and color contrast (Gaskett et al 2017;Osorio and Vorobyev 2005;Renoult et al 2013). They can select quantitative variation in floral coloration, possibly determining its evolution (Lynn et al 2005).…”