Abstract
Many Batesian mimics are considered to be inaccurate copies of their models, including a number of hoverfly species which appear to be poor mimics of bees and wasps. This inaccuracy is surprising since more similar mimics are expected to deceive predators more frequently and therefore have greater survival. One suggested explanation is that mimics which appear inaccurate to human eyes may be perceived differently by birds, the probable agents of selection. For example, if patterns contain an ultra-violet (UV) component, this would be visible to birds but overlooked by humans. So far, indirect comparisons have been made using human and bird responses to mimetic stimuli, but direct colour measurements of mimetic hoverflies are lacking. We took spectral readings from a wide range of hoverfly and wasp patterns. They show very low reflectance in the UV range, and do not display any human-invisible colour boundaries. We modelled how the recorded spectra would be perceived by both birds and humans. While colour differences between wasps and hoverflies are slightly more distinct according to human visual abilities, bird vision is capable of discriminating the two taxa in almost all cases. We discuss a number of factors that might make the discrimination task more challenging for a predator in the field, which could explain the apparent lack of selection for accurate colour mimicry.
Highlights
Colour is widely used by animals as a signal, for example to attract mates (Andersson 1994)or as an anti-predator warning display (Ruxton et al 2004)
Some harmless organisms attempt to deceive predators by mimicking the display of a more dangerous “model”, in a process known as Batesian mimicry (Bates 1862)
To human eyes there is great variation in the degree of resemblance between mimics and models in nature, which raises the question of why the less accurate mimics persist in the face of predicted selection towards perfect resemblance (Edmunds 2000, Kikuchi and Pfennig 2013)
Summary
Colour is widely used by animals as a signal, for example to attract mates (Andersson 1994)or as an anti-predator warning display (Ruxton et al 2004). Colour is widely used by animals as a signal, for example to attract mates (Andersson 1994). Male blue tits (Cyanistes caeruleus) use an ultra-violet (UV) signal to attract a mate, which is striking to female conspecifics but invisible to humans (Andersson et al 1998). The butterfly Heliconius numata displays a colourful wing pattern that conveys different signals to other butterflies and to potential predators (Llaurens et al 2014). Cases like these demonstrate the importance of considering the signal receiver when assessing the colour component of any biological signal, and show that doing so can shed new light on well-studied systems. To human eyes there is great variation in the degree of resemblance between mimics and models in nature, which raises the question of why the less accurate mimics persist in the face of predicted selection towards perfect resemblance (Edmunds 2000, Kikuchi and Pfennig 2013)
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