Why many Batesian mimics are inaccurate: evidence from hoverfly colour patterns

Mimicry is considered a classic example of the elaborate adaptations that natural selection can produce, yet often similarity between Batesian (harmless) mimics and their unpalatable models is far from perfect. Variation in mimetic accuracy is a puzzle, since natural selection should favour mimics that are hardest to distinguish from their models. Numerous hypotheses exist to explain the persistence of inaccurate mimics, but most have rarely or never been tested against empirical observations from wild populations. One reason for this is the difficulty in measuring pattern similarity, a key aspect of mimicry. Here, we use a recently developed method, based on the distance transform of binary images, to quantify pattern similarity both within and among species for a group of hoverflies and their hymenopteran models. This allowed us to test several key hypotheses regarding inaccurate mimicry. We find that levels of phenotypic variation are similar across most hoverfly species and do not correlate with body size, providing evidence against the idea that selection is more relaxed in less accurate mimics. We also show that mimics do not have to compromise between accuracy to multiple model species. We find that darker-coloured hoverflies are less accurate mimics, which could lead to a trade-off between mimicry and thermoregulation in temperate regions. Our results shed light on a classic problem concerning the limitations of natural selection