Cracks in the mirror hypothesis: High specularity does not reduce detection or predation risk

Abstract

1. Some animals, including certain fish, beetles, spiders and Lepidoptera chrysalises, have such shiny or glossy surfaces that they appear almost mirror-like. A compelling but unsubstantiated hypothesis is that a highly specular or mirror-like appearance enhances survival by reflecting the surrounding environment and reducing detectability.2. We tested this hypothesis by asking human participants to wear a mobile eyetracking device and locate highly realistic mirror-green and diffuse-green replica beetles against a variety of backgrounds in a natural forest environment. We also tested whether a mirror-like appearance enhances survival to wild predators by monitoring the survival of mirror-green and diffuse-green replica beetles in a forested habitat and an open habitat.3. Human participants showed no difference in the detection probability or detection latency of mirror versus diffuse replica beetles, indicating that mirror-like appearance does not impair prey capture. The field predation experiment found no difference in survival between the mirror and diffuse replica beetles in forested environments. Similarly, there was no difference in survival when beetles were deployed in the open habitat where there is no background to reflect, indicating that predators detect and do not actively avoid mirror-like beetles. 4. Our results suggest that a mirror-like appearance does not reduce attack by predators. Instead, highly specular, mirror-like surfaces may have evolved for an alternate visual function or as a secondary consequence of selection for a non-visual function, such as thermoregulation.

On metallic luster and iridescence in animal coloration

“…Specifically, these studies compared the attack rates (by bird predators) in the wild on replica beetles with either glossy or matte elytra. In one study, survival rates between glossy and matte beetles of a similar green hue placed in natural environments did not differ (Franklin et al, 2022), while a more recent study found that glossy beetles of three different hues were consistently easier to detect for bird predators than matte replicas (Thomas et al, 2014). Thus, while there is not yet much direct evidence to suggest that animals pay attention to glossy objects, it has not been tested in many species or contexts.…”

Pretty Cool Beetles: Can Manipulation of Visible and Near-Infrared Sunlight Prevent Overheating?

“…Although the biological significance of these highly metallic appearances is still unclear ( Thomas et al. 2007 ; Franklin et al. 2021 ), it may be premature to exclude a thermal function.…”