Functional implications of temperature‐correlated colour polymorphism in an iridescent, scarabaeine dung beetle

Abstract

1. In many Coleoptera, iridescent colouration is generated by exoskeleton ultra-structure, within which multilayer interference reflects only certain wavelengths. Published work indicates that the colour polymorphism shown by some iridescent beetles is under genetic control. However, the present study suggests environmental involvement in the polymorphic southern African dung beetle, Gymnopleurus humanus Macleay.2. At 24 study sites across a 1000-km latitudinal temperature gradient, population samples of G. humanus were dominated by blue individuals in the cooler south, by cupreous individuals in the warmer north, and by locally co-occurring blue, green and cupreous individuals in intermediate situations.3. Using digital reflectance spectrophotometry, we measured wavelength intensity values across the visible spectrum (400 -800 nm) and used the 70 measured specimens to estimate maximum reflectance from a further 3912 beetles. Differences in mean reflectance values between 24 populations were strongly correlated with average annual temperatures at study sites.4. Much stronger correlations between mean reflectance values and average temperatures of the cool dry season months suggest that the cross-climatic patterns may be related to interaction between breeding seasonality and development under different cooler temperatures.5. Published evidence suggests that inherent physical properties of cholesteric liquid crystals and their responses to different thermal conditions could, potentially, generate the different exocuticle ultra-structure responsible for different reflected colour wavelengths. Furthermore, colour polymorphism could be advantageous across a gradient from cooler to warmer climate as a result of the different thermal properties of different colours.6. Given the correlation with temperature, it is predicted that the prevailing reflected colour balance in southern populations would shift in response to global climatic change.

The physical mechanism of cuticular color in Phelotrupes auratus (Coleoptera, Geotrupidae)

“…2007). Based on these facts, Davis et al . (2008) hypothesized that the cuticular color of G. humanus was possibly under environmental control, influenced by thermal conditions experienced during its development.…”

“…Davis et al . (2008) showed that the southern African dung beetle Gymnopleurus humanus MacLeay exhibited geographic color polymorphism: blue individuals dominate the cooler south; cupreous ones the warmer north; and blue, green and cupreous ones co‐occur in intermediate situations.…”

“…(2008) also revealed that the cuticular color of this beetle (measured by wavelength at the reflectance peak) was significantly correlated with average monthly temperatures of the cool dry season at beetle collection sites. According to Davis et al . (2008), responding to different thermal conditions, cholesteric liquid crystals could potentially generate different structures responsible for different reflected colors.…”

The effect of skin reflectance on thermal traits in a small heliothermic ectotherm

“…Convincing support for the association between thermal traits and colour has been found in many invertebrate species (Davis et al, 2008;Forsman, 2000;Forsman et al, 2002;Harris et al, 2012;Jong et al, 1996;Kingsolver and Huey, 1998;Parkash et al, 2010;Telonis-Scott et al, 2011). For instance, in pygmy grasshoppers (Tetrix subdulata) darker phenotypes were shown to heat up faster and prefer higher body temperatures than paler phenotypes (Forsman, 2000).…”

Sources of variation in reflectance spectrophotometric data: a quantitative analysis using avian eggshell colours

“…The application of spectrophotometers has also spanned diverse habitat types and included the study of both terrestrial and aquatic vertebrate and invertebrate species (e.g. Marshall 2000; Siefferman & Hill 2003; Davis et al. 2008; Detto & Backwell 2009).…”