The wide variety of colours exhibited by animals have long been a source of fascination and conjecture, but procedural and theoretical limitations have reduced our ability to explore the wide range of possible relationships between colour traits and environmental variables. In this study, we developed a novel procedure to identify and quantify melanin pigments, using a combination of Raman microspectroscopy and cuticle sectioning, in order to explore whether cuticular melanisation in ant species changed with position on a gradient of 14.9 to 20.7 MJ/m2 average annual solar exposure in southern Australia. We tested the relationships among species traits, environmental variables (temperature, humidity, solar radiation and luminance) and cuticular melanisation measured over the bioclimatic gradient using generalised linear mixed models (GLMM) with a Gaussian distribution.
We also compared commonly employed colour traits thought to be associated with cuticular eumelanin concentrations (‘lightness’ scores and colour metrics obtained through multispectral photography) to our more robust measure.
Results/Conclusions
Our novel procedure was able to successfully identify eumelanin (Raman bands at approximately 1400 and 1600cm-1) and/or pheomelanin (500, 1500 and 2000cm-1), as well as provide an indication of pigment distribution and a relative measure of melanin concentration within ant cuticle sections.
Using traditional colour trait measurements, we found that, with the notable exception of thermophilic species (Melophorus spp.), ants with ‘darker’ bodies were active at higher levels of solar radiation and tended to be diurnal. This largely agrees with the findings of previous studies, but also provided significant underestimates of the relative quantity of eumelanin in several iridescent and thermophilic species, when compared to our new approach. Additionally, a wide range of species contained pheomelanin, which is not of any known photo-protective benefit or well understood functionally.
Our study suggests that eumelanin is positively associated with solar exposure in diurnal species due to the protection from UV-radiation attributed to melanin-based colours. Past measurements, whether through digital photography or visual assessment, could not reliably indicate the quantity or indeed presence of cuticular melanins for the species studied.
Empirical investigations of trait function are critical to ensure that the ecological importance of trait-environment relationships are better understood.