A project undertaken at the School of Biosciences, The University of Melbourne, and supervised by Dr Devi Stuart-Fox
Animals have evolved remarkable and unique ways of controlling light (e.g. fluorescence, iridescence). Until now, biologists have focused almost exclusively on how ultraviolet (UV) and human-visible wavelengths are manipulated to produce the diversity of colours and optical effects we see in nature. However, the UV and visible parts of the spectrum represent less than half of the energy in sunlight. The majority of the energy in sunlight that is absorbed or reflected by animal surfaces is in near-infrared wavelengths invisible to animals. The way near-infrared wavelengths are controlled to affect body temperature is critical to survival; yet the adaptive significance of near-infrared variation remains almost entirely unexplored.
The objective of our project was to understand the adaptive significance of near-infrared (NIR) variation in Australian Christmas beetles (subfamily Rutelinae) and discover the nano-scale cuticle architectures that generate this variation.
Figure 1. Human-visible (400 – 700 nm) and near-infrared (NIR) reflectance spectra and photos of green patches from three beetle species. Although all three patches are similar colours (shades of green), they have radically different NIR reflectance and corresponding light vs. dark appearance in NIR photos (700 – 1100 nm).
We characterised how Christmas beetles and relatives (35 species) reflect light from the ultraviolet through human-visible to near-infrared wavelengths. We developed a method and set of parameters to characterise the full diversity of optical effects produced by natural materials, including angle-dependent effects (e.g. iridescence, metallic and glossy appearances), using Christmas beetles as case study.
We discovered that Christmas beetles have previously unrecognised diversity of near-infrared properties – including near-infrared iridescence! Scarabs such as Christmas beetles also reflect circularly polarised light. The degree to which they do so varies, despite using the same materials in their cuticle. Based on electron microscopy analyses, we have proposed and modelled alternative nano-structural arrangements that can account for these different near-infrared and polarisation properties. This has potential bioinspired applications, for example for sensors and coloured coatings.
Figure 2. The iridescent Callidemum hypochalceum beetle (top left) with close-up of one section (bottom left) and scanning electron micrograph of surface sculpturing and sub-surface cross-section (right).
Image 2: The great diversity of visual appearances in the group of Christmas beetles tells a story about the evolution of the nanoscale structures required to master light manipulation.
What is the biological significance of the near-infrared properties we have discovered? We have shown that the near-infrared properties of Christmas beetles significantly affect their heating rates. Reflecting a lot of near-infrared light could prevent Christmas beetles from overheating. Conversely, reflecting little near-infrared light could enable beetles to warm up rapidly to reach active temperatures. If near-infrared properties are important for regulating body temperature, we would expect near-infrared properties of different species to be correlated with the climate in which they occur. This is precisely what we found – but body size is also important.
This project has revealed hidden diversity in the unexplored realm of near-infrared reflectance in animals. It has identified new nano-structures to manipulate light and heat and contributed to fundamental knowledge of thermal adaptations in animals. Beyond scientific discovery, this project will provide biological models for the development of energy-efficient materials and is the basis for a project on ‘Super-iridescent, strong and beautiful materials inspired by the most successful animal on earth: Beetles’ sponsored by the University of Melbourne Hallmark Research Initiative in Bioinspiration.
