Beyond colour: discovering how beetles manipulate near-infrared light to control their temperature

A project undertaken at the School of Biosciences, The University of Melbourne, and supervised by Dr Devi Stuart-Fox

Significance: 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 the near-infrared, and 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.

Aims: Our project aims to:

  1. Characterise variation in UV, visible and near-infrared reflectance in Australian beetles using digital imaging and spectrometry.
  2. Test whether this variation is adaptive by assessing correlations with climate, and combining biophysical models of energy exchange in varying microclimates with laboratory experiments to assess potential effects on fitness.
  3. Discover the sub-micron scale structures that generate variation in near-infrared reflectance using electron microscopy and optical modelling.

This project will reveal hidden diversity in the unexplored realm of near-infrared reflectance in animals; identify new nano-structures to manipulate light and heat; and contribute to fundamental knowledge of thermal adaptations. Beyond scientific discovery, this project will provide biological models for the development of the next generation of energy-efficient materials where the goal is to manipulate both optical and thermal properties (e.g. ‘cool coatings’) or direct specific wavelengths of light to maximise energy capture (e.g. layered next-generation solar cells).

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).

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).