The hidden world of colours: a thermal story

This blog is part of our colourful countdown to the holiday season where we’re celebrating the diversity and beauty of the natural world. In this post, Johnathan Goldenberg of Ghent University unpicks the fascinating role of scale colouration in lizard temperature control and what this means under climate change.

Jonathan - Colour blindness test
Test yourself – can you spot a number? For many, this may be an easy questions, but for colour blind persons that is not the case. Credits: Wikipedia Commons Public Domain

Colour is around us everyday, and we often talk about colours like everyone has the same experience as us. Take this image on the side for example, do you see a number?

If so, you are likely not colour blind. Indeed, people affected by such vision deficiency have a decreased ability to ascertain colours, due to malfunctions with photoreceptor cells called cones located in the retina of eyes.

So, how can a colour blind and a normal colour vision person agree on a definition of colour? Following a lively debate, they agree that it is a perceptual attribute that depends on the spectral receptor type(s) and the neural processing. Despite this definition settling the matter, it unfolds two key points: colouration depends on 1) the receptor type and 2) the neural processing of the organism. As animals have different receptors and neural processing from ours, they likely perceive and experience their surrounding differently from us.  

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Colour blindness. We often give for granted that all of us can distinguish colours similarly. Fortunately, in recent years publishers migrated towards colour blind friendly prints, which significantly contributed to correctly interpret messages and graphics among all readers. Credits: characters produced by Warner Bros, text and layout by JG.

Animals’ colouration is used for functions as diverse as crypsis or communication. Moreover, the coloured surface itself (be it skin, feathers, scales) may serve for thermal purposes as well. This last function is exactly my field of interest, where I am exploring how the coloured surface of organisms influences their ecology and evolution in light of climate change.

To understand how colouration is linked to thermoregulation, it is useful to describe the sun’s electromagnetic radiation. Among its entire range falling on the earth’s surface only the Ultraviolet (UV: 250-380 nm), human Visible (Vis: 380-750 nm) and Near Infrared (Nir: 750-2500 nm) ranges are primarily responsible for heating surfaces. What we commonly refer to as colours is thus only a small range of sun’s energy-rich radiation. The ranges beyond (Nir) and before (UV) the Vis are hidden to our eyes, but still perform vital functions by contributing to absorb and reflect the incoming solar radiation. 

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Electromagnetic spectrum. What we visually perceive, the visible light, is only a small fraction of the entire electromagnetic spectrum. Credits: “emspect”, Cyberphysics, https://www.cyberphysics.co.uk/topics/light/emspect.htm, [November 11, 2021]

All things considered, a dark material such as a dark rock, absorbs more solar radiation – and therefore heat – than a bright material. This is also applicable to the surface of animals, where the pigments and structures that produce colouration also affect the absorption and reflection of solar radiation. In most cases, this is regulated by a class of multifunctional macromolecules called melanins, which can ultimately affect the body temperature of organisms by causing darker individuals to heat up faster. 

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Simplified flow of energy between dark-bright organisms and their environment. The schematic shows that darker surfaces (melanin rich) absorb more heat and, vice versa, brighter surfaces (melanin poor) reflect more. Drawing by Karen Bisschop. Source: Goldenberg et al. (2021). Adapted from Porter & Gates (1969) and Porter et al. (1973).

Colouration can thus help species to cope with different climatic conditions. In the rapidly changing environment that we currently inhabit, understanding the climatic patterns driving colour variation of organisms is vital because it allows conservation strategists to preventively take actions by protecting species or even populations displaying unique colour features (think about the arctic fox with its summer and winter coat and how important that is for its ecology).

Across the tree of life, researchers have focused their attention on ectotherms when studying the impact of colouration on the thermal balance of individuals, because differently from endotherms*, this group of organisms depends on their surrounding environment to achieve optimal body temperatures. Thus, changing climates may impose high constraints on their ecology.

A large group of ectotherms which occupy virtually all levels of the trophic chain and play a key role in the balance of an ecosystem are squamates (or scaled reptiles). And among the squamates, I have been fortunate enough to work with one of the most unique lizards currently alive: the cordylids of South Africa, an endemic southern African group of 70 species that evolved in the last 35 million years. We initially predicted that populations with darker scales will suffer most in forecasted increasing temperatures owing to their higher solar radiation absorption rates.

To test this, we first acquired morphological and physiological data from 12 cordylids species (272 individuals from 26 populations), and then we modelled their activity time (our proxy for viability) under future climatic scenarios. Contrary to our expectations, our findings suggest that darker integuments in these species will be favoured under global warming, and we believe that this linked with their current relatively cold environment which likely limits their viability. 

However, this is not the full story as climate change will cause not only warmer temperatures, but also an increase in temperature fluctuations, and less predictable weather. How species will cope with such conditions sparks conflicting views in the scientific community, where some propose that global warming will favour brightly coloured lizards to better reflect the heat, while others suggest that darker coloured lizards will be selected for in warm and humid environments as a protection mechanism against microbial infections (melanin can provide antimicrobial protection). 

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Cordylids colour variation. From this series of images, it is possible to appreciate colour variations across cordylid species. Some display colourful integuments (e.g. Pseudocordylus microlepidotus, Cordylus vittifer), others have a plain colour (e.g. C. niger). From top left to right, down on the right, bottom right to left, centre: C. cordylus (Western Cape), Smaug giganteus (Free State), C. macropholis (Western Cape), C. niger (Western Cape), S. vandami (Mpumalanga), P. microlepidotus (Eastern Cape), C. vittifer (Mpumalanga). Photos: JG.

All of this leaves us with the question of whether we can find global patterns under which climate selects for colouration. Research shows that species respond differently to environmental pressures, due to unique evolutionary trajectories which can ultimately influence adaptation success. While it may be possible to identify general environmental trends that select for specific colourations, it is likely harmful to evaluate conservation status based on those as each species has its distinctive evolutionary signature.

With this blog I hope I introduced you to the unique hidden world of colours, where thermoregulation is a central function for animals, especially ectotherms. Not only from a biological perspective, understanding the architecture and the ultimate-and-proximate causes regulating thermoregulation can provide us with new tools to develop bio-inspired materials to keep us cool. This is a fascinating avenue of exploration and should be of considerable global interest given the current climate trend. As such, another good reason to preserve as much biodiversity as possible is because we never know where the next bio-inspired discovery might come from. 

Discover more posts as part of our Colour Countdown series here.

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