Thermal consequences of colour and near-infrared reflectance

Abstract

The importance of colour for temperature regulation in animals remains controversial. Colour can affect an animal's temperature because all else being equal, dark surfaces absorb more solar energy than do light surfaces, and that energy is converted into heat. However, in reality, the relationship between colour and thermoregulation is complex and varied because it depends on environmental conditions and the physical properties, behaviour and physiology of the animal. Furthermore, the thermal effects of colour depend as much on absorptance of near-infrared ((NIR), 700-2500 nm) as visible (300-700 nm) wavelengths of direct sunlight; yet the NIR is very rarely considered or measured. The few available data on NIR reflectance in animals indicate that the visible reflectance is often a poor predictor of NIR reflectance. Adaptive variation in animal coloration (visible reflectance) reflects a compromise between multiple competing functions such as camouflage, signalling and thermoregulation. By contrast, adaptive variation in NIR reflectance should primarily reflect thermoregulatory requirements because animal visual systems are generally insensitive to NIR wavelengths. Here, we assess evidence and identify key research questions regarding the thermoregulatory function of animal coloration, and specifically consider evidence for adaptive variation in NIR reflectance.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Keywords: animal coloration; energy budget; infrared; melanism; thermal melanism; thermoregulation.

Figure 1.

Examples of thermal melanism. The red form of the two-spot beetle (Adalia bipunctata) (a) cannot maintain as high body temperatures as the melanistic form (b), resulting in activity level differences [4]. Light-coloured species of girdled lizards (Cordylus) such as C. cordylus (c) occur in warmer and less overcast areas than darker species such as C. niger (d) which have smaller, usually cooler ranges [5]. North American warblers (Parulidae) with melanized legs, such as the palm warbler (Setophaga palmarum) (f) tend to remain at higher latitudes for longer and have more northerly distributed breeding ranges than warblers with pale legs, such as the Connecticut warbler (Oporornis agilis) (e) [6]. Image credits: (a) Ettore Balocchi; (b) Pavel Kirillov; (c) and (d) Susana Clusella-Trullas; (e) Matt Stratmoen; (f) David Inman.

Figure 2.

Sunlight reaching the Earth's surface (ASTM G173–03 standard irradiance spectrum for dry air) showing ultraviolet (UV), visible and near-infrared (NIR) components. NIR light contributes about 55% of the total energy in sunlight at sea level.

Figure 3.

(a) Comparison of reflectance spectra in two bird and two lizard species, showing the variable relationship between visible and NIR reflectance. (i) Male red-capped robin (Petroica goodenovii) red breast feathers; (ii) little black cormorant (Phalacrocorax sulcirostris) black breast feathers; (iii) tawny dragon lizard (Ctenophorus decresii) blue-grey dorsal surface; (iv) Cape girdled lizard (Cordylus cordylus) dorsal area. C. cordylus: S Clusella-Trullas 2012, unpublished data; all other spectra: E Newton 2016, unpublished data. Image credits: (i) Julie Burgher; (ii) Jon Sullivan; (iii) Claire McLean, (iv) S.C.-T. (b) Examples of intra-specific variation in full-spectrum reflectance with demonstrated effect on body temperatures. The higher reflectance of (i) white elytra in a subspecies of tiger beetle (Cicindela formosa gibsoni) compared with (ii) the structural red elytra in a second subspecies (C. f. pigmentosignata) results in a mean equilibrium temperature difference of 2.2°C [60]. Spectra (iii) and (iv) show temperature-dependent reflectance change from (iii) 40°C to (iv) 15°C in the central bearded dragon (Pogona vitticeps). Biophysical models suggest that this 15% change on overall reflectance could reduce the time taken to reach active body temperature by an average of 22 min per active day, saving 85 h of basking time throughout the activity season [21]. Tiger Beetle spectral data reproduced with permission from Schultz & Hadley [60]. Bearded dragon spectral data reproduced with permission from Smith et al. [21]. Image credits: (i) Ted MacRae; (ii) David Rogers; (iii) and (iv) Kathleen Smith.