Reflection of near-infrared light confers thermal protection in birds

Abstract

Biologists have focused their attention on the optical functions of light reflected at ultraviolet and human-visible wavelengths. However, most radiant energy in sunlight occurs in ‘unseen’ near-infrared (NIR) wavelengths. The capacity to reflect solar radiation at NIR wavelengths may enable animals to control heat gain and remain within their critical thermal limits. Here, using a continent-wide phylogenetic analysis of Australian birds, we show that species occupying hot, arid environments reflect more radiant energy in NIR wavelengths than species in thermally benign environments, even when controlling for variation in visible colour. Biophysical models confirm that smaller species gain a greater advantage from high NIR reflectivity in hot, arid environments, reducing water loss from compensatory evaporative cooling by up to 2% body mass per hour. These results highlight the importance of NIR reflectivity for thermal protection, which may become increasingly critical as the frequency of extreme climatic events increases.

Reconciling ecogeographical rules: rainfall and temperature predict global colour variation in the largest bird radiation

“…; Medina et al . ). These conflicting results have also been found by some studies of intraspecific variation (Aldrich & James ; Morales et al .…”

“…Indeed, recent comparative analyses on birds have revealed patterns that seemingly oppose some aspects of Gloger's rule, namely that species with darker plumage are often found in colder rather than warmer regions (Friedman & Reme s 2017;Delhey 2018;Galv an et al 2018;Medina et al 2018). These conflicting results have also been found by some studies of intraspecific variation (Aldrich & James 1991;Morales et al 2017;Fargallo et al 2018).…”

Body coloration and mechanisms of colour production in Archelosauria: The case of deirocheline turtles

“…Therefore, the function of dermal collagen fibres may be to prevent long infrared wavelengths from reaching the body core to avoid overheating. A thermoprotective function of colour-producing structures has been recently suggested for the deep iridophores found in chameleons [55] or plumage of birds [131]. The role of melanin in thermoregulation is widespread across a broad range of organisms [132].…”

Macroevolutionary bursts and constraints generate a rainbow in a clade of tropical birds

“…An OU model best fit most wing and body patches, which suggests either a constraint on evolution to new states, or selection not captured by our PGLS analysis with colour and climate. In the forest canopy, green body and wing colour may serve the purpose of camouflage against predation [6,55], while brighter plumage colours may serve as signals. The tradeoff between 375 psittacofulvin-based signaling and crypsis has been observed in the reversed sexually dichromatic parrot Eclectus roratus, where bright-red female plumage advertises nesting sites, and green plumage helps foraging males avoid predation through camouflage [21].…”

“…Animals and plants express a dazzling range of colours. Colour has a direct impact on fitness through signaling [1][2][3][4][5], camouflage [2][3][4], and thermoregulation [6][7][8], and is therefore a key component of biological diversification. For birds in particular, plumage colour is integral to life history and evolution.…”