Reflection of near-infrared light confers thermal protection in birds

Iliana Medina,Elizabeth Newton,Warren P Porter,Raoul A Mulder,Devi Stuart-Fox,Michael R Kearney

doi:10.1038/s41467-018-05898-8

Iliana Medina, Elizabeth Newton + Show 4 more

Open Access

https://doi.org/10.1038/s41467-018-05898-8

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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.

Highlights

Biologists have focused their attention on the optical functions of light reflected at ultraviolet and human-visible wavelengths
We tested whether NIR reflectivity varies in relation to thermal environment in a continent-wide analysis of 90 species (12%) of Australian birds from all major habitat types, including sea and shore birds, waterbirds, forest or arid specialists and habitat generalists (Supplementary Data 1)
Around 70% of the Australian continent is characterised by hot, dry environments where it is the upper rather than lower critical thermal limits that drive thermal adaptations[7]

Summary

Introduction

Biologists have focused their attention on the optical functions of light reflected at ultraviolet and human-visible wavelengths. 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. Smaller species, rather than large ones, may obtain larger thermal benefits from reflectivity in this part of the spectrum

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