The microstructure of white feathers predicts their visible and near-infrared reflectance properties.

Devi Stuart-Fox,Matthew D Shawkey,Elizabeth Newton,Branislav Igic,Liliana D’Alba,Raoul A Mulder,Daniel Osorio

doi:10.1371/journal.pone.0199129

Devi Stuart-Fox, Matthew D Shawkey + Show 5 more

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https://doi.org/10.1371/journal.pone.0199129

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Abstract

Research on the optical properties of animal integuments, including fur, feather, skin and cuticle, has focussed almost exclusively on animal-visible wavelengths within the narrow range of 300–700 nm. By contrast, the near-infrared (NIR) portion of direct sunlight, spanning 700–2600 nm, has been largely ignored despite its potentially important thermal consequences. We quantified variation in visible and NIR reflectance and transmission for white body contour feathers of 50 bird species, and examined how well they are predicted by feather macro- and micro-structural morphology. Both visible and NIR reflectance of the feathers varied substantially across species. Larger, thicker, and sparser feathers that are characteristic of larger species, and feathers with rounder barbs and more closely spaced barbules, had high average reflectance, particularly within avian-visible wavelengths (300–700 nm). Feathers with rounder barbs and more closely situated barbules also had high average reflectance, particularly for NIR wavelengths. Barb roundness and barbule density were the only predictors of NIR reflectance after accounting for variation in visible reflectance and body size. Our results highlight the potential for adaptive variation in NIR reflectance mediated by feather structure, which may inform the design of functional materials to control light and heat.

Highlights

Animal integuments reflect sunlight, which comprises ultraviolet (UV, 300–400 nm), humanvisible (VIS, 400–700 nm), and near-infrared (NIR, 700–2600 nm) wavelengths, which are invisible to the human eye
Variation in reflectance properties and feather morphology among species Both visible and NIR reflectance spectra for white feather patches were similar in shape across species but varied substantially in their mean values
Average visible reflectance ranged from 25.8% for the hoary redpoll, Carduelis hornemanni, to 52.4% for the American white pelican, Pelecanus erythrorhynchos, whereas average NIR reflectance ranged from 29.1% for the hoary redpoll, Carduelis hornemanni, to 61.3% for the belted kingfisher, Megaceryle alcyon

Summary

Introduction

Animal integuments reflect sunlight, which comprises ultraviolet (UV, 300–400 nm), humanvisible (VIS, 400–700 nm), and near-infrared (NIR, 700–2600 nm) wavelengths, which are invisible to the human eye. Reflection or absorption of solar radiation within NIR wavelengths affect heat gain at an animal’s surface and critical thermal limits for survival. This is because surface heat gain depends on the proportion of total incident solar energy that is absorbed or reflected by the body surface (in the absence of convection), and about 55% of the energy in sunlight falls within NIR wavelengths (45% within UV-VIS) [1]. NIR reflectance influences the thermal balance of organisms, its potential function in visual communication (e.g. camouflage, communication) are likely minimal or absent (reviewed in [2]).

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