Color and Near-Infrared Reflectance Covary in Distinct Ways Across Taxa.

Solar radiation reaching Earth spans 300-2500 nm, covering ultraviolet (UV), visible (Vis), and near-infrared (NIR) wavelengths. Its reflectance is critical for animal signaling, camouflage, and thermoregulation, likely leading to selective trade-offs that shape diverse reflectance patterns. Nonetheless, little is known about associations among these spectral regions across different taxa. Here, we investigate if the reflectance in different regions of the spectrum (UV, Vis, NIR) is correlated using data from integuments and eggs of 322 endotherms and ectotherms. To disentangle these relationships, we examined correlation strength, slope, and intercept, thereby providing nuance on consistency of association, magnitude of change across wavelengths, and baseline differences among taxa. We find that UV and Vis reflectance are significantly positively correlated across all taxa, whereas Vis-NIR correlation strengths are variable. For instance, in integuments, while the slopes of the Vis-NIR reflectance are positively correlated, their intercepts differ among taxa, suggesting that baseline NIR reflectance relative to Vis is influenced by macromolecular composition and/or structure. Moreover, in eggshells, UV-Vis and Vis-NIR slopes are also positively correlated, except for birds that show no significant Vis-NIR correlation. Our findings suggest that reflectance across wavelengths can be decoupled, laying a foundation for understanding how reflectance traits evolve under physiological and ecological pressures.