Signal evolution and morphological complexity in hummingbirds (Aves:Trochilidae)

Abstract

Understanding how animal signals are produced is critical for understanding their evolution because complexity and modularity in the underlying morphology can affect evolutionary patterns. Hummingbird feathers show some of the brightest and most iridescent colors in nature. These are produced by optically complex stacks of hollow, platelet‐shaped organelles called melanosomes. Neither how these morphologies produce colors nor their evolution has been systematically studied. We first used nanoscale morphological measurements and optical modeling to identify the physical basis of color production in 34 hummingbird species. We found that, in general, the melanosome stacks function as multilayer reflectors, with platelet thickness and air space size explaining variation in hue (color) and saturation (color purity). Additionally, light rays reflected from the outer keratin surface interact with those reflected by small, superficial melanosomes to cause secondary reflectance peaks, primarily in short (blue) wavelengths. We then compared variation of both the morphological components and the colors they produce. The outer keratin cortex evolves independently and is more variable than other morphological traits, possibly due to functional constraints on melanosome packing. Intriguingly, shorter wavelength colors evolve faster than longer wavelength colors, perhaps due to developmental processes that enables greater lability of the shapes of small melanosomes. Together, these data indicate that increased structural complexity of feather tissues is associated with greater variation in morphology and iridescent coloration.

Mechanisms involved in the production of differently colored feathers in the structurally colored swallow tanager (Tersina viridis; Aves: Thraupidae)

“…For instance, iridescence (i.e., the change in color with incidence and/or observation angle and feather orientation) is considered a structural phenomenon. In bird feathers, iridescence is most frequently the result of constructive interference of light with a regular layered or crystal‐like arrangement of melanin granules (Eliason et al, 2020; Prum, 2006). The broadband absorption of light by melanin is also important for the elaboration of the iridescent color effect (Shawkey & D'Alba, 2017).…”

“…The study of the evolution of plumage coloration is increasingly focused on understanding how the different feather elements are combined and genetically regulated (Eliason et al, 2020; Fan et al, 2019; Funk & Taylor, 2019). Small adjustments in the regulatory processes leading to pigment deposition in growing feathers result in conspicuous plumage color differences between populations or species (Campagna et al, 2017; Lopes et al, 2016; Toews et al, 2017).…”

Evolution of brilliant iridescent feather nanostructures

“…Indeed, Gruson et al, 2019 found color production to be similar among patches with different melanosome types in hummingbirds. We speculate that high variability in melanosome type in hummingbirds and birds-of-paradise is not related to general optical benefits of specific melanosome types or modifications, but rather to general high rates of color change in these groups (Eliason et al, 2020; Parra, 2010). Our optical modeling results (Figure 6B-E) show that there are multiple ways to reach the same areas of color space—using different melanosome types.…”

Biophotonics of diversely coloured peacock tail feathers

“…[18][19][20][21] The extreme iridescence of hummingbird feathers is created by stacks of air-lled melanosome platelets in the barbules. 1,[22][23][24][25] Peacock feather barbules contain a rectangular lattice of solid melanosomes and air channels, which is a unique arrangement compared to other iridescent birds. The anatomy of peacock feathers has been studied in extensive detail by Durrer.…”