Manakins can produce iridescent and bright feather colours without melanosomes

Males of many species often use colourful and conspicuous ornaments to attract females. Among these, male manakins (family: Pipridae) provide classic examples of sexual selection favouring the evolution of bright and colourful plumage coloration. The highly iridescent feather colours of birds are most commonly produced by the periodic arrangement of melanin-containing organelles (melanosomes) within barbules. Melanin increases the saturation of iridescent colours seen from optimal viewing angles by absorbing back-scattered light; however, this may reduce the wide-angle brightness of these signals, contributing to a dark background appearance. We examined the nanostructure of four manakin species (Lepidothrix isidorei, L. iris, L. nattereri and L. coeruleocapilla) to identify how they produce their bright plumage colours. Feather barbs of all four species were characterized by dense and fibrous internal spongy matrices that likely increase scattering of light within the barb. The iridescent, yet pale or whitish colours of L. iris and L. nattereri feathers were produced not by periodically arranged melanosomes within barbules, but by periodic matrices of air and beta-keratin within barbs. Lepidothrix iris crown feathers were able to produce a dazzling display of colours with small shifts in viewing geometry, likely because of a periodic nanostructure, a flattened barb morphology and disorder at a microstructural level. We hypothesize that iridescent plumage ornaments of male L. iris and L. nattereri are under selection to increase brightness or luminance across wide viewing angles, which may potentially increase their detectability by females during dynamic and fast-paced courtship displays in dim light environments

Exposure to UV radiance predicts repeated evolution of concealed black skin in birds

Plumage is among the most well-studied components of integumentary colouration. However, plumage conceals most skin in birds, and as a result the presence, evolution and function of skin colour remains unexplored. Here we show, using a database of 2259 species encompassing >99% of bird genera, that melanin-rich, black skin is found in a small but sizeable percentage (5%) of birds, and that it evolved over 100 times. The spatial distribution of black skin follows Gloger's rule, which states that pigmentation of endothermic animals increases towards the equator. Furthermore, most black-skinned birds inhabit high irradiation regions, and tend to be bald and/or have white feathers. Thus, taken together, our results suggest that melanin-rich, black skin helps to protect birds against ultraviolet irradiation. More generally, our results illustrate that feathered skin colour varies taxonomically, ontogenetically and temporally, providing an additional dimension for avian colour research

Contributions of feather microstructure to eider down insulation properties

Insulation is an essential component of nest structure that helps provide incubation requirements for birds. Many species of waterfowl breed in high latitudes where rapid heat loss can necessitate a high energetic input from parents and use down feathers to line their nests. Common eider Somateria mollissima nest down has exceptional insulating properties but the microstructural mechanisms behind the feather properties have not been thoroughly examined. Here, we hypothesized that insulating properties of nest down are correlated to down feather (plumule) microstructure. We tested the thermal efficiency (fill power) and cohesion of plumules from nests of two Icelandic colonies of wild common eiders and compared them to properties of plumules of wild greylag goose Anser anser. We then used electron microscopy to examine the morphological basis of feather insulating properties. We found that greylag goose down has higher fill power (i.e. traps more air) but much lower cohesion (i.e. less prone to stick together) compared to common eider down. These differences were related to interspecific variation in feather microstructure. Down cohesion increased with the number of barbule microstructures (prongs) that create strong points of contact among feathers. Eider down feathers also had longer barbules than greylag goose down feathers, likely increasing their air-trapping capacity. Feather properties of these two species might reflect the demands of their contrasting evolutionary history. In greylag goose, a temperate, terrestrial species, plumule microstructure may optimize heat trapping. In common eiders, a diving duck that nests in arctic and subarctic waters, plumule structure may have evolved to maximize cohesion over thermal insulation, which would both reduce buoyancy during their foraging dives and enable nest down to withstand strong arctic winds

Contributions of feather microstructure to eider down insulation properties

Insulation is an essential component of nest structure that helps provide incubation requirements for birds. Many species of waterfowl breed in high latitudes where rapid heat loss can necessitate a high energetic input from parents and use down feathers to line their nests. Common eider Somateria mollissima nest down has exceptional insulating properties but the microstructural mechanisms behind the feather properties have not been thoroughly examined. Here, we hypothesized that insulating properties of nest down are correlated to down feather (plumule) microstructure. We tested the thermal efficiency (fill power) and cohesion of plumules from nests of two Icelandic colonies of wild common eiders and compared them to properties of plumules of wild greylag goose Anser anser. We then used electron microscopy to examine the morphological basis of feather insulating properties. We found that greylag goose down has higher fill power (i.e. traps more air) but much lower cohesion (i.e. less prone to stick together) compared to common eider down. These differences were related to interspecific variation in feather microstructure. Down cohesion increased with the number of barbule microstructures (prongs) that create strong points of contact among feathers. Eider down feathers also had longer barbules than greylag goose down feathers, likely increasing their air-trapping capacity. Feather properties of these two species might reflect the demands of their contrasting evolutionary history. In greylag goose, a temperate, terrestrial species, plumule microstructure may optimize heat trapping. In common eiders, a diving duck that nests in arctic and subarctic waters, plumule structure may have evolved to maximize cohesion over thermal insulation, which would both reduce buoyancy during their foraging dives and enable nest down to withstand strong arctic winds

A bony-crested Jurassic dinosaur with evidence of iridescent plumage highlights complexity in early paravian evolution

The Jurassic Yanliao theropods have offered rare glimpses of the early paravian evolution and particularly of bird origins, but, with the exception of the bizarre scansoriopterygids, they have shown similar skeletal and integumentary morphologies. Here we report a distinctive new Yanliao theropod species bearing prominent lacrimal crests, bony ornaments previously known from more basal theropods. It shows longer arm and leg feathers than Anchiornis and tail feathers with asymmetrical vanes forming a tail surface area even larger than that in Archaeopteryx. Nanostructures, interpreted as melanosomes, are morphologically similar to organized, platelet-shaped organelles that produce bright iridescent colours in extant birds. The new species indicates the presence of bony ornaments, feather colour and flight- related features consistent with proposed rapid character evolution and significant diversity in signalling and locomotor strategies near bird origins

Interactions between colour-producing mechanisms and their effects on the integumentary colour palette

Animal integumentary coloration plays a crucial role in visual communication and camouflage, and varies extensively among and within species and populations. To understand the pressures underlying such diversity, it is essential to elucidate the mechanisms by which animals have created novel integumentary coloration. Colours can be produced by selective absorption of light by skin pigments, through light scattering by structured or unstructured tissues, or by a combination of pigments and nanostructures. In this review, we highlight our current understanding of the interactions between pigments and structural integumentary tissues and molecules. We analyse the available evidence suggesting that these combined mechanisms are capable of creating colours and optical properties unachievable by either mechanism alone, thereby effectively expanding the animal colour palette. Moreover, structural and pigmentary colour mechanisms frequently interact in unexpected and overlooked ways, suggesting that classification of colours as being of any particular type may be difficult. Finally, we discuss how these mixtures are useful for investigating the largely unknown genetic, developmental and physical processes generating phenotypic diversity

Seasonal egg-mass variation and laying sequence in a bird with facultative brood reduction

Females may maximize their lifetime reproductive output by adjusting their investment in each breeding event to the perceived likelihood Of Success. The Blue-footed Booby (Sula nebouxii) is a long-lived seabird with facultative siblicide. We examined whether there is differential resource allocation to eggs with laying order and whether greater egg mass increases hatching probability, chick survival, hatching interval, and mass and size at hatching. We found that the relative investment in first and second eggs decreased as the season advanced: second eggs were slightly (1.5%) heavier than first eggs in early Clutches; by contrast, first eggs were 2% heavier than second eggs in late clutches. Accordingly, hatching probability increased with laying date for first eggs and decreased for second eggs. The mass of the egg increased hatching probability, and no effect on chick survival was detected. Laying interval increased after a heavier egg was laid, and heavier eggs produced heavier hatchlings. Hatching intervals were positively related to laying intervals, but egg mass was unrelated to the length of the incubation period and the hatching interval. Our results suggest that egg mass influences embryo survival and that Blue-footed Booby females may adaptively allocate resources to eggs of different laying order according to breeding conditions

Melanosomes : biogenesis, properties, and evolution of an ancient organelle

Melanosomes are organelles that produce and store melanin, a widespread biological pigment with a unique suite of properties including high refractive index, semiconducting capabilities, material stiffness, and high fossilization potential. They are involved in numerous critical biological functions in organisms across the tree of life. Individual components such as melanin chemistry and melanosome development have recently been addressed, but a broad synthesis is needed. Here, we review the hierarchical structure, development, functions, and evolution of melanosomes. We highlight variation in melanin chemistry and melanosome morphology and how these may relate to function. For example, we review what is known of the chemical differences between different melanin types (eumelanin, pheomelanin, allomelanin) and whether/how melanosome morphology relates to chemistry and color. We integrate the distribution of melanin across living organisms with what is known from the fossil record and produce hypotheses on its evolution. We suggest that melanin was present in life forms early in evolutionary history and that melanosomes evolved at the origin of organelles. Throughout, we discuss the (sometimes gaping) holes in our knowledge and suggest areas that need particular attention as we move forward in our understanding of these still-mysterious organelles and the materials that they contain