Bird Integumentary Melanins: Biosynthesis, Forms, Function and Evolution

Abstract

Melanins are the ubiquitous pigments distributed in nature. They are one of the main pigments responsible for colors in living cells. Birds are among the most diverse animals regarding melanin-based coloration, especially in the plumage, although they also pigment bare parts of the integument. This review is devoted to the main characteristics of bird melanins, including updated views of the formation and nature of melanin granules, whose interest has been raised in the last years for inferring the color of extinct birds and non-avian theropod dinosaurs using resistant fossil feathers. The molecular structure of the two main types of melanin, eumelanin and pheomelanin, and the environmental and genetic factors that regulate avian melanogenesis are also presented, establishing the main relationship between them. Finally, the special functions of melanin in bird feathers are also discussed, emphasizing the aspects more closely related to these animals, such as honest signaling, and the factors that may drive the evolution of pheomelanin and pheomelanin-based color traits, an issue for which birds have been pioneer study models.

Critical Analysis of the Melanogenic Pathway in Insects and Higher Animals

“…Pheomelanin is a branch point in the overall melanin biosynthetic process [2,3,5,8,9,42]. Dopaquinone as well as dopamine quinone that are formed by the action of tyrosinase exhibit instantaneous nonenzymatic addition reactions with endogenous thiols such as cysteine producing thiolated catecholamines.…”

“…The cysteinyl amino group is situated in a suitable position to form a cyclic adduct. This reaction gives rise to a quinone imine that could either undergo decarboxylative aromatization or simple isomerization coupled aromatization producing the benzothiazine and its 3-carboxylic derivative respectively [2,3,5,8,9,42,51,52,53]. Oxidative polymerization of these two derivatives leads to the majority of the red to brown colored pheomelanin pigments observed in animals (Figure 11).…”

“…Neuromelanin is produced in substantia nigra and is made up of dopamine derived polymeric materials. Studies on the biosynthesis, properties, function and structure of melanins have been thoroughly examined by numerous research groups worldwide for several decades [1,2,3,4,5,6,7,8,9]. However, several aspects of melanin biochemistry including biosynthesis, metabolism, structure, biological and molecular biological aspects of the enzymes associated with the process are yet to be unraveled.…”

TIME-OF-DAY BUT NOT SEASONALITY INFLUENCES FEEDING AND DIURNAL TIME ACTIVITY BUDGET OF ADULT RED-CAPPED LARK (Calandrella cinerea).

“…For the Red-capped Lark, 1242 preening rates were significantly higher during the non-breeding than breeding season. Since temperatures tend to be higher during the non-breeding season when it becomes hot and dry, some of the factors that may explain differences in preening include ; the need to increase preening effectiveness when feathers are exposed more to the sunlight given that most ecto-parasites are negatively phototactic (Koop et al, 2012); and the need to improve maintenance/strengthen the feathers to reduce the damaging effects of UV radiation (Galvan & Solano, 2016) when birds are exposed to increased temperatures in the open grassland area.…”

Introductory Chapter: Melanin, a Versatile Guardian

“…In insects, melanin is used even more resourcefully, not only for pigmentation of the exoskeleton but also for cuticle hardening, wound healing and in their innate immune responses [5]. Melanin in birds is responsible for coloration of the plumage and the exposed bare parts of the integument [6].…”