How is the striking coloration of the plumage of some birds produced?

    A study reveals that the production of colored ornaments mediated by carotenoids in birds is metabolically associated with the regulation of cellular respiration, so that the intensity of the coloration developed can be a direct indicator of the physiological state of an individual.


    In the world of birds, plumage coloration is often used as a means of communication between conspecifics. A good example is found in the males of some bird species that have developed striking coloration compared to females. This physical trait, which obeys the evolutionary force of sexual selection, allows males to "inform" females about their reproductive quality, their state of health or their ability to survive diseases or adverse environmental conditions, so that those with a more intense coloration they acquire a competitive advantage in reproduction as they are perceived by females as the best parents for their offspring.

    When the striking coloration of a physical ornament is due to the presence of yellow, orange and/or red tones, it is normally produced by a series of complex metabolic mechanisms involving a group of organic pigments known as carotenoids, which can only be acquired through diet. On the other hand, these pigments also fulfill various physiological functions, such as neutralizing harmful free radicals that are produced as a result of cellular metabolism, thanks to their antioxidant capacity. But how is it explained that pigments ingested through the diet, which also perform these other physiological functions, can determine the reproductive success of a bird?

     

    In the case of the common crossbill (loxia curvirostra) there is no evidence that the striking reddish coloration of the males has an effect on sexual selection, so this species is ideal for studying the metabolic processes that regulate the transformation of carotenoids, which is key to understanding the evolutionary basis of sexual selection for these color traits.

    La first hypothesis Formulated to explain the evolution of striking colors produced by carotenoids in vertebrates, it assumes that carotenoids are not easy to obtain in nature as they are scarce in the diet. Acquiring irresistible color ornaments for females would imply the need for males to spend time and energy searching for foods rich in carotenoids.

    Based on this hypothesis, other researchers suggested that, in addition to the effort to find carotenoids with food, there would be a need to offsetting carotenoids that are assigned to showy coloration versus those that are assigned to important physiological functions.

    Accordingly, “inverting” carotenoids in plumage coloration could be a cost for survival, since these carotenoids, which are only obtained with great effort, would not be available to serve as an antioxidant defense and prevent diseases. Only males with abundant carotenoids would be capable of resolving this physiological commitment well and showing intense coloration, thus being chosen by females as the best.

    But a new investigation, carried out by scientists from the National Museum of Natural Sciences (MNCN-CSIC), the University of Turku (Finland), the Instituto de Investigación en Recursos Cinegéticos (IREC – CSIC, UCLM, JCCM) and the Society of Sciences "Aranzadi", has provided experimental evidence of what is known as "the shared path hypothesis”, using male crossbills (loxia curvirostra) as a study model. This hypothesis mentions that it has not been proven that carotenoids are scarce in the diet, at least in that of birds, so neither their achievement nor use would be associated with a physiological cost. Therefore, the colors produced by carotenoids could reflect individual quality in another way.

    The shared pathway hypothesis focuses on those birds whose Red colors are produced by enzymatic transformations of yellow carotenoids from the diet. It suggests that this transformation could perhaps be done in the mitochondria, which are the organelles responsible for cellular respiration. Therefore, the transformation into a red carotenoid would directly reflect the general metabolic quality of the individual, without depending on the abundance of pigment.

     

    Natural variation of plumage coloration in adult male Crossbills (Photo: Daniel Alonso).

    The authors of the study administered subcutaneously, to a group of crossbills, antioxidant compounds designed to enter the membrane of mitochondria. What they observed is that one of these compounds (called mythTEMPO) caused birds deposit more red pigment in their feathers and achieve redder plumage. However, this only happened in individuals who were totally red before the experiment. It is necessary to think that in this species the males show an enormous diversity of coloration from yellow to intense red.

    Therefore, the authors concluded that the mitochondria may indeed be involved in the production of said red color, but also that only the best individuals (the reddest) are capable of generating those intense red colors.

    Thus, the red ornaments produced by carotenoids would be associated with the same biochemical pathways that control homeostasis and cell function. The intensity of the color of these ornaments would be the product of a vital cellular process and could be considered as a direct indicator of the quality of the individual who has developed them.

    This new information is not only of great value for understanding the evolutionary process of sexual selection mediated by the development of colored ornaments in vertebrates, but it may also help to understand the processes by which pollution or stress in different environments negatively affect the coloration of birds and other vertebrates.

    The scientific publication of this research is available at: