Carotenoid biotransformation pathways and oxidative stress in a bird: an experimental approach and its implications for color signaling

Abstract

Colorful ornaments have been the focus of sexual selection studies since the work of Darwin. Yellow to red coloration is often produced by carotenoid pigments. Different hypotheses have been formulated to explain the evolution of these traits as signals of individual quality. Many of these hypotheses involve the existence of a signal production cost. The carotenoids necessary for signaling can only be obtained from food. In this line, carotenoid-based signals could reveal an individual's capacity to find sufficient dietary pigments. However, the ingested carotenoids are often yellow and became transformed by the organism to produce pigments of more intense color (red ketocarotenoids).Biotransformation often involves oxidation reactions. We tested the hypothesis that biotransformation could be costly because a certain level of oxidative stress is required.Thus, the carotenoid-based signals could reveal the efficiency of the owner in successfully managing this challenge. In a bird with ketocarotenoid-based ornaments (the red-legged partridge; Alectoris rufa), the availability of different carotenoids in the diet and oxidative stress were manipulated. We found that color and pigment levels in the ornaments depended on the relative quantity in the food of those carotenoids used as substrates in biotransformation (i.e. zeaxanthin and lutein). Moreover, we found that birds exposed to certain levels of a free radical generator (diquat) developed redder bills and deposited higher amounts of ketocarotenoids (astaxanthin) in ornaments, thus supporting the hypothesis. However, the effect also depended on the relative abundance of substrate carotenoids in the diet. This last result suggests the involvement of a resource allocation trade-off, which would support, to some extent, a signaling cost linked to carotenoid carotenoid-based signals could reveal an individual's capacity to find sufficient dietary 7 pigments. However, the ingested carotenoids are often yellow and became transformed by 8 the organism to produce pigments of more intense color (red ketocarotenoids). 9Biotransformation often involves oxidation reactions. We tested the hypothesis that 10 biotransformation could be costly because a certain level of oxidative stress is required.