The physiological costs of being colourful: Nutritional control of carotenoid utilization in the American goldfinch, Carduelis tristis

Many sexually selected traits are thought to be costly to produce, which ensures that they communicate information honestly to conspecifics. Carotenoid pigmentation is classically considered as a costly sexual signal in many fish and birds. It is often argued that carotenoid colours are 'condition dependent', with an individual's nutritional or health state directly determining trait expression. However, few studies have investigated precisely how a compromised nutritional state affects an animal's capacity to develop these sexually attractive colours. Here, we studied the effect of food restriction on the ability of male American goldfinches to physiologically process carotenoids during the period of feather growth. We used high-performance liquid-chromatography to determine the types and amounts of carotenoids circulating in blood during moult as well as in newly grown colourful feathers. We show that nutritional deprivation affects the degree to which male goldfinches transport carotenoids through the bloodstream. Food-restricted males circulated significantly less blood carotenoids than controls. They also incorporated less carotenoids into feathers and grew less colourful plumage, but the decrease in plumage carotenoids did not significantly exceed the depressed amounts already present in blood. These results suggest that the means by which these yellow-coloured passerines either extract carotenoids from food (e.g. via lipoidal micelles) or transport them through blood (e.g. via lipoproteins) are more sensitive to changes in nutritional/energy state than are the mechanisms for metabolizing dietary pigments or depositing metabolites into feathers.