1. Carotenoid pigments are deposited into egg yolk by female birds, but little is known of the ecological regulation of this process. In particular, the relative importance of pigment acquisition and utilization (i.e. physiological discrimination: differential uptake, transport, deposition or metabolic conversions) is poorly understood.
2. Carotenoids are antioxidants, capable of neutralizing reactive free radicals and thus preventing lipid peroxidation. But animals must ultimately obtain carotenoids in the diet, and metabolic conversion of carotenoids is inefficient and expends energy. It has therefore been suggested that carotenoid supply may be limiting to animals.
3. Prelaying, wild Lesser Black-Backed Gulls (Larus fuscus L.) were given supplementary feed with a cocktail of four carotenoids, or a carotenoid-free (control) supplement. The yolk carotenoid profile and susceptibility to lipid peroxidation were then compared in eggs that they laid.
4. In comparison with controls, there was an increase in the yolk concentrations of seven carotenoids, and also unidentified carotenoids in eggs produced by carotenoid-supplemented females. However, the relative proportions of five classes of carotenoid did not differ significantly, and consequently the percentage profile of yolk carotenoids was positively correlated, between feeding treatments, possibly indicating metabolic transformations and differential transfer of carotenoids from maternal diet to yolk.
5. Moreover, in control eggs relatively few possible correlations between carotenoid concentrations were significantly positive, and some correlations between carotenoid proportions were significantly negative, suggesting physiological discrimination.
6. Egg yolk produced by carotenoid-supplemented females was significantly less susceptible to lipid peroxidation in comparison with controls.
7. These results suggest that yolk enrichment with carotenoids involves physiological discrimination in gulls, highlighting a previously unstudied, potential cost of reproduction.