Fat stores in birds: an overlooked sink for carotenoid pigments?

1. Carotenoids are responsible for the most striking colours in birds, but also play an important role as enhancers of the immune system. Consequently , a trade-off between the ornamental and health functions of carotenoids in birds has been proposed. 2. Although it is well known that birds can store carotenoids in different organs and tissues, including the fat stores, until now all field studies o f the regulation of carotenoid stores have focused on plasma carotenoids. 3. Carotenoids in the fat of 44 wild Greylag Geese (Anser anser L.) winteri ng in south-western Spain were identified and quantified. In addition, the relationships between carotenoids and the size of the fat stores, as well a s the sex and age of the geese, were analysed. 4. The major carotenoid in goose fat was lutein. This and related carotenoi ds are also the most prevalent pigments in bird plasma and secondary sexual characters. We also detected beta -cryptoxanthin, beta -carotene, neochrom e and neoxanthin. Total carotenoid concentration was negatively correlated with the size of the fat stores. Males had higher concentrations of caroten oids than females. 5. A possible explanation for these patterns is that male birds tend to hav e higher plasma carotenoids than females, a difference that could be transp osed to fat if carotenoids diffused passively from the blood into adipose t issues. Carotenoids, however, may tend to remain in the fat stores. If this were true, fat would not be a reservoir of carotenoids, but a sink where a significant amount of these pigments would be sequestered, being no longer available for other functions.