Understanding Ornamental Carotenoid Metabolism in the House Finch (Haemorhous mexicanus)

Abstract

Carotenoids are the most common source of yellow, orange and red coloration in birds. However, like other vertebrates, birds cannot synthesize carotenoids de novo. To produce carotenoid-based coloration they must ingest carotenoids from their diet. Most songbirds that deposit red carotenoids in their feathers, skin or beaks ingest only yellow or orange dietary carotenoids, which they further oxidized to red ketocarotenoids via a ketolation reaction. Despite of the extensive effort to understand carotenoid-based coloration in birds, fundamental questions regarding ornamental carotenoid metabolism remain unanswered. In this dissertation, I used house finches (Haemorhous mexicanus) as a model animal to help us better understand this carotenoid metabolism in birds. I collected tissues and extracted subcellular fractions from wild house finches and surveyed the ornamental ketoarotenoid content from both tissue and subcellular fractions to locate the potential site of carotenoid ketolation. I then compared the carotenoid content across sexes and seasons to get a bigger picture of how carotenoid metabolism might be like in the house finch. Last but not least, I collected other bird species with red carotenoid coloration but different display strategies to try to understand the general pattern of carotenoid metabolism in the avian world. We found the highest concentration of ketocarotenoid inside the hepatic mitochondria, or more specifically, in the inner mitochondria membrane in the house finch, suggesting liver as the primary site of carotenoid ketolation. Ketocarotenoids were detected in both males and females during the molting season, but no ketocarotenoid was detected out of the molting window. And the patterns of ketocarotenoid distribution varied among birds with different red carotenoid color displays, indicating the possibility of multiple strategies of ornamental carotenoid metabolism in birds.