Fig. 3
Structure of carotenoid metabolic network establishes rules of avian carotenoid diversification. a Gains and losses (in millions of years ± 1SE) of plumage carotenoids during avian evolution in relation to their distance (in reactions) from dietary starting points. Numbers above bars are average frequency in millions of years. Insert (lower right): Hypothesis that observed metabolic elongation is sustained by recurrent acquisition of a new pathway linked to an additional dietary (external) compound. b Metabolic elongation (longest path from dietary to plumage carotenoids, in reactions) is only achieved by species utilizing >2 different dietary carotenoids (F 1,165 = 35.9, P < 10−4). Numbers above bars are numbers of species in each category. Species with only direct (non-metabolized) deposition of dietary carotenoids into plumage have path length = 0. “Maximum possible elaboration” (dashed line) is the longest metabolic pathway structurally possible in connected avian carotenoid network (Fig. 6a) that can be accomplished from most dietary starting points. c Network robustness (an average ratio of compounds that retain their expression in the plumage when any one enzymatic reaction in the network is deleted – a measure of redundancy) enables metabolic elongation. This metric excludes dietary compounds (Additional file 8: Table S1). Independent phylogenetic linear contrasts are shown