Costly sexual signals: are carotenoids rare, risky or required?

Theories of animal signalling emphasize the importance of costliness - to be effective, signals must be dependable; to be dependable, signals must carry costs and carotenoid-based signals are a favoured example. The traditional view that carotenoids are costly because they are scarce still carries weight. However, biomedical research has led to alternative views on costliness, mainly related to beneficial, but also to detrimental, effects of carotenoids. Recent improvements in our understanding of carotenoids suggest that the relative importance of these mechanisms will soon be determined, leading to a fresh outlook on cost-based signalling.