Melanin- versus carotenoid-based sexual signals: is the difference really so black and red? SIMON C. GRIFFITH * , TIMOTHY H. PARKER † & VALE ´ RIE A. OLSON ‡ *School of Biological, Earth & Environmental Sciences, University of New South Wales yDivision of Biology, Kansas State University zInstitute of Zoology, Zoological Society of London (Received 5 April 2005; initial acceptance 25 June 2005; final acceptance 26 July 2005; published online 10 February 2006; MS. Number: RV-61) A large number of coloured sexually selected ornamental traits in the animal kingdom are based on carot- enoid and melanin pigments. The biochemical differences between these two classes of pigment, together with their different physiological roles, have led to the general belief that there will be a fundamental dif- ference in the way in which they are used in animal signals. Specifically, it has been argued that caroten- oid-based colours will have higher levels of condition dependence and that melanin-based traits will be under tighter genetic control. We present a meta-analysis of studies that have experimentally investigated the signalling quality of the two kinds of colour in birds and show that there is no evidence of a difference between them. Furthermore, we show that the available data are currently very limited, both in the num- ber of studies and in the quality of many of the studies that have attempted to examine this question, and we suggest directions for future work. Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Melanins and carotenoids are the two pigments responsi- ble for most animal coloration. They differ fundamentally because, while animals are capable of synthesizing mela- nins from the aromatic amino acids phenylalanine and tyrosine (Hearing 1993), they are unable to synthesize ca- rotenoid pigments and must acquire them, intact, from the environment (Goodwin 1984). Therefore, animals might be expected to utilize melanin and carotenoid pig- ments differently in their sexual signals (Kodric-Brown & Brown 1984). Many sexually selected traits appear to have evolved as honest signals of quality. Thus, individ- uals that choose mates with the most extreme trait expres- sion in the population will enjoy a fitness advantage, for either themselves or their offspring (reviewed in Ander- sson 1994). A variety of mechanisms have been proposed to explain the routes by which honest signalling with pig- mentation may evolve (Zahavi 1975; Andersson 1986). These models are based on variation in the ability of indi- viduals to acquire and absorb the pigments and their pre- cursors (Hill 1992), to process them, and to bear the risk of predation or social interactions related to variation in their expression (e.g. Rohwer 1975; Endler 1980). More re- cently, authors have suggested that honest signalling may result from the trade-off between the expression of pig- ments in sexually selected traits and their use in physio- logical processes such as immunity (e.g. Abbas et al. 1991; Owens & Wilson 1999). Given that melanin pigments can be readily synthesized by animals, in contrast to carotenoids, and are not thought to be involved in as many physiological pro- cesses, it is perhaps reasonable to expect that sexually selected traits based on colour will less often be pigmented by melanins than by carotenoids (Gray 1996). For exam- ple, it would seem intuitive that the expression of a mela- nin-based sexually selected trait is less likely to reveal something about foraging ability than a carotenoid-based signal would. Our purpose here is to argue that these intu- itive hypotheses are based on fairly weak grounds and that there are no reliable a priori reasons to expect clear-cut dif- ferences between melanin-based and carotenoid-based sig- nals in terms of the information they contain. In this review, we focus primarily on avian coloration, as this is Correspondence: S. C. Griffith, School of Biological, Earth & Environ- mental Sciences, University of New South Wales, Kensington, Sydney, NSW 2052, Australia (email: s.griffith@unsw.edu.au). T. H. Parker is at the Division of Biology, Kansas State University, Manhattan, KS 66506, U.S.A. V. A. Olson is at the Institute of Zoology, Zoological Society of London, Regent’s Park, London NW1 4RY, U.K. 749 0003–3472/06/$30.00/0 Ó 2006 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2006, 71, 749–763 doi:10.1016/j.anbehav.2005.07.016