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Differential effects of endoparasitism



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doi 10 1098 rspb 2000 1174 Differential effects of endoparasitism on the expression of carotenoid and melanin based ornamental coloration Kevin J McGraw and Geo rey E Hill Department of Biological Sciences and Alabama Agricultural Experiment Station 331 Funchess Hall Auburn University Auburn AL 36849 USA The striking diversity of sexual dimorphisms in nature begs the question Why are there so many signal types One possibility is that ornamental traits convey di erent sets of information about the quality of the sender to the receiver The colourful pigmented feathers of male birds seem to meet the predictions of this hypothesis Evidence suggests that carotenoid pigmentation re ects the nutritional condition of males during moult whereas in many instances melanin pigmentation is a reliable indicator of social status However as of yet there have been no experimental tests to determine how these two ornament types respond to the same form of environmental stress In this study we tested the e ect of endoparasitic infection by intestinal coccidians Isospora sp on the expression of both carotenoid and melanin based ornamental coloration in captive male American gold nches Carduelis tristis We found that the carotenoid based plumage and bill coloration of parasitized males was less saturated than that developed by unparasitized males but that the brightness and size of melanin based black caps did not di er between the groups These ndings provide the most robust empirical support to date for the notion that carotenoid and melanin ornaments reveal di erent information to conspeci cs Keywords American gold nch Carduelis tristis coccidia honest advertisement Isospora plumage coloration 1 INTRODUCTION Since Darwin 1871 developed his theory of sexual selection to explain the remarkable sex di erences in size shape colour and behaviour that exist among animals in nature biologists have been fascinated by the evolution of sexually dimorphic traits Yet the information content of



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