Behavioral Ecology and Sociobiology Geographic variation in breeding system and environment predicts melanin-based plumage ornamentation of male and female Kentish plovers

1 Sexual selection determines the elaboration of morphological and behavioural traits and thus 2 drives the evolution of phenotypes. Sexual selection on males and females can differ between 3 populations, especially when populations exhibit different breeding systems. A substantial body 4 of literature describes how breeding systems shape ornamentation across species, with a strong 5 emphasis on male ornamentation and female preference. However, whether breeding system 6 predicts ornamentation within species, and whether similar mechanisms as in males also shape 7 the phenotype of females remains unclear. Here, we investigate how different breeding systems 8 are associated with male and female ornamentation in five geographically distinct populations 9 of Kentish plovers Charadrius alexandrinus. We predicted that polygamous populations would 10 exhibit more elaborate ornaments and stronger sexual dimorphism than monogamous 11 populations. By estimating size and intensity of male (n = 162) and female (n = 174) melanin12 based plumage ornaments, i.e. breast bands and ear coverts, we show that plumage 13 ornamentation is predicted by breeding system in both sexes. A difference in especially male 14 ornamentation between polygamous (darker and smaller ornaments) and monogamous (lighter 15 and larger) populations causes the greatest sexual dimorphism to be associated with polygamy. 16 The non-social environment, however, may also influence the degree of ornamentation, for 17 instance through availability of food. We found that, in addition to breeding system, a key 18 environmental parameter, rainfall, predicted a seasonal change of ornamentation in a sex19 specific manner. Our results emphasise that to understand the phenotype of animals, it is 20 important to consider both natural and sexual selection acting on both males and females. 21 22