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Protandry and sexual dimorphism in trans-Saharan migratory birds
Authors:Rubolini  Diego; Spina  Fernando; Saino  Nicola
Institution:a Dipartimento di Biologia Animale, Università degli Studi di Pavia, p.zza Botta 9, I-27100 Pavia, Italy, b Istituto Nazionale per la Fauna Selvatica, via Ca' Fornacetta 9, I-40064 Ozzano Emilia (BO), Italy, and c Dipartimento di Biologia, Sez. Zoologia-Scienze Naturali, Università degli Studi di Milano, via Celoria 26, I-20133 Milano, Italy
Abstract:Earlier arrival to reproductive sites of males relative to females(protandry) is widespread among migratory organisms. Diversemechanisms have been proposed that may select for protandry,including competition for limiting resources (e.g., territories)or mates. In species with large variation in male reproductivesuccess, such as polygamous species and those with intense spermcompetition, early arriving males may accrue a fitness advantagebecause they acquire more mates or have larger chances of paternity.Comparative studies of birds have shown that sexual size dimorphism(SSD) is positively associated with the level of polygyny, whereasintense sperm competition is associated with sexual dichromatism(SD). Positive correlations between protandry and SSD or SDcan therefore be expected to exist across avian species. Becauselarge males are predicted to be better able to cope with adverseecological conditions early in the breeding season, selectionfor protandry, in turn, may have a correlated response on SSDamong migratory species breeding in boreal latitudes. Althoughprevious studies of birds have analyzed the association betweenSSD and protandry, none has analyzed SD in relation to protandry.Here we analyze the association between protandry during springmigration, SSD, and SD in 21 trans-Saharan monogamous migratorybird species. The difference in median migration dates betweenfemales and males, reflecting protandry, was positively associatedwith SD but not with SSD. Because dichromatism is positivelyrelated to sperm competition across species, present resultsare consistent with predictions derived from sexual selectionhypotheses for the evolution of protandry mediated by spermcompetition.
Keywords:arrival date  bird migration  comparative analysis  sexual dichromatism  sexual selection  sperm competition  
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