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Latitudinal Distribution, Migration, and Testosterone Levels in Birds

1. Department of Biology, University of Antwerp, Universiteitsplein 1, B‐2610 Wilrijk, Belgium;2. Konrad Lorenz Forschungsstelle, A‐4645 Grunau 11, Vienna, Austria;3. Department of Limnology, University of Pannonia, Pf. 158, H‐8201 Veszprém, Hungary;4. Génétique et Evolution des Maladies Infectieuses, Unité Mixte de Recherche (UMR) Centre National de la Recherche Scientifique (CNRS)–Institut de Recherche pour le Développement 2724, B.P. 64501, 911 Avenue Agropolis, F‐34394 Montpellier Cédex 5, France;5. Laboratoire de Parasitologie Evolutive, CNRS UMR 7103, Université Pierre et Marie Curie, Bâtiment A, 7ème étage, 7 Quai St. Bernard, Case 237, F‐75252 Paris Cédex 5, France;6. Instituto Superior de Psicologia Aplicada, Rua Jardim do Tabaco 34, 1149–041 Lisboa, Portugal;7. Department of Biology, University of Washington, Seattle, Washington 98195

Tropical bird species usually have lower testosterone (T) levels during breeding than temperate species. However, the potential mechanisms behind the positive interspecific correlation between T and latitude remain unexplored. In a comparative study of more than 100 bird species, we examined whether social constraints during male‐male competition arising from migration and breeding synchrony are responsible for the latitude effects. Species that breed at higher latitudes are more likely to migrate and experience more intense intrasexual competition upon spring arrival than nonmigrant species from lower latitudes. Additionally, species from higher latitudes cope with shorter breeding seasons and thus with more synchronous breeding, which selects for high T titers via increased male‐male conflicts. Accordingly, peak T levels were associated with migration and the duration of the egg laying period that reflects breeding synchrony. Because migration and breeding synchrony were related to latitudinal distribution, they appear to be important components of the latitude effects on T. A multivariate model controlling for covariation of predictor variables revealed that latitude remained the strongest predictor of peak T. Therefore, selection due to migration and breeding synchrony may partially cause the latitude effect, but other geographically varying factors may also play a role in mediating peak T levels at different latitudes.