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Journal of Ornithology

, Volume 154, Issue 1, pp 91–97 | Cite as

Primary sex ratios vary with clutch size in the size-dimorphic White-throated Dipper Cinclus cinclus

  • Trond Øigarden
  • Jan T. Lifjeld
Original Article

Abstract

Sex allocation theory explains how parents should allocate reproductive effort to the rearing of sons and daughters. A central premise is that there is room for individual optimization of brood sex ratios as a function of the relative rearing costs of sons and daughters, the fitness return curves for sons and daughters, and the total amount of reproductive resources available. When sons are more expensive to raise than daughters, and fitness returns per unit investment are equal for the two sexes, we show with a simple graphical model that large clutches should be female-biased. We tested this prediction in a field study of the White-throated Dipper Cinclus cinclus in Norway. In this species, males are nearly 20 % heavier than females, and the predominant mating system is social monogamy with little or no extrapair paternity, indicative of similar reproductive variances for the two sexes. We found a significant deficit of males (27 % males) for the largest clutch size (=6 eggs) in the population and no deviation from parity for five-egg clutches (47 % males). This contrast could not be explained by any sex-biased mortality rate at the egg or nestling stages, and thus suggests facultative adjustment of the primary sex ratio. We also verified a sexual dimorphism in growth rates at the nestling stage. Our results indicate that different rearing costs of sons and daughters may select for clutch-size dependent sex-ratio adjustment in size-dimorphic species.

Keywords

Growth rate Parental expenditure Sex allocation Sexual dimorphism Sex ratio adjustment 

Zusammenfassung

Das primäre Geschlechterverhältnis variiert mit der Gelegegröße bei der Wasseramsel ( Cinclus cinclus ), einer Art mit sexuellem Größendimorphismus

Die Theorie der Geschlechtsallokation erklärt, wie Eltern ihren Fortpflanzungsaufwand auf die Aufzucht von Söhnen und Töchtern verteilen sollten. Eine zentrale Voraussetzung ist, dass die Gelegenheit zur individuellen Optimierung des Geschlechterverhältnisses innerhalb der Brut besteht, in Abhängigkeit von den relativen Aufzuchtskosten von Söhnen und Töchtern, den Fitnessertragskurven für Söhne und Töchter und der Gesamtmenge verfügbarer Fortpflanzungsressourcen. Wir zeigen mit einem einfachen graphischen Modell, dass, wenn die Aufzucht von Söhnen kostenintensiver ist als die von Töchtern und die Fitnesserträge pro Investitionseinheit für die beiden Geschlechter gleich sind, große Gelege zu Weibchen hin verschoben sein sollten. Wir haben diese Vorhersage in einer Feldstudie an der Wasseramsel (Cinclus cinclus) in Norwegen getestet. Bei dieser Art sind die Männchen fast 20 % schwerer als die Weibchen, und das vorherrschende Paarungssystem ist soziale Monogamie mit wenig oder gar keiner Fremdvaterschaft, was auf ähnliche reproduktive Varianz für die beiden Geschlechter schließen lässt. Wir fanden ein signifikantes Männchendefizit (27 % Männchen) für die höchste Gelegegröße (= 6 Eier) in der Population und keine Abweichung von einem ausgeglichenen Geschlechterverhältnis für Gelege mit fünf Eiern (47 % Männchen). Dieser Unterschied konnte nicht mit höherer Mortalität eines Geschlechts im Ei- oder Nestlingsstadium erklärt werden und deutet daher auf eine fakultative Anpassung des primären Geschlechterverhältnisses hin. Wir haben außerdem Geschlechtsdimorphismus in der Wachstumsrate im Nestlingsstadium bestätigt. Unsere Ergebnisse deuten darauf hin, dass unterschiedliche Aufzuchtskosten für Söhne und Töchter bei Arten mit Größendimorphismus eine von der Gelegegröße abhängige Anpassung des Geschlechterverhältnisses begünstigen dürften.

Notes

Acknowledgments

We thank A. Mæhlen for assistance in the field, local members of the Norwegian Ornithological Society for access to their nest boxes, Lars Erik Johannessen for help with statistics, and Arild Johnsen, Kevin Omland, Thomas Friedl, and two anonymous reviewers for comments on the manuscript. Financial support was provided by the County Governor of Hedmark.

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Copyright information

© Dt. Ornithologen-Gesellschaft e.V. 2012

Authors and Affiliations

  1. 1.Natural History MuseumUniversity of OsloOsloNorway

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