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

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.