Manipulation of primary sex-ratio: an updated review

Abstract

Some bird species would be able to manipulate primary sex ratio, i.e. the proportion of male and female offspring at the laying time. Such trait could be considered being mostly under maternal control. Nevertheless, the exact mechanism involved remains undetermined. Knowledge of the mechanism would contribute to any assessment of its cost in terms of resources andyor time, which would be important to formulate predictions of those scenarios where it could have evolved. In fact, small costs of sex ratio control could overcome the adaptive value of adjusting the proportion of sexes in the progeny. Pike and Petrie (2003) published an excellent review on the potential mechanism involved. However, many new experimental and correlational evidences have appeared from that date, providing new and interesting perspectives. Thus, hormonal control is obtaining strong support from recent findings. The present review updates the current knowledge on this subject with emphasis on the question of whether sex ratio manipulation is a widespread trait among avian taxa. Finally, future directions for research, including not only those related to the mechanism per se, but also those linked to conditions necessary for the evolution of this trait, are presented. Thus, the necessity of determining fitness functions of male and female offspring and both parents, as well as the problem of if this trait is the result of a evolutionary constraint or strategy, are also addressed.