Gender and environmental sensitivity in nestling Collared Flycatchers
Sheldon BC., Merilä J., Lindgren G., Ellegren H.
In many vertebrates, males are apparently more affected by adverse environmental conditions, particularly during early stages of development, than are females. Three explanations have been proposed for this pattern. First, sexual size dimorphism (SSD) may result in sexes having different nutritional requirements to achieve the same viability, and males are more commonly the larger sex. Second, reduced performance of males could result from possession of an unguarded sex chromosome combined with environmental dependence in expression of deleterious recessives. Third, the observed difference may be a consequence of possession of a male phenotype, for example, due to higher circulating levels of androgens associated with development of male reproductive organs acting antagonistically on other systems. We used experimental manipulations of rearing conditions, coupled with a molecular genetic technique for gender identification, to test these hypotheses in a population of Collared Flycatchers, Ficedula albicollis, a migrant, hole-nesting passerine bird. Nestlings of that species exhibit sexual size monomorphism, and as with other birds, females are heterogametic. As a consequence, the three hypotheses make different predictions about the way in which gender and the environment will interact. Comparisons of brothers and sisters in a split-brood, partial cross-fostering design revealed no evidence of gender x environment interaction on body size, wing length, body mass, or recruitment to the breeding population in this size monomorphic species. Our results therefore support the first hypothesis, namely, that sex differences in performance in sexually dimorphic species are most likely to be caused by different nutritional requirements. Our experiments allow us to investigate the existence of sex-specific fitness differences across an environmental gradient; such data are important for generating and testing hypotheses relating to adaptive sex allocation. The absence of gender x environment interactions demonstrated here supports recent studies of this species indicating a lack of sex ratio adjustment in response to a related, natural, environmental gradient. The possibility of gender x environment interactions along environmental gradients other than those investigated here should be addressed experimentally.