Resumen de Avian Sex Ratio and Sex-Specific Traits in Offspring
Sex allocation (SA) refers to the proportion of resources allocated to male and female reproduction; relatedly, sex ratio (SR) refers to the numbers of each sex that are produced. Hence, their scientific study deals with the variation in the quantity and quality of males and females and its ultimate and proximate causes. Adaptive theories make predictions about sex allocation under the assumption that facultative adjustment will be favoured when the fitness benefits compensate the fitness costs. However, although SA theory is one of the great successes of evolutionary biology and its mechanisms have been successfully applied to a number of taxa, they are still poorly understood in vertebrates. This thesis investigated causes of variation in avian sex allocation, with a focus on the role of sexual size dimorphism (SSD) and other features of avian life-history, associated with differences in the reproductive value of sons and daughters. In the first section, we present the results of tests of a sex ratio population model which involve species with sexual size dimorphism, in which it is possible to estimate the strength of selection for adjustments. Both species-level and phylogenetically controlled analyses showed that, across bird species, hatching and fledging sex ratios are influenced by the degree of SSD. Dimorphic species show a higher proportion of the smaller sex, but the effect is particularly strong in species with female-biased SSD. Parents adjust offspring sex ratio by differential production, possibly the less costly mechanism, rather than by sex-specific mortality. The degree of SSD is also correlated with offspring sex-biased vulnerability (expressed as a greater mortality and mass reduction of the larger sex). The survival and viability costs involved in achieving a larger body size support the size-dependent explanations of vulnerability. However, they should be combined with sex-dependent explanations, as growing large is mainly disadvantageous when coupled with the male-phenotype. Other life-history characteristics of the species, however, appear as potential modifiers of the relationship between SR and SSD. This reflects the composite matrix of factors that are acting on avian sex ratio evolution and questions the validity of “equal-investment” theories in this context. In the second section, the common tern Sterna hirundo was used as model species to investigate, via observational and experimental studies, the factors that could shape sex allocation in this slightly dimorphic species. In search for indications of different reproductive value/costs of each sex and possible differential parental allocation, we describe the sexual dimorphism in phenotype and developmental strategies of the offspring, related to environmental and parental traits. The results underline (1) the potential of parents to affect the development of their offspring by differential supply of particularly important resources, such as carotenoids; and (2) the influence of parental reproductive quality on the offspring sexual differences, even if these are slight. Furthermore, we performed a comprehensive study with complete information over a long period (7 years), to examine sex ratio and sex-specific mortality in common terns. Population patterns revealed no deviations from parity, but environmental factors and parental condition and quality affected sex ratio adjustment at individual level. Overall, this thesis highlights the intricate relationships between facultative sex ratios and individual and life-history traits which may drive the evolution of sex allocation in birds.
