| Summary: | Sexual selection is a strong evolutionary force shaping the traits determining individual reproductive success. Such traits can be crucial for reproductive success before or after copulation (pre- versus post-copulatory sexual selection). However, little is known about how primary and secondary sexual traits vary in relation to male age in socially monogamous but genetically promiscuous (i.e. extra-pair) species. This is a particularly pressing question because extra-pair sires are more often older rather than younger males, which suggests that male age reflects genetic quality to females. Yet, older males are senescent males and produce lower quality offspring. Thus, females should actually avoid mating with older males. Furthermore, it is assumed that males that mate with multiple females achieve fitness benefits (higher lifetime reproductive success) compared to males that mate monogamously. This assumption underlies sexual selection theory. Additionally, it provides a straightforward explanation of why males in socially monogamous species engage in extra-pair mating: it maximises their reproductive success. Surprisingly, this classic claim has not been thoroughly quantified and thus awaits validation. In this thesis, I examined age-related variation in reproduction. Particularly, I studied how male age relates to mating strategies, sperm traits and reproductive success. I further quantified the fitness consequences of a males mating promiscuously over males mating monogamously. My study species was the house sparrow, Passer domesticus, a predominantly socially monogamous species. I studied captive and wild sparrows, both of exact known ages. Avian studies commonly rely on discriminating between first-year and older breeders only. Such a dichotomous discrimination of age prohibits analyses at all life history stages. Further, I quantified the lifetime reproductive success of wild sparrows. Knowledge of lifetime reproductive success is crucial for understanding, for instance, the fitness consequences of mating ...
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