Male ornamentation and immune function in two species of passerines

Abstract

Male ornamentation may have evolved as a result of sexual selection or to signal dominance outside the breeding season. The aim of this thesis was to examine the function of male plumage variability and how immune function would contribute to the honesty of sexual signalling in the great tit and in the pied flycatcher. Immune function may affect sexual signalling because immune function is costly and traded off against sexual advertisement. Alternatively genetic variation in specific immune responsiveness may generate variation in condition that may be reflected in the expression of sexual traits. Our results support both of the hypotheses. We found large antigen specific genetic variation in humoral immune responsivenss suggesting potential for host-parasite coevolution to work as suggested by the Hamilton & Zuk hypothesis. Instead, general immune effectiveness was strongly condition dependent with no significant genetic variation. We also found both correlative and experimental evidence that immune function may be traded off against sexual advertisement. Our results indicate that sex differences in immune function may arise as a consequence of higher mating effort of males. Juvenile dominance in great tits was environmentally determined during the early development with no heritable variation. We found little evidence that black breast stripe would signal dominance in the great tit. Instead the breast stripe was flexible in its expression and may be used to signal aggressive motivation. Flexibility of ornament expression may have evolved to track changes in condition or status and need to be considered in studies of sexual selection and status signalling.