Changes in phytohaemagglutinin skin-swelling responses during the breeding season in a multi-brooded species, the Eurasian Tree Sparrow: do males with higher testosterone levels show stronger immune responses?

Life-history theory assumes that the fitness costs of immunity may have negative effects on reproductive success. Similarly, the immunocompetence handicap hypothesis is based on findings that testosterone (T) has immunosuppressive effects, although the basis of this hypothesis has recently been challenged. As much of the work examining the relationship between T levels and immune function has been carried out in captive-housed species, these results may not accurately reflect the situation of animals living in natural environments. To better understand the relationship between plasma T levels and immune function, studies focusing on free-living animals are needed. A previous study by our group determined the changes in both baseline and stress-induced T levels in free-living Eurasian Tree Sparrows ( Passer montanus) across different annual cycle stages. In this study, we further report the phytohaemagglutinin skin-swelling (PHA) immune response in this multi-brooded species during different breeding sub-stages, and then determine the relationships between the PHA response and both baseline and stress-induced T levels. Our results show that the PHA response varied across the different sub-stages and differed significantly between the first and second brood stage. Furthermore, T levels in male Tree Sparrows are positively correlated with the PHA response during the breeding season, whereas this relationship is negative in females, suggesting that the biological function of T differs between the sexes. Therefore, our results suggest that free-living animals have evolved the ability to orchestrate trade-offs between reproduction and immune functions based on changes in physiology and the environment, which should provide further opportunities to study the flexibility and plasticity of physiological and ecological adaptations in natural environments. [ABSTRACT FROM AUTHOR]