Abstract

Evolutionary theory predicts that exposure to more diverse pathogens will lead to the evolution of more effective immune responses. The innate immune system defends the host from pathogens in a non-specific manner and is an important first-line of defense. We predicted that female Tree Swallows have more robust innate immunocompetence than males because females are exposed to more microbes during the breeding season than are males. This is because (a) females participate in extra-pair copulations with multiple males exposing them to sexually transmitted microbes (STMs) (e.g., bacteria, fungi, viruses), (b) the transmission of STMs during copulation is asymmetrical because ejaculates move from males to females, (c) Tree Swallow semen contains potentially pathogenic STMs, and (d) females spend more time in the nest than males. Additionally, elevated testosterone in males is a known correlate with suppressed immune function. We tested our prediction in the 2009 breeding season by conducting an assay of the innate immune system using whole-blood samples. A microbicidal assay using E. coli produced an index of the capacity of the blood to kill bacteria. Tree Swallow whole blood readily lyses E. coli, but there was no difference in mean E. coli lysis levels between males and females. However, females with higher lysis levels had less louse damage to feathers and older females with higher lysis levels had greater fledging success. These results suggest that while female Tree Swallows may not experience greater pathogen exposure, female innate immunocompetence may predict reproductive success.