Sexual dimorphism in susceptibility to parasites and cell-mediated immunity in great tit nestlings

1. Parasites can affect host fitness, provoke host responses, and thereby mediate host life history evolution. As life history strategies are often sex-specific, immunological or behavioural responses of the host aiming to reduce the impact of parasites may be sexually dimorphic, e.g. as a consequence of sex differences in the resource allocation trade-off between parasite defence, morphological traits and body functions. Parasites may therefore affect males and females differently leading to sex specific patterns of parasite susceptibility. 2. In an experimental field study, we manipulated the ectoparasite load of great tit nests (Parus major) and investigated its effects on male and female nestlings. As susceptibility to parasites may be linked to the ability of the nestlings to fight off parasites immunologically, we further investigated sex differences in cell-mediated immunity using a phytohaemagglutinin (PHA) assay. 3. Body mass, metatarsus length and overall body size, but not feather length, showed a sexual dimorphism at the end of the nestling period. A significant interaction between the effects of sex and parasite treatment on the sexually dimorphic traits indicates that the parasite effect is sex-specific. While no differences in morphological traits were found in females raised in infested and uninfested nests, parasitized males were significantly smaller and lighter than males raised in uninfested nests. Further, we found a pronounced sexual dimorphism in the response to the PHA assay with males showing a reduced cellular immunity. The parasite treatment had a non-significant effect on the PHA response and the PHA response of males and females were not influenced differently by parasites. 4. Our study shows that sexual dimorphism in susceptibility to parasites and immunocompetence develops early in life, and suggests sex-specific strategies in the allocation of limited resources. Possible mechanisms of sex differences in susceptibility to parasites and immunocompetence during postnatal growth and the consequences for optimal sex allocation strategies of the parents are discussed.