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

Abstract

Summary 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 tradeoff 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.

The effect of rearing environment on blue structural coloration of eastern bluebirds (Sialia sialis)

“…The greater environmental sensitivity of male eastern bluebird nestlings is similar to the response of great tit (Parus major) nestlings; male great tits are more susceptible to parasites and less able to mount cell-mediated immunity compared to female great tits (Tschirren et al 2003). Moreover, as in our study of bluebirds, the UV-blue structurally colored tail feathers of male but not female blue tit nestlings are adversely affected by brood enlargement (Jacot and Kempenaers 2007).…”

Trypanosomes, fleas and field voles: ecological dynamics of a host-vector–parasite interaction

“…Male M. agrestis, being highly polygynous, may encounter increasingly physiologically stressful situations during the breeding season when population density increases and territorial defence and competition for mates becomes intense (Pusenius and Viitala, 1993). Older males may therefore not only be physiologically more susceptible to flea infestation, but may also allow an ectoparasite to more readily obtain a longer bloodmeal and thus increase its fecundity levels (Lehmann, 1992 ;Tschirren, Fitze and Richner, 2003).…”

Sex-specific development of cell-mediated immunity under experimentally altered rearing conditions in blue tit nestlings

“…In Eurasian kestrels (Falco tinnunculus), a species showing reversed sexual size dimorphism, cell-mediated immune response in male nestlings was enhanced when rearing conditions were improved by brood reduction as compared with control conditions, while the immune response of female nestlings was unaffected (Fargallo et al 2002). In great tits (Parus major), adverse rearing conditions evoked by an experimental flea infestation had non-significant effect on nestling cell-mediated immunity and the effect did not differ between the sexes (Tschirren et al 2003). However, as fleas do not only influence conditions experienced by the nestlings, but also activate immune function, it is not possible reach straightforward conclusions regarding sexrelated development of immune function.…”