Summary 1.The timing and mechanism of the maternal response to ectoparasites is investigated in a host-parasite system consisting of great tits and a haematophageous ectoparasite, the hen flea. It has been demonstrated previously that a maternal response to this parasite enhances survival and fertility of the offspring. This may have arisen by either a maternally transferred protection via the egg, or by a parental response affecting the common rearing environment after hatching. Two experiments aimed to differentiate between the two possibilities are reported here. 2. First, mothers were either exposed to or kept free of ectoparasite during egg production, and subsequently the newborn nestlings were cross-fostered between the two treatments. The experimental design discriminates as to whether the maternal effect arises before or after hatching. Within the same nest, the nestlings originating from previously exposed mothers grew faster than nestlings of unexposed mothers.3. Secondly, we tested for the transfer of parasite-induced immunoglobulins (IgG) via the egg. Mothers were kept free of ectoparasites until they had laid the first egg and were then either exposed to or kept free of ectoparasites to the end of laying. The IgG-concentration significantly increased from the first to the eighth egg of exposed mothers, but not in eggs of unexposed ones. 4. In summary, the first experiment shows that ectoparasites can induce a beneficial maternal response at egg laying, and the second experiment suggests that the maternal effect is due to immunoglobulins transferred via the egg. Maternal responses to other parasites, e.g. blood parasites, are known for chicken in captivity. In natural populations of birds both the timing and mechanism of the response are poorly understood, despite their relevance for behavioural and population ecology.
Immune system activation may benefit hosts by generating resistance to parasites. However, natural resources are usually limited, causing a trade-off between the investment in immunity and that in other life-history or sexually selected traits. Despite its importance for the evolution of host defense, state-dependent fitness costs of immunity received little attention under natural conditions. In a field experiment we manipulated the nutritional condition of male field crickets Gryllus campestris and subsequently investigated the effect of an induced immune response through inoculation of bacterial lipopolysaccharides. Immune system activation caused a condition-dependent reduction in body condition, which was proportional to the condition-gain during the preceding food-supplementation period. Independent of nutritional condition, the immune insult induced an enduring reduction in daily calling rate, whereas control-injected males fully regained their baseline level of sexual signaling following a temporary decline. Since daily calling rate affects female mate choice under natural conditions, this suggests a decline in male mating success as a cost of induced immunity. Food supplementation enhanced male life span, whereas the immune insult reduced longevity, independent of nutritional status. Thus, immune system activation ultimately curtails male fitness due to a combined decline in sexual display and life span. Our field study thus indicates a key role for fitness costs of induced immunity in the evolution of host defense. In particular, costs expressed in sexually selected traits might warrant the honest advertisement of male health status, thus representing an important mechanism in parasite-mediated sexual selection.
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