In the presence of parasites, parents can increase the amount of resources allocated to parasite defense and thereby enhance their chances of survival and future reproduction or allocate more resources to current reproduction and thus increase the condition of their offspring. Here we test how a common ectoparasite affects parental behavior and the trade-off between parasite defense and reproduction in a wild bird population. To avoid confounding effects of the parasite infestation on the offspring and to test purely for the effect of the parasite on parents, we exposed parents to parasites before the young hatched only, that is, in the early phases of reproduction. Experimental great tit (Parus major) nests were infested with hen fleas (Ceratophyllus gallinae) until the start of incubation, whereas control nests were left parasite free. Parasite-induced maternal and genetic effects were then eliminated by replacing all clutches with the clutches of unexposed parents. All fleas were removed after clutch completion and hence nestlings grew up in practically flea-free nests. The experimental flea infestation before incubation did not change parental feeding rates but reduced the frequencies of brooding and nestling care. Tick prevalence increased and tarsus growth was reduced in nestlings reared by previously exposed parents. It suggests that a fraction of the costs for parents of the flea exposure before incubation is shifted to offspring via reduced parental care. The flea infestation before incubation did not affect parental body condition. However, flea-exposed parents had higher tick loads, which may impair parental health and survival.
Newborn animals do not have a fully functional immune system and are thus impaired in their ability to fight parasites. Mothers can therefore increase the survival probability of their young by providing them with passive immunity, e.g. in the form of maternal antibodies transferred via the placenta or the eggs. The maternal responses are only induced when parasites are present, and have been observed not only in vertebrates but also in invertebrates. However, while these parasite‐induced maternal effects are known to reduce the harmful effects of common parasites, they may also impose costs for the young, either because the maternal response impairs parental performance, or because maternally transmitted products moderate offspring development. We experimentally tested these two hypotheses in a wild great tit population. We exposed birds to a common ectoparasite before egg‐laying to induce the maternal response, and thereafter separated egg‐mediated maternal effects from effects on post‐laying parental performance by cross‐fostering whole clutches. To assess the costs of this response without its confounding benefits, we kept nests free of parasites after hatching. Since the costs of maternal effects can be expressed differently under relaxed and harsh rearing conditions, we simultaneously manipulated clutch size. First, parasite‐exposed parents raised lighter young, suggesting that parasite defence or the induced maternal response are costly to the parents and reduce their capacity to raise young. Second, under relaxed but not under harsh rearing conditions, young with the flea‐induced maternal effect were heavier and were in better body condition than controls, suggesting that the maternally transferred products can be allocated to physiological functions beyond parasite defence.
Parents can increase their reproductive success by assisting their neonate offspring in parasite defence. In birds, parental tactics include post-hatching parental responses such as increased parental care and pre-hatching maternal effects such as the transfer of maternal antibodies via the egg. These parasite-induced parental responses are known to reduce the effects of parasites on offspring, but their costs for the parasite are largely unknown. In two separate experiments on great tits Parus major we assessed these costs for hen fleas Ceratophyllus gallinae. Half of the parents where exposed to fleas during egg-laying to induce the parental response, while control nests were left flea-free. In experiment 1 parents raised their own young and we measured the effect of combined pre-and post-hatching parental effects, while in experiment 2 a crossfoster design allowed us to assess the effects of pre-hatching maternal effects alone. In both experiments we let fleas take a blood meal on nestlings from either flea-exposed or unexposed parents. We then measured flea-feeding duration, the quantity of extracted blood, and the fleas' subsequent survival time. We found in both experiments that on the largest nestlings of a brood flea survival was significantly reduced by the parental effects, whereas on the smaller nestlings it was independent of parental effects. The pre-and post-hatching parental responses did neither affect duration nor size of a flea blood meal. These results suggest first that the pre-hatching maternal effects, i.e. the substances transferred to the nestling via the egg, have the potential to harm fleas without reducing flea feeding capacity, and second that the strength of the maternal response varies between the nestlings, either because maternal products are unequally distributed among eggs within a clutch, or because large nestlings can build up a response that enhances the effect of the maternal products.
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