Parental care is advantageous because it typically increases the survival of genetically related young. In contrast, parental care given to unrelated young incurs no benefit. A further cost of parental investment is that it reduces the future reproductive potential of the caregiver. I examined whether eastern phoebes’ Sayornis phoebe future reproductive effort was related to interspecific brood parasitism by brown-headed cowbirds Molothrus ater in prior broods. In 2000 absolute and relative measures of change in clutch sizes from first to second breeding attempts were similar in parasitized and non-parasitized broods, while the latency to renest was several days shorter for parasitized broods. In addition, the relative change in clutch size was more negative for phoebe nests with more cowbird chicks per brood. In 2001 these statistical relationships between absolute and relative measures of residual effort and prior parasite load were also confirmed in control but not in experimentally manipulated clutches. The experimental data support previous findings that parasitism per se does not seem to influence residual reproductive output of adult phoebe hosts. These data also emphasize that intragenerational residual costs of parental care should be measured by the use of a relative measure of reproductive effort or other statistical methods that take into account the biological and statistical non-independence of clutch sizes from subsequent breeding attempts.
Toxoplasma gondii, a parasite infecting 20-60% of humans in various countries, influences the behaviour of infected animal and human hosts. Infected human subjects have changes in several of Cattell's and Cloninger's personality factors. Recently, three independent studies have shown that Rh-positive subjects are protected against the T. gondii-induced changes of reaction times and increased risk of traffic accidents. Here we searched for evidence of similar effects of RhD phenotype on toxoplasmosis- or aging-associated changes in the personality profile of about 302 blood donors. We found that Rh-positive and Rh-negative subjects responded differently to toxoplasmosis. In addition to the already known effects of toxoplasmosis on novelty seeking, self transcendence, superego strength and protension, we also found effects of RhD phenotype on ego strength, protension, and praxernia, as well as opposite effects of toxoplasmosis on ego strength, praxernia, ergic tension and cooperativeness in Rh-positive and Rh-negative subjects. Moreover, our results indicate that RhD phenotype might influence not only the effect of toxoplasmosis but also the effect of aging on specific personality traits.