Theoretical studies predict that parasitic infection may impact host longevity and ultimately modify the trade-off between reproduction and survival. Indeed, a host may adjust its energy allocation in current reproduction to balance the negative effects of parasitism on its survival prospects. However, very few empirical studies tested this prediction. Avian haemosporidian parasites provide an excellent opportunity to assess the influence of parasitic infection on both host survival and reproduction. They are represented by three main genera (Plasmodium, Haemoproteus and Leucocytozoon) and are highly prevalent in many bird populations. Here we provide the first known long-term field study (12 years) to explore the effects of haemosporidian parasite infection and co-infection on fitness in two populations of great tits (Parus major), using a multistate modelling framework. We found that while coinfection decreased survival probability, both infection and co-infection increased reproductive success. This study provides evidence that co-infections can be more virulent than single infections. It also provides support for the life-history theory which predicts that reproductive effort can be adjusted to balance one's fitness when survival prospects are challenged.

Sex-biased infections are a recurrent observation in vertebrates. In many species, males are more parasitized than females. Two potentially complementary mechanisms are often suggested to explain this pattern: sexual differences in susceptibility mainly caused by the effect of sex hormones on immunity and differential exposure to parasites. Exposure is mostly a consequence of host behavioural traits, but vector-borne parasitic infections involve another degree of complexity due to the active role of vectors in transmission. Blood-sucking insects may make choices based on cues produced by hosts. Regarding malaria, several studies highlighted a male-biased infection by Plasmodium sp in great tits ( Parus major ). We hypothesize that the mosquito vector, Culex pipiens , might at least partially cause this bias by being more attracted to male birds. Intrinsic variation associated to bird sex would explain a preference of mosquitoes for males. To test this hypothesis, we provide uninfected mosquitoes with a choice between uninfected male and female nestlings. Mosquito choice is assessed by sex typing of the ingested blood. We did not observe any preference for a given sex. This result does not support our prediction of a preference of mosquitoes for male great tits during the nestling period. In conclusion, mosquitoes do not seem to have an intrinsic preference for male nestlings. However, sexually divergent traits ( e . g . behaviour, odour, metabolic rate) present in adults may play a role in the attraction of mosquitoes and should be investigated.

A large diversity of parasites manipulates their hosts in various ways to complete their own life cycle. Enhancing the attractiveness of their host to vectors has been suggested as a strategy allowing vector-borne parasites to increase their transmission. Indeed, a higher attraction of hematophagous, arthropod vectors to infected vertebrates compared to uninfected individuals has been found in many systems (e.g., Trypanosoma-tsetse flies, Leishmania-sand flies, Borrelia-ticks) but was most often verified in the Plasmodium-mosquitoes model. However, a number of studies found no difference in attractiveness, or a higher attractiveness of uninfected hosts. In this review, we present studies reporting a comparison of the attractiveness and/or the biting rate of infected and uninfected vertebrates. We then discuss several biological factors and experimental design aspects that can explain discrepancies between studies. Finally, we stress the importance of investigating the mechanisms of parasiteinduced increased attractiveness of infected hosts to conclude that such observations are cases of adaptive manipulation.

Several factors shape lifetime reproductive success, including genetic background, body condition, environmental conditions and ecological interactions such as parasitism. Adults often show higher reproductive success than their young conspecifics, especially in longlived bird species, and this may be explained by the cumulative effects of an increase in reproductive experience and the selection of high-quality individuals from one year to Accepted ArticleThis article is protected by copyright. All rights reserved.another. To test whether this pattern also exists in short-lived bird species, we used 13 years of monitoring data from two Great Tit Parus major populations. The effects of male and female age on several reproductive parameters were analysed in 419 pairs of Great Tits, whilst accounting for body condition and infection by haemosporidian parasites.Reproductive success was mainly affected by the age-class of males. Pairs containing a subadult male fledged one-third fewer chicks than pairs containing an adult male. The difference was not caused by variation in male fertility but could have been caused by better parental care provided by adult birds. In addition to lower reproductive success, firstyear males also had reduced access to mating compared to adult males, suggesting an avoidance of sub-adult males by females. Nestling body condition was positively correlated with parental body condition, and the body condition of male and female members of breeding pairs was positively correlated. Finally, the number of fledged chicks was mainly affected by the infection status of males. This results temper our previously published results showing an effect of infection on Great Tit reproduction regardless of their sex. In this previous study, and as in most cases, the status of the partner was not taken into account and we show here that this is essential because it can lead to a biased interpretation of the results.