Costes reproductivos y de supervivencia del parasitismo de críaUn estudio a largo plazo

Resumo

Brood parasitism reduces the reproductive success of their hosts, therefore exerting a strong selective pressure which favours the evolution of defences to avoid parasitism. This, in turn, selects for counter-adaptations in parasites, and thus parasites and hosts are immersed in a coevolutionary arms race. The extent of the costs of parasitism differs in distinct host-parasite systems, and an exact estimation of these costs should consider the effects of parasitism on the reproductive success of the hosts across their lifetime (LRS, lifetime reproductive success). Furthermore, brood parasitism could entail costs or consequences distinct to those directly related to the reduction of breeding success in the parasitized broods, also known as extra costs, such as consequences for host survival, which could manifest through the lifetime of individuals. Despite this, the lifetime costs of parasitism have not been explored in most of the systems and the consequences of these costs in terms of overall reproductive success have not been explored. The majority of studies have focused on estimating the short term costs and therefore, in general, we don’t know the true costs of brood parasitism for the majority of hosts and thus we lack a precise estimation of the magnitude of the pressure that parasites exert on the evolution hosts’ defences. This thesis aims to explore short and mid-term costs or consequences of brood parasitism on the system formed by the great spotted cuckoo (Clamator glandarius) and the magpie (Pica pica), its main host in Europe, determining its extra costs (not directly related with the decrease of reproductive success) and the its effect on the longevity of magpies and their LRS. The negative effects of brood parasitism on the reproductive success of this host and the existence of defence mechanisms such as recognition and rejection of parasitic eggs have been documented in this system, but up to now, the lifetime costs of brood parasitism for hosts has not yet been determined. On one hand, the short-term costs of brood parasitism have been estimated via two different approaches: a) a behavioural and correlational approach, and b) a molecular and experimental approach using telomere dynamics as a biomarker of the reproductive (and parasitism) costs. On the other hand, the possible association of parasitism with survival and mortality trajectories, and longevity of the hosts has been explored through a longitudinal study with marked individuals which have been monitored through their lives and which were either never or sometimes parasitized during their reproductive events. The results of this thesis suggest that parasitism does not represent an extra cost for the hosts in the short-to-mid-term, compared to the reproductive effort invested in raising non-parasitized broods. In fact, it is probable that in most of cases parasitism represents a smaller cost than a non-parasitized brood. It also brings to light that parasitism results in reduced telomere shortening in some (probably younger) individuals, which could be related to a greater probability of survival and longevity in parasitized individuals compared to those which are never parasitized. It is suggested that these short -term costs have long-term consequences on life expectancy and host longevity given that the analysis of survival trajectories and mortality shows that those individuals which were parasitized at least once through their lives live longer than those individuals which were never parasitized, as they suffered a lower mortality rate. These differences in longevity amongst individuals with distinct parasitism status imply that individuals sometimes parasitized have a similar LRS than those individuals never been parasitized during their lives. Parasitized individuals who live longer are able to compensate the loss of reproductive success from parasitized reproductive events. This maybe a consequence of a larger number of breeding events but could also be mediated by the development of this host’s main defence against brood parasitism (recognition and rejection of parasitic eggs), which appears at older ages.