DISCUSSION
Parasitic infection by the carnid fly and sibling competition interacted to negatively affect telomere dynamics in developing individuals. Telomere shortening was accelerated in parasitized nestling jackdaws reared in enlarged broods but not in parasitized nestlings reared in reduced broods, indicating a synergistic effect of these stressors. Furthermore, ectoparasitic infections by Carnus hemapterus did not affect nestling growth but nestlings reared in enlarged broods gained less weight, in accordance with previous findings in the same population .
Telomeres shortened on average 264 bp over the 25-day development period, but when nestlings in enlarged broods suffered from infections by the parasitic carnid fly this was increased to 415 bp, a 57% increase relative to the overall mean. This increase could be a direct effect of enhanced cell proliferation to replenish the red blood cells (RBCs) depleted by the hematophagous fly, as reported for species parasitized by blow fly larvae . Alternatively, but not mutually exclusive, increased telomere loss could be an indirect effect of the parasitic infection, for example through an increase in oxidative stress . The latter possibility is supported by studies reporting increased oxidative stress in birds infected by hematophagous ectoparasites . However, an effect of parasitic infections on telomere dynamics via oxidative stress remains speculative at this stage. Future studies combining brood size manipulation with anti-parasite medication would further confirm a direct effect of parasitic infections on telomere dynamics (i.e. when infected nestlings shorten their telomeres more than non-infected siblings reared in reduced broods). Additionally, measuring physiological parameters such as haemoglobin and markers of oxidative damage could shed light on the mechanism through which blood-sucking parasites increase telomere loss.
The effect of parasitic infection on telomere shortening was contingent on brood size, with Carnus presence accelerating telomere shortening in enlarged broods, but not in reduced broods. This result is in line with findings in other studies showing that the negative effects of adversity on telomere dynamics are more evident under ecological hardship (e.g. in the American alligator, . The finding that nestlings in enlarged broods were more susceptible to effects of Carnusinfestation is an example of a reverse Matthew effect, i.e. individuals that are in a poorer state suffer more from additional adversity than individuals in a better state. This appears to be a widespread phenomenon (e.g. . Occurrence of the reverse Matthew effect during early life may contribute to the fitness consequences of adverse developmental conditions, in a sense that offspring that are in a poorer state because of a particular adverse aspect of their environment may end up more disadvantaged relative to individuals in a better state due to additional challenges.
Environmental adversity generally decreases growth rate, and adverse conditions accelerate telomere shortening. One could therefore expect fast growth to be associated with long telomeres . However, reality is more complex, because studies find contrasting relationships between growth and telomere length . We found TL to decrease with increasing body mass (scaled – see methods), and this association was independent of age, brood size manipulation and Carnus infestation. The fact that the mass effect on TL did not change with age, despite the approximately four-fold increase in body mass, could be taken to mean that the mass effect at fledging age was in fact the day 5 mass effect that was carried over to fledging age. This underlines our earlier finding that individual variation in telomere length is largely determined very early in life (Boonekamp et al 2014). Telomere dynamics during development is the outcome of resource allocation between growth and somatic maintenance, and depending on variation in resource allocation and availability positive and negative associations between growth and TL can arise . Apparently, jackdaw nestlings canalized resources to early growth at the expense of telomere maintenance, possibly because day 5 mass has strong fitness consequences, with survival up to fledging increasing strongly with increasing day 5 mass .
Had we restricted our study of the effects of Carnus infection to growth, we would have concluded that Carnus infection does not noticeably affect jackdaw nestlings, and it is only with the extension of our study to telomere dynamics that effects of Carnusinfection were revealed. Given the earlier finding that telomere shortening predicts post-fledging survival to recruitment in this population (Boonekamp et al 2014), we assume that Carnusinfection thereby affects offspring fitness prospects, but we acknowledge that this remains to be established. Here, we indicate that the effects of early life adversity depend on environmental conditions (i.e. parasite presence), complicating predictions of early-life effects on fitness. Thus, the implications of parasitic infections in combination with brood competition in the longer term remain to be explored under different scenarios. Telomere length and dynamics are proving a useful tool for this purpose (e.g. ; DNA methylation is another promising candidate in this context . For example, it might be that extrinsic stressors have an additive effect on survival, further confirming the impact that early-life adversity can have on fitness prospects . Alternatively, when the negative effects of parasitic infections are more canalised , one might expect no direct effect ofCarnus parasites on survival. Furthermore, cryptic sub-lethal effects of developmental conditions that are expressed beyond the time frame of the study highlight the need to evaluate the effects of developmental conditions beyond morphological parameters.