INTRODUCTION
Individual variation in fitness is usually large, and at least part of that variation is caused by variation in phenotypic quality, as evidenced by positive associations between reproductive success and parental survival . The early-life environment has been suggested to play a major role in shaping variation in phenotypic quality . Studies of the link between early-life conditions and fitness prospects either measure fitness components directly, by following individuals for a significant part of their lives, or use markers of morphological, physiological and/or molecular state as fitness proxy. Unfortunately, fitness prospects can be difficult to infer using such proxies, because responses to environmental perturbations typically differ between markers . In this context, the length and dynamics of telomeres, complexes of proteins and repetitive DNA at the end of eukaryotic chromosomes , have emerged as relatively robust markers of past experiences. Telomere length and dynamics have been shown to be susceptible to early-life environmental conditions , and longer telomeres and/or lower rates of telomere shortening predict longevity and/or other fitness components in humans and model and non-model organisms .
Variation in telomere length is already present early in life in humans and non-model organisms . While the variation present at birth is mostly maintained throughout adulthood , variation in telomere shortening in early life is associated with morbidity and mortality . However, developing individuals live in complex environments, and despite a recent surge in research effort , little is known about the overall effects of early-life conditions on telomere length (reviewed in . In particular the outcome of the interplay between multiple stressors during growth and their effect on telomere dynamics is poorly understood. The combined effect of stressors on telomere shortening may be additive, whereby the combined result is the sum of the effect of each stressor separately, or synergistic, when multiple stressors interact to produce an effect that is greater than the sum of the individual effects in isolation. Understanding whether the combined effect of multiple stressors is additive or synergistic is of importance, because developing animals will only rarely grow up in conditions that are optimal in every way; it is more likely that some of the many environmental aspects that affect development are at a sub-optimal level, and as such can be considered a stressor.
To address the question whether effects of different stressors are additive or synergistic we examined how/whether two extrinsic stressors, manipulated brood size and parasitic infections, interact to affect growth and telomere shortening of nestling jackdaws Corvusmonedula . A stressful social environment (increased sibling competition) has proven to negatively affect growth and telomere dynamics in this and other bird populations . The effects of parasitic infections on telomere shortening are increasingly attracting attention, with some studies finding that parasitized individuals had shorter telomeres or higher rates of telomere loss , while other studies did not find a clear relationship between parasitic infections and telomere dynamics (; ; ; . Nestling jackdaws in our population are frequently parasitized by the carnid fly Carnus hemapterus , a blood-sucking ectoparasite that is commonly found infecting nestlings from various medium to big size bird species (i.e. starlings, bee-eaters, tawny owls, raptors . Gravid females lay their eggs within the bird’s nest material on which the larvae then feed. The imago emerges the next year around the time the birds’ eggs hatch and parasitize nestlings through a large part of the development period . Due to the end replication problem during cell division and/or loss of the single strand overhang , telomeres shorten in length as cell division progresses. Thus, because Carnus hemapterus feeds on blood, the parasite could increase red blood cell replacement rate, thereby directly affecting telomere dynamics, or indirectly affect telomere dynamics by triggering related physiological processes (i.e. oxidative stress). Nevertheless, we anticipated that parasites would accelerate telomere shortening in their hosts, as reported for great tit nestlings reared in nests experimentally infested with hematophagous hen fleas . Summarizing, we expect parasitized nestlings and nestlings reared in enlarged brood to show reduced growth and faster rates of telomere shortening, and we tested whether these effects were additive or synergistic.