Abstract
Parasitism experienced early in ontogeny can have a major impact on host growth, development and future fitness, but whether siblings are affected equally by parasitism is poorly understood. In birds, hatching asynchrony induced by hormonal or behavioural mechanisms largely under parental control might predispose young to respond to infection in different ways. Here we show that parasites can have different consequences for offspring depending on their position in the family hierarchy. We experimentally treated European Shag (Phalacrocorax aristoteli) nestlings with the broad-spectrum anti-parasite drug ivermectin and compared their growth rates with nestlings from control broods. Average growth rates measured over the period of linear growth (10 days to 30 days of age) and survival did not differ for nestlings from treated and control broods. However, when considering individuals within broods, parasite treatment reversed the patterns of growth for individual family members: last-hatched nestlings grew significantly slower than their siblings in control nests but grew faster in treated nests. This was at the expense of their earlier-hatched brood-mates, who showed an overall growth rate reduction relative to last-hatched nestlings in treated nests. These results highlight the importance of exploring individual variation in the costs of infection and suggest that parasites could be a key factor modulating within-family dynamics, sibling competition and developmental trajectories from an early age.
Highlights
Environmental and social conditions experienced at critical early life stages can impact juvenile growth and development in vertebrates, with potentially long-lasting effects on health and performance [1,2,3,4]
Nestling growth rates Overall, there was no significant effect of the ivermectin treatment on nestling growth rates (Table 1; main analysis based on nests where all 3 three nestlings survived)
While we found no overall differences in growth rates or fledging success between treated and untreated broods, treatment with a broad spectrum anti-parasite drug reversed the patterns of growth within a brood
Summary
Environmental and social conditions experienced at critical early life stages can impact juvenile growth and development in vertebrates, with potentially long-lasting effects on health and performance [1,2,3,4]. Stressful rearing conditions such as food and nutrient limitation, inclement weather and sibling competition can depress nestling growth rates [5,6,7,8], and reduced size and/or body condition at fledging has been shown across species to negatively impact post-fledging survival and recruitment success [9,10,11,12]. Young birds are potentially more severely affected by parasitic infection than adults as a result of a less efficient immune system, which is not fully developed at hatching [19,20], and exposure to nest-dwelling parasites [21]. Parasitism can alter early developmental trajectories [25,26,27,28], as resources otherwise allocated to growth are diverted to fight infection [29]
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