Abstract
Conditions experienced during early life can have long-term individual consequences by influencing dispersal, survival, recruitment and productivity. Resource allocation during development can have strong carry-over effects onto these key parameters and is directly determined by the quality of parental care. In the black sparrowhawk (Accipiter melanoleucus), a colour-polymorphic raptor, parental morphs influence nestling somatic growth and survival, with pairs consisting of different colour morphs (‘mixed-morph pairs’) producing offspring with lower body mass indices, but higher local apparent survival rates. Resource allocation theory could explain this relationship, with nestlings of mixed-morph pairs trading off a more effective innate immune system against somatic growth. We quantified several innate immune parameters of nestlings (hemagglutination, hemolysis, bacteria-killing capacity and haptoglobin concentration) and triggered an immune response by injecting lipopolysaccharides. Although we found that nestlings with lower body mass index had higher local survival rates, we found no support for the proposed hypothesis: neither baseline immune function nor the induced immune response of nestlings was associated with parental morph combination. Our results suggest that these immune parameters are unlikely to be involved in providing a selective advantage for the different colour morphs’ offspring, and thus innate immunity does not appear to be traded off against a greater allocation of resources to somatic growth. Alternative hypotheses explaining the mechanism of a low nestling body mass index leading to subsequent higher local survival could be related to the post-fledgling dependency period or differences in dispersal patterns for the offspring from different morph combinations.
Highlights
Conditions experienced during early life can have long-term individual consequences by influencing dispersal, survival, recruitment and productivity
We proposed an alternative resource allocation hypothesis to explain why black sparrowhawk offspring from mixed morph parents produced nestlings with a low body mass index but with higher apparent survival rates
We found no support of our second prediction, that birds with higher immune function will have a leaner body; only one immune parameter was associated with nestling body mass index, and this relationship ran counter to our predictions, with higher hemagglutination levels for nestlings with a heavier body
Summary
Conditions experienced during early life can have long-term individual consequences by influencing dispersal, survival, recruitment and productivity. In the black sparrowhawk (Accipiter melanoleucus), a colour-polymorphic raptor, parental morphs influence nestling somatic growth and survival, with pairs consisting of different colour morphs (‘mixed-morph pairs’) producing offspring with lower body mass indices, but higher local apparent survival rates. We found that nestlings with lower body mass index had higher local survival rates, we found no support for the proposed hypothesis: neither baseline immune function nor the induced immune response of nestlings was associated with parental morph combination. Nestlings of altricial bird species are completely reliant on the resources provided by their p arents[25,26] When these resources are limited, individuals must allocate them amongst competing functions, such as somatic growth or the development of the immune system[27,28,29]. By combining multiple assays as well as measuring both baseline innate (constitutive) immune function and an experimentally induced innate immune response, which are regulated differently and have different costs, our study aims for a more complete view of the immune system[59,61,62,63]
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