Abstract
BackgroundLife-history studies of wild bird populations often focus on the relationship between an individual’s condition and its capacity to mount an immune response, as measured by a commonly-employed assay of cutaneous immunity, the PHA skin test. In addition, haematocrit, the packed cell volume in relation to total blood volume, is often measured as an indicator of physiological performance. A multi-year study of a wild population of house wrens has recently revealed that those exhibiting the highest condition and strongest PHA responses as nestlings are most likely to be recruited to the breeding population and to breed through two years of age; in contrast, intermediate haematocrit values result in the highest recruitment to the population. Selection theory would predict, therefore, that most of the underlying genetic variation in these traits should be exhausted resulting in low heritability, although such traits may also exhibit low heritability because of increased residual variance. Here, we examine the genetic and environmental variation in condition, cutaneous immunity, and haematocrit using an animal model based on a pedigree of approximately 2,800 house wrens.ResultsEnvironmental effects played a paramount role in shaping the expression of the fitness-related traits measured in this wild population, but two of them, condition and haematocrit, retained significant heritable variation. Condition was also positively correlated with both the PHA response and haematocrit, but in the absence of any significant genetic correlations, it appears that this covariance arises through parallel effects of the environment acting on this suite of traits.ConclusionsThe maintenance of genetic variation in different measures of condition appears to be a pervasive feature of wild bird populations, in contradiction of conventional selection theory. A major challenge in future studies will be to explain how such variation persists in the face of the directional selection acting on condition in house wrens and other species.
Highlights
Introduction to Quantitative GeneticsNew York: Longman; 1981. 29
Notwithstanding this large amount of environmental variation, both nestling haematocrit and condition were significantly heritable, with additive genetic variance accounting for about 15% of the phenotypic variation in each case (Table 3)
Nestling condition differed across years, and male nestlings were of significantly higher body condition than female nestlings (Table 2)
Summary
Introduction to Quantitative GeneticsNew York: Longman; 1981. 29. Jensen H, Sæther B-E, Ringby TH, Tufto J, Griffith SC, Ellegren H: Sexual variation in heritability and genetic correlations of morphological traits in house sparrow (Passer domesticus). A multi-year study of a wild population of house wrens has recently revealed that those exhibiting the highest condition and strongest PHA responses as nestlings are most likely to be recruited to the breeding population and to breed through two years of age; in contrast, intermediate haematocrit values result in the highest recruitment to the population. We examine the genetic and environmental variation in condition, cutaneous immunity, and haematocrit using an animal model based on a pedigree of approximately 2,800 house wrens. In field studies of birds and mammals, an individual’s condition is often measured as body mass corrected for structural body size, which is often derived from the residuals of a regression of mass on body size [4]. Bodymass based measures appear to be tightly linked with fitness in a number of vertebrate species [6]
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