Abstract
Parent-offspring conflicts lead the offspring to evolve reliable signals of individual quality, including parasite burden, which may allow parents to adaptively modulate investment in the progeny. Sex-related variation in offspring reproductive value, however, may entail differential investment in sons and daughters. Here, we experimentally manipulated offspring condition in the barn swallow (Hirundo rustica) by subjecting nestlings to an immune challenge (injection with bacterial lipopolysaccharide, LPS) that simulates a bacterial infection, and assessed the effects on growth, feather quality, expression of morphological (gape coloration) and behavioral (posture) begging displays involved in parent-offspring communication, as well as on food allocation by parents. Compared to sham-injected controls, LPS-treated chicks suffered a depression of body mass and a reduction of palate color saturation. In addition, LPS treatment resulted in lower feather quality, with an increase in the occurrence of fault bars on wing feathers. The color of beak flanges, feather growth and the intensity of postural begging were affected by LPS treatment only in females, suggesting that chicks of either sex are differently susceptible to the immune challenge. However, irrespective of the effects of LPS, parents equally allocated food among control and challenged offspring both under normal food provisioning and after a short period of food deprivation of the chicks. These results indicate that bacterial infection and the associated immune response entail different costs to offspring of either sex, but a decrease in nestling conditions does not affect parental care allocation, possibly because the barn swallow adopts a brood-survival strategy. Finally, we showed that physiological stress induced by pathogens impairs plumage quality, a previously neglected major negative impact of bacterial infection which could severely affect fitness, particularly among long-distance migratory birds.
Highlights
Theoretical models of conflicts among family members posit that offspring are selected to obtain a larger share of parental resources than their siblings and to attract more care than would be optimal for parents to provide [1,2,3]
Natural selection may have promoted the parental ability to allocate resources according to variation in offspring signals of need and condition
Parasites can negatively influence the physiological state of their hosts by causing disease and reducing food intake and resource assimilation [17,18], or imposing an energy cost due to mounting an immune response which may have to be traded against competing physiological functions [19,20,21]
Summary
Theoretical models of conflicts among family members posit that offspring are selected to obtain a larger share of parental resources than their siblings and to attract more care than would be optimal for parents to provide [1,2,3]. Natural selection may have promoted the parental ability to allocate resources according to variation in offspring signals of need (e.g. hunger) and condition (general state) (see [9]). Both theoretical models and experimental studies have supported this prediction, and have suggested that multi-trait begging displays convey reliable information over offspring quality to attending parents [2,3,7,10,11,12,13]. The marginal fitness return of investing in offspring of different condition may vary according to contingent need of food by individual nestlings [15,25,27,28]
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